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- libpng.txt - a description on how to use and modify libpng
- libpng version 0.99b
- Updated and distributed by Glenn Randers-Pehrson <randeg@alumni.rpi.edu>
- Copyright (c) 1998, Glenn Randers-Pehrson
- February 3, 1998
- based on:
- libpng 1.0 beta 6 version 0.96
- Updated and distributed by Andreas Dilger
- Copyright (c) 1996, 1997 Andreas Dilger
- May 28, 1997
- libpng 1.0 beta 2 - version 0.88
- For conditions of distribution and use, see copyright notice in png.h
- Copyright (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc.
- January 26, 1996
- Updated/rewritten per request in the libpng FAQ
- Copyright (c) 1995 Frank J. T. Wojcik
- December 18, 1995 && January 20, 1996
- I. Introduction
- This file describes how to use and modify the PNG reference library
- (known as libpng) for your own use. There are five sections to this
- file: introduction, structures, reading, writing, and modification and
- configuration notes for various special platforms. In addition to this
- file, example.c is a good starting point for using the library, as
- it is heavily commented and should include everything most people
- will need.
- Libpng was written as a companion to the PNG specification, as a way
- of reducing the amount of time and effort it takes to support the PNG
- file format in application programs. The PNG specification is available
- as RFC 2083 <ftp://ftp.uu.net/graphics/png/documents/> and as a
- W3C Recommendation <http://www.w3.org/pub/WWW/TR/REC.png.html>. Some
- additional chunks are described in the special-purpose public chunks
- documents at <ftp://ftp.uu.net/graphics/png/documents/>. Other information
- about PNG can be found at the PNG home page, <http://www.cdrom.com/pub/png/>.
- Most users will not have to modify the library significantly; advanced
- users may want to modify it more. All attempts were made to make it as
- complete as possible, while keeping the code easy to understand.
- Currently, this library only supports C. Support for other languages
- is being considered.
- Libpng has been designed to handle multiple sessions at one time,
- to be easily modifiable, to be portable to the vast majority of
- machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy
- to use. The ultimate goal of libpng is to promote the acceptance of
- the PNG file format in whatever way possible. While there is still
- work to be done (see the TODO file), libpng should cover the
- majority of the needs of its users.
- Libpng uses zlib for its compression and decompression of PNG files.
- The zlib compression utility is a general purpose utility that is
- useful for more than PNG files, and can be used without libpng.
- See the documentation delivered with zlib for more details.
- You can usually find the source files for the zlib utility wherever you
- find the libpng source files.
- Libpng is thread safe, provided the threads are using different
- instances of the structures. Each thread should have its own
- png_struct and png_info instances, and thus its own image.
- Libpng does not protect itself against two threads using the
- same instance of a structure.
- II. Structures
- There are two main structures that are important to libpng, png_struct
- and png_info. The first, png_struct, is an internal structure that
- will not, for the most part, be used by a user except as the first
- variable passed to every libpng function call. It is not actually
- used in many of the functions; do not be alarmed about compiler
- warnings that say something to the effect that "png_ptr is not used."
- The png_info structure is designed to provide information about the
- PNG file. At one time, the fields of png_info were intended to be
- directly accessible to the user. However, this tended to cause problems
- with applications using dynamically loaded libraries, and as a result
- a set of interface functions for png_info was developed. The fields
- of png_info are still available for older applications, but it is
- suggested that applications use the new interfaces if at all possible.
- The png.h header file is an invaluable reference for programming with libpng.
- And while I'm on the topic, make sure you include the libpng header file:
- #include <png.h>
- III. Reading
- Reading PNG files:
- We'll now walk you through the possible functions to call when reading
- in a PNG file, briefly explaining the syntax and purpose of each one.
- See example.c and png.h for more detail. While Progressive reading
- is covered in the next section, you will still need some of the
- functions discussed in this section to read a PNG file.
- You will want to do the I/O initialization(*) before you get into libpng,
- so if it doesn't work, you don't have much to undo. Of course, you
- will also want to insure that you are, in fact, dealing with a PNG
- file. Libpng provides a simple check to see if a file is a PNG file.
- To use it, pass in the first 1 to 8 bytes of the file, and it will
- return true or false (1 or 0) depending on whether the bytes could be
- part of a PNG file. Of course, the more bytes you pass in, the
- greater the accuracy of the prediction.
- If you are intending to keep the file pointer open for use in libpng,
- you must ensure you don't read more than 8 bytes from the beginning
- of the file, and you also have to make a call to png_set_sig_bytes_read()
- with the number of bytes you read from the beginning. Libpng will
- then only check the bytes (if any) that your program didn't read.
- (*): If you are not using the standard I/O functions, you will need
- to replace them with custom functions. See the discussion under
- Customizing libpng.
- FILE *fp = fopen(file_name, "rb");
- if (!fp)
- {
- return;
- }
- fread(header, 1, number, fp);
- is_png = png_check_sig(header, 0, number);
- if (!is_png)
- {
- return;
- }
- Next, png_struct and png_info need to be allocated and initialized. In
- order to ensure that the size of these structures is correct even with a
- dynamically linked libpng, there are functions to initialize and
- allocate the structures. We also pass the library version, optional
- pointers to error handling functions, and a pointer to a data struct for
- use by the error functions, if necessary (the pointer and functions can
- be NULL if the default error handlers are to be used). See the section
- on Changes to Libpng below regarding the old initialization functions.
- png_structp png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,
- (void *)user_error_ptr, user_error_fn, user_warning_fn);
- if (!png_ptr)
- return;
- png_infop info_ptr = png_create_info_struct(png_ptr);
- if (!info_ptr)
- {
- png_destroy_read_struct(&png_ptr, (png_infopp)NULL, (png_infopp)NULL);
- return;
- }
- png_infop end_info = png_create_info_struct(png_ptr);
- if (!end_info)
- {
- png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL);
- return;
- }
- The error handling routines passed to png_create_read_struct() are only
- necessary if you are not using the libpng supplied error handling
- functions. When libpng encounters an error, it expects to longjmp back
- to your routine. Therefore, you will need to call setjmp and pass the
- jmpbuf field of your png_struct. If you read the file from different
- routines, you will need to update the jmpbuf field every time you enter
- a new routine that will call a png_ function.
- See your documentation of setjmp/longjmp for your compiler for more
- information on setjmp/longjmp. See the discussion on libpng error
- handling in the Customizing Libpng section below for more information on
- the libpng error handling. If an error occurs, and libpng longjmp's
- back to your setjmp, you will want to call png_destroy_read_struct() to
- free any memory.
- if (setjmp(png_ptr->jmpbuf))
- {
- png_destroy_read_struct(&png_ptr, &info_ptr, &end_info);
- fclose(fp);
- return;
- }
- Now you need to set up the input code. The default for libpng is to
- use the C function fread(). If you use this, you will need to pass a
- valid FILE * in the function png_init_io(). Be sure that the file is
- opened in binary mode. If you wish to handle reading data in another
- way, you need not call the png_init_io() function, but you must then
- implement the libpng I/O methods discussed in the Customizing Libpng
- section below.
- png_init_io(png_ptr, fp);
- If you had previously opened the file and read any of the signature from
- the beginning in order to see if this was a PNG file, you need to let
- libpng know that there are some bytes missing from the start of the file.
- png_set_sig_bytes(png_ptr, number);
- In PNG files, the alpha channel in an image is the level of opacity.
- If you need the alpha channel in an image to be the level of transparency
- instead of opacity, you can invert the alpha channel (or the tRNS chunk
- data) after it's read, so that 0 is fully opaque and 255 (in 8-bit or
- paletted images) or 65535 (in 16-bit images) is fully transparent, with
- png_set_invert_alpha(png_ptr);
- This has to appear here rather than later with the other transformations
- because the tRNS chunk data must be modified in the case of paletted images.
- If your image is not a paletted image, the tRNS data (which in such cases
- represents a single color to be rendered as transparent) won't be changed.
- You are now ready to read all the file information up to the actual
- image data. You do this with a call to png_read_info().
- png_read_info(png_ptr, info_ptr);
- Functions are used to get the information from the info_ptr:
- png_get_IHDR(png_ptr, info_ptr, &width, &height, &bit_depth, &color_type,
- &interlace_type, &compression_type, &filter_type);
- width - holds the width of the image in pixels (up to 2^31).
- height - holds the height of the image in pixels (up to 2^31).
- bit_depth - holds the bit depth of one of the image channels.
- (valid values are 1, 2, 4, 8, 16 and depend also on the
- color_type. See also significant bits (sBIT) below).
- color_type - describes which color/alpha channels are present.
- PNG_COLOR_TYPE_GRAY (bit depths 1, 2, 4, 8, 16)
- PNG_COLOR_TYPE_GRAY_ALPHA (bit depths 8, 16)
- PNG_COLOR_TYPE_PALETTE (bit depths 1, 2, 4, 8)
- PNG_COLOR_TYPE_RGB (bit_depths 8, 16)
- PNG_COLOR_TYPE_RGB_ALPHA (bit_depths 8, 16)
- PNG_COLOR_MASK_PALETTE
- PNG_COLOR_MASK_COLOR
- PNG_COLOR_MASK_ALPHA
- filter_type - (must be PNG_FILTER_TYPE_BASE for PNG 1.0)
- compression_type - (must be PNG_COMPRESSION_TYPE_BASE for PNG 1.0)
- interlace_type - (PNG_INTERLACE_TYPE_NONE or PNG_INTERLACE_TYPE_ADAM7)
- Any or all of interlace_type, compression_type, of filter_type can be
- NULL if you are not interested in their values.
- channels = png_get_channels(png_ptr, info_ptr);
- channels - number of channels of info for the color type
- (valid values are 1 (GRAY, PALETTE), 2 (GRAY_ALPHA),
- 3 (RGB), 4 (RGB_ALPHA or RGB + filler byte))
- rowbytes = png_get_rowbytes(png_ptr, info_ptr);
- rowbytes - number of bytes needed to hold a row
- signature = png_get_signature(png_ptr, info_ptr);
- signature - holds the signature read from the file (if any). The
- data is kept in the same offset it would be if the
- whole signature were read (ie if an application had
- already read in 4 bytes of signature before staring
- libpng, the remaining 4 bytes would be in signature[4]
- through signature[7] (see png_set_sig_bytes())).
-
- Information from each if the IHDR fields can be retrieve separately as well:
- width = png_get_image_width(png_ptr, info_ptr);
- height = png_get_image_height(png_ptr, info_ptr);
- bit_depth = png_get_bit_depth(png_ptr, info_ptr);
- color_type = png_get_color_type(png_ptr, info_ptr);
- filter_type = png_get_filter_type(png_ptr, info_ptr);
- compression_type = png_get_compression_type(png_ptr, info_ptr);
- interlace_type = png_get_interlace_type(png_ptr, info_ptr);
- These are also important, but their validity depends on whether the chunk
- has been read. The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and
- png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the
- data has been read, or zero if it is missing. The parameters to the
- png_get_<chunk> are set directly if they are simple data types, or a pointer
- into the info_ptr is returned for any complex types.
- png_get_PLTE(png_ptr, info_ptr, &palette, &num_palette);
- palette - the palette for the file (array of png_color)
- num_palette - number of entries in the palette
- png_get_gAMA(png_ptr, info_ptr, &gamma);
- gamma - the gamma the file is written at (PNG_INFO_gAMA)
- png_get_sRGB(png_ptr, info_ptr, &srgb_intent);
- srgb_intent - the rendering intent (PNG_INFO_sRGB)
- The presence of the sRGB chunk means that the pixel
- data is in the sRGB color space. This chunk also
- implies specific values of gAMA and cHRM.
- png_get_sBIT(png_ptr, info_ptr, &sig_bit);
- sig_bit - the number of significant bits for (PNG_INFO_sBIT)
- the gray, red, green, and blue channels, whichever
- are appropriate for the given color type (png_color_16)
- png_get_tRNS(png_ptr, info_ptr, &trans, &num_trans, &trans_values);
- trans - array of transparent entries for palette (PNG_INFO_tRNS)
- trans_values - transparent pixel for non-paletted images (PNG_INFO_tRNS)
- num_trans - number of transparent entries (PNG_INFO_tRNS)
- png_get_hIST(png_ptr, info_ptr, &hist); (PNG_INFO_hIST)
- hist - histogram of palette (array of png_color_16)
- png_get_tIME(png_ptr, info_ptr, &mod_time);
- mod_time - time image was last modified (PNG_VALID_tIME)
- png_get_bKGD(png_ptr, info_ptr, &background);
- background - background color (PNG_VALID_bKGD)
- num_text = png_get_text(png_ptr, info_ptr, &text_ptr);
- text_ptr - array of png_text holding image comments
- text_ptr[i]->key - keyword for comment.
- text_ptr[i]->text - text comments for current keyword.
- text_ptr[i]->compression - type of compression used on "text"
- PNG_TEXT_COMPRESSION_NONE or
- PNG_TEXT_COMPRESSION_zTXt
- num_text - number of comments
- png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y, &unit_type);
- offset_x - positive offset from the left edge of the screen
- offset_y - positive offset from the top edge of the screen
- unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
- png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y, &unit_type);
- res_x - pixels/unit physical resolution in x direction
- res_y - pixels/unit physical resolution in x direction
- unit_type - PNG_RESOLUTION_UNKOWN, PNG_RESOLUTION_METER
- The data from the pHYs chunk can be retrieved in several convenient
- forms:
- res_x_and_y = png_get_pixels_per_meter(png_ptr, info_ptr)
- aspect_ratio = png_get_pixel_aspect_ratio(png_ptr, info_ptr)
- (Each of these returns 0 [signifying "unknown"] if the data is not
- present or if res_x is 0; res_x_and_y is 0 if res_x != res_y)
- For more information, see the png_info definition in png.h and the
- PNG specification for chunk contents. Be careful with trusting
- rowbytes, as some of the transformations could increase the space
- needed to hold a row (expand, filler, gray_to_rgb, etc.).
- See png_read_update_info(), below.
- A quick word about text_ptr and num_text. PNG stores comments in
- keyword/text pairs, one pair per chunk, with no limit on the number
- of text chunks, and a 2^31 byte limit on their size. While there are
- suggested keywords, there is no requirement to restrict the use to these
- strings. It is strongly suggested that keywords and text be sensible
- to humans (that's the point), so don't use abbreviations. Non-printing
- symbols are not allowed. See the PNG specification for more details.
- There is also no requirement to have text after the keyword.
- Keywords should be limited to 79 Latin-1 characters without leading or
- trailing spaces, but non-consecutive spaces are allowed within the
- keyword. It is possible to have the same keyword any number of times.
- The text_ptr is an array of png_text structures, each holding pointer
- to a keyword and a pointer to a text string. Only the text string may
- be null. The keyword/text pairs are put into the array in the order
- that they are received. However, some or all of the text chunks may be
- after the image, so, to make sure you have read all the text chunks,
- don't mess with these until after you read the stuff after the image.
- This will be mentioned again below in the discussion that goes with
- png_read_end().
- After you've read the header information, you can set up the library
- to handle any special transformations of the image data. The various
- ways to transform the data will be described in the order that they
- should occur. This is important, as some of these change the color
- type and/or bit depth of the data, and some others only work on
- certain color types and bit depths. Even though each transformation
- checks to see if it has data that it can do somthing with, you should
- make sure to only enable a transformation if it will be valid for the
- data. For example, don't swap red and blue on grayscale data.
- The colors used for the background and transparency values should be
- supplied in the same format/depth as the current image data. They
- are stored in the same format/depth as the image data in a bKGD or tRNS
- chunk, so this is what libpng expects for this data. The colors are
- transformed to keep in sync with the image data when an application
- calls the png_read_update_info() routine (see below).
- Data will be decoded into the supplied row buffers packed into bytes
- unless the library has been told to transform it into another format.
- For example, 4 bit/pixel paletted or grayscale data will be returned
- 2 pixels/byte with the leftmost pixel in the high-order bits of the
- byte, unless png_set_packing() is called. 8-bit RGB data will be stored
- in RGBRGBRGB format unless png_set_filler() is called to insert filler
- bytes, either before or after each RGB triplet. 16-bit RGB data will
- be returned RRGGBBRRGGBB, with the most significant byte of the color
- value first, unless png_set_strip_16() is called to transform it to
- regular RGBRGB triplets.
- The following code transforms grayscale images of less than 8 to 8 bits,
- changes paletted images to RGB, and adds a full alpha channel if there is
- transparency information in a tRNS chunk. This is most useful on
- grayscale images with bit depths of 2 or 4 or if there is a multiple-image
- viewing application that wishes to treat all images in the same way.
- if (color_type == PNG_COLOR_TYPE_PALETTE && bit_depth <= 8)
- png_set_expand(png_ptr);
- if (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8)
- png_set_expand(png_ptr);
- if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS))
- png_set_expand(png_ptr);
- PNG can have files with 16 bits per channel. If you only can handle
- 8 bits per channel, this will strip the pixels down to 8 bit.
- if (bit_depth == 16)
- png_set_strip_16(png_ptr);
- The png_set_background() function tells libpng to composite images
- with alpha or simple transparency against the supplied background
- color. If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid),
- you may use this color, or supply another color more suitable for
- the current display (e.g., the background color from a web page). You
- need to tell libpng whether the color is in the gamma space of the
- display (PNG_BACKGROUND_GAMMA_SCREEN for colors you supply), the file
- (PNG_BACKGROUND_GAMMA_FILE for colors from the bKGD chunk), or one
- that is neither of these gammas (PNG_BACKGROUND_GAMMA_UNIQUE - I don't
- know why anyone would use this, but it's here).
- If, for some reason, you don't need the alpha channel on an image,
- and you want to remove it rather than combining it with the background
- (but the image author certainly had in mind that you *would* combine
- it with the background, so that's what you should probably do):
- if (color_type & PNG_COLOR_MASK_ALPHA)
- png_set_strip_alpha(png_ptr);
- PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
- they can, resulting in, for example, 8 pixels per byte for 1 bit
- files. This code expands to 1 pixel per byte without changing the
- values of the pixels:
- if (bit_depth < 8)
- png_set_packing(png_ptr);
- PNG files have possible bit depths of 1, 2, 4, 8, and 16. All pixels
- stored in a PNG image have been "scaled" or "shifted" up to the next
- higher possible bit depth (eg from 5 bits/sample in the range [0,31] to
- 8 bits/sample in the range [0, 255]). However, it is also possible to
- convert the PNG pixel data back to the original bit depth of the image.
- This call reduces the pixels back down to the original bit depth:
- png_color_16p sig_bit;
- if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
- png_set_shift(png_ptr, sig_bit);
- PNG files store 3-color pixels in red, green, blue order. This code
- changes the storage of the pixels to blue, green, red:
- if (color_type == PNG_COLOR_TYPE_RGB ||
- color_type == PNG_COLOR_TYPE_RGB_ALPHA)
- png_set_bgr(png_ptr);
- PNG files store RGB pixels packed into 3 bytes. This code expands them
- into 4 bytes for windowing systems that need them in this format:
- if (bit_depth == 8 && color_type == PNG_COLOR_TYPE_RGB)
- png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);
- where "filler" is the number to fill with, and the location is
- either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
- you want the filler before the RGB or after. This transformation
- does not affect images that already have full alpha channels.
- If you are reading an image with an alpha channel, and you need the
- data as ARGB instead of the normal PNG format RGBA:
- if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
- png_set_swap_alpha(png_ptr);
- For some uses, you may want a grayscale image to be represented as
- RGB. This code will do that conversion:
- if (color_type == PNG_COLOR_TYPE_GRAY ||
- color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
- png_set_gray_to_rgb(png_ptr);
- If you have a grayscale and you are using png_set_expand() to change to
- a higher bit-depth you must indicate if the supplied background gray
- is supplied in the original file bit depth (need_expand = 1) or in the
- expanded bit depth (need_expand = 0). Similarly, if you are reading
- a paletted image, you must indicate if you have supplied the background
- as a palette index that needs to be expanded (need_expand = 1). You can
- also specify an RGB triplet that isn't in the palette when setting your
- background for a paletted image.
- png_color_16 my_background;
- png_color_16p image_background;
- if (png_get_bKGD(png_ptr, info_ptr, &image_background))
- png_set_background(png_ptr, image_background),
- PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
- else
- png_set_background(png_ptr, &my_background,
- PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);
- To properly display PNG images on any kind of system, the application needs
- to know what the display gamma is. Ideally, the user will know this, and
- the application will allow them to set it. One method of allowing the user
- to set the display gamma separately for each system is to check for the
- DISPLAY_GAMMA and VIEWING_GAMMA environment variables or for a SCREEN_GAMMA
- environment variable, which will hopefully be correctly set.
- Note that display_gamma is the gamma of your display, while screen_gamma is
- the overall gamma correction required to produce pleasing results,
- which depends on the lighting conditions in the surrounding environment.
- Screen_gamma is display_gamma/viewing_gamma, where viewing_gamma is
- the amount of additional gamma correction needed to compensate for
- a dim (viewing_gamma=1.125) or dark (viewing_gamma=1.25) environment.
- In a brightly lit room, no compensation other than the display_gamma
- is needed (viewing_gamma=1.0).
- if (/* We have a user-defined screen gamma value */)
- {
- screen_gamma = user_defined_screen_gamma;
- }
- /* One way that applications can share the same screen gamma value */
- else if ((gamma_str = getenv("SCREEN_GAMMA")) != NULL)
- {
- screen_gamma = atof(gamma_str);
- }
- /* If we don't have another value */
- else
- {
- screen_gamma = 2.5; /* A good guess for a PC monitor in a bright office */
- screen_gamma = 2.2; /* A good guess for a PC monitor in a dim room */
- screen_gamma = 2.0; /* A good guess for a PC monitor in a dark room */
- screen_gamma = 1.7 or 1.0; /* A good guess for Mac systems */
- }
- The png_set_gamma() function handles gamma transformations of the data.
- Pass both the file gamma and the current screen_gamma. If the file does
- not have a gamma value, you can pass one anyway if you have an idea what
- it is (usually 0.50 is a good guess for GIF images on PCs). Note
- that file gammas are inverted from screen gammas. See the discussions
- on gamma in the PNG specification for an excellent description of what
- gamma is, and why all applications should support it. It is strongly
- recommended that PNG viewers support gamma correction.
- if (png_get_gAMA(png_ptr, info_ptr, &gamma))
- png_set_gamma(png_ptr, screen_gamma, gamma);
- else
- png_set_gamma(png_ptr, screen_gamma, 0.50);
- If you need to reduce an RGB file to a paletted file, or if a paletted
- file has more entries then will fit on your screen, png_set_dither()
- will do that. Note that this is a simple match dither that merely
- finds the closest color available. This should work fairly well with
- optimized palettes, and fairly badly with linear color cubes. If you
- pass a palette that is larger then maximum_colors, the file will
- reduce the number of colors in the palette so it will fit into
- maximum_colors. If there is a histogram, it will use it to make
- more intelligent choices when reducing the palette. If there is no
- histogram, it may not do as good a job.
- if (color_type & PNG_COLOR_MASK_COLOR)
- {
- if (png_get_valid(png_ptr, info_ptr, PNG_INFO_PLTE))
- {
- png_color_16p histogram;
- png_get_hIST(png_ptr, info_ptr, &histogram);
- png_set_dither(png_ptr, palette, num_palette, max_screen_colors,
- histogram, 1);
- }
- else
- {
- png_color std_color_cube[MAX_SCREEN_COLORS] =
- { ... colors ... };
- png_set_dither(png_ptr, std_color_cube, MAX_SCREEN_COLORS,
- MAX_SCREEN_COLORS, NULL,0);
- }
- }
- PNG files describe monochrome as black being zero and white being one.
- The following code will reverse this (make black be one and white be
- zero):
- if (bit_depth == 1 && color_type == PNG_COLOR_GRAY)
- png_set_invert_mono(png_ptr);
- PNG files store 16 bit pixels in network byte order (big-endian,
- ie. most significant bits first). This code chages the storage to the
- other way (little-endian, ie. least significant bits first, eg. the
- way PCs store them):
- if (bit_depth == 16)
- png_set_swap(png_ptr);
- If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
- need to change the order the pixels are packed into bytes, you can use:
- if (bit_depth < 8)
- png_set_packswap(png_ptr);
- The last thing to handle is interlacing; this is covered in detail below,
- but you must call the function here if you want libpng to handle expansion
- of the interlaced image.
- number_of_passes = png_set_interlace_handling(png_ptr);
- After setting the transformations, libpng can update your png_info
- structure to reflect any transformations you've requested with this
- call. This is most useful to update the info structure's rowbytes
- field so you can use it to allocate your image memory. This function
- will also update your palette with the correct screen_gamma and
- background if these have been given with the calls above.
- png_read_update_info(png_ptr, info_ptr);
- After you call png_read_update_info(), you can allocate any
- memory you need to hold the image. The row data is simply
- raw byte data for all forms of images. As the actual allocation
- varies among applications, no example will be given. If you
- are allocating one large chunk, you will need to build an
- array of pointers to each row, as it will be needed for some
- of the functions below.
- After you've allocated memory, you can read the image data.
- The simplest way to do this is in one function call. If you are
- allocating enough memory to hold the whole image, you can just
- call png_read_image() and libpng will read in all the image data
- and put it in the memory area supplied. You will need to pass in
- an array of pointers to each row.
- This function automatically handles interlacing, so you don't need
- to call png_set_interlace_handling() or call this function multiple
- times, or any of that other stuff necessary with png_read_rows().
- png_read_image(png_ptr, row_pointers);
- where row_pointers is:
- png_bytep row_pointers[height];
- You can point to void or char or whatever you use for pixels.
- If you don't want to read in the whole image at once, you can
- use png_read_rows() instead. If there is no interlacing (check
- interlace_type == PNG_INTERLACE_TYPE_NONE), this is simple:
- png_read_rows(png_ptr, row_pointers, NULL, number_of_rows);
- where row_pointers is the same as in the png_read_image() call.
- If you are doing this just one row at a time, you can do this with
- row_pointers:
- png_bytep row_pointers = row;
- png_read_row(png_ptr, &row_pointers, NULL);
- If the file is interlaced (info_ptr->interlace_type != 0), things get
- somewhat harder. The only current (PNG Specification version 1.0)
- interlacing type for PNG is (interlace_type == PNG_INTERLACE_TYPE_ADAM7)
- is a somewhat complicated 2D interlace scheme, known as Adam7, that
- breaks down an image into seven smaller images of varying size, based
- on an 8x8 grid.
- libpng can fill out those images or it can give them to you "as is".
- If you want them filled out, there are two ways to do that. The one
- mentioned in the PNG specification is to expand each pixel to cover
- those pixels that have not been read yet (the "rectangle" method).
- This results in a blocky image for the first pass, which gradually
- smooths out as more pixels are read. The other method is the "sparkle"
- method, where pixels are drawn only in their final locations, with the
- rest of the image remaining whatever colors they were initialized to
- before the start of the read. The first method usually looks better,
- but tends to be slower, as there are more pixels to put in the rows.
- If you don't want libpng to handle the interlacing details, just call
- png_read_rows() seven times to read in all seven images. Each of the
- images is a valid image by itself, or they can all be combined on an
- 8x8 grid to form a single image (although if you intend to combine them
- you would be far better off using the libpng interlace handling).
- The first pass will return an image 1/8 as wide as the entire image
- (every 8th column starting in column 0) and 1/8 as high as the original
- (every 8th row starting in row 0), the second will be 1/8 as wide
- (starting in column 4) and 1/8 as high (also starting in row 0). The
- third pass will be 1/4 as wide (every 4th pixel starting in column 0) and
- 1/8 as high (every 8th row starting in row 4), and the fourth pass will
- be 1/4 as wide and 1/4 as high (every 4th column starting in column 2,
- and every 4th row starting in row 0). The fifth pass will return an
- image 1/2 as wide, and 1/4 as high (starting at column 0 and row 2),
- while the sixth pass will be 1/2 as wide and 1/2 as high as the original
- (starting in column 1 and row 0). The seventh and final pass will be as
- wide as the original, and 1/2 as high, containing all of the odd
- numbered scanlines. Phew!
- If you want libpng to expand the images, call this before calling
- png_start_read_image() or png_read_update_info():
- if (interlace_type == PNG_INTERLACE_TYPE_ADAM7)
- number_of_passes = png_set_interlace_handling(png_ptr);
- This will return the number of passes needed. Currently, this
- is seven, but may change if another interlace type is added.
- This function can be called even if the file is not interlaced,
- where it will return one pass.
- If you are not going to display the image after each pass, but are
- going to wait until the entire image is read in, use the sparkle
- effect. This effect is faster and the end result of either method
- is exactly the same. If you are planning on displaying the image
- after each pass, the "rectangle" effect is generally considered the
- better looking one.
- If you only want the "sparkle" effect, just call png_read_rows() as
- normal, with the third parameter NULL. Make sure you make pass over
- the image number_of_passes times, and you don't change the data in the
- rows between calls. You can change the locations of the data, just
- not the data. Each pass only writes the pixels appropriate for that
- pass, and assumes the data from previous passes is still valid.
- png_read_rows(png_ptr, row_pointers, NULL, number_of_rows);
- If you only want the first effect (the rectangles), do the same as
- before except pass the row buffer in the third parameter, and leave
- the second parameter NULL.
- png_read_rows(png_ptr, NULL, row_pointers, number_of_rows);
- After you are finished reading the image, you can finish reading
- the file. If you are interested in comments or time, which may be
- stored either before or after the image data, you should pass the
- info_ptr pointer from the png_read_info() call, or you can pass a
- separate png_info struct if you want to keep the comments from
- before and after the image separate. If you are not interested, you
- can pass NULL.
- png_read_end(png_ptr, end_info);
- When you are done, you can free all memory allocated by libpng like this:
- png_destroy_read_struct(&png_ptr, &info_ptr, &end_info);
- For a more compact example of reading a PNG image, see the file example.c.
- Reading PNG files progressively:
- The progressive reader is slightly different then the non-progressive
- reader. Instead of calling png_read_info(), png_read_rows(), and
- png_read_end(), you make one call to png_process_data(), which calls
- callbacks when it has the info, a row, or the end of the image. You
- set up these callbacks with png_set_progressive_read_fn(). You don't
- have to worry about the input/output functions of libpng, as you are
- giving the library the data directly in png_process_data(). I will
- assume that you have read the section on reading PNG files above,
- so I will only highlight the differences (although I will show
- all of the code).
- png_structp png_ptr;
- png_infop info_ptr;
- /* An example code fragment of how you would initialize the progressive
- reader in your application. */
- int
- initialize_png_reader()
- {
- png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,
- (void *)user_error_ptr, user_error_fn, user_warning_fn);
- if (!png_ptr)
- return -1;
- info_ptr = png_create_info_struct(png_ptr);
- if (!info_ptr)
- {
- png_destroy_read_struct(&png_ptr, (png_infopp)NULL, (png_infopp)NULL);
- return -1;
- }
- if (setjmp(png_ptr->jmpbuf))
- {
- png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL);
- return -1;
- }
- /* This one's new. You can provide functions to be called
- when the header info is valid, when each row is completed,
- and when the image is finished. If you aren't using all
- functions, you can specify a NULL parameter. You can use
- any struct as the user_ptr (cast to a void pointer for the
- function call), and retrieve the pointer from inside the
- callbacks using the function png_get_progressive_ptr(png_ptr);
- which will return a void pointer, which you have to cast
- appropriately.
- */
- png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
- info_callback, row_callback, end_callback);
- return 0;
- }
- /* A code fragment that you call as you recieve blocks of data */
- int
- process_data(png_bytep buffer, png_uint_32 length)
- {
- if (setjmp(png_ptr->jmpbuf))
- {
- png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL);
- return -1;
- }
- /* This one's new also. Simply give it a chunk of data
- from the file stream (in order, of course). On machines
- with segmented memory models machines, don't give it any
- more than 64K. The library seems to run fine with sizes
- of 4K. Although you can give it much less if necessary
- (I assume you can give it chunks of 1 byte, I haven't
- tried less then 256 bytes yet). When this function returns,
- you may want to display any rows that were generated in the
- row callback if you don't already do so there.
- */
- png_process_data(png_ptr, info_ptr, buffer, length);
- return 0;
- }
- /* This function is called (as set by png_set_progressive_fn() above)
- when enough data has been supplied so all of the header has been read.
- */
- void
- info_callback(png_structp png_ptr, png_infop info)
- {
- /* Do any setup here, including setting any of the transformations
- mentioned in the Reading PNG files section. For now, you _must_
- call either png_start_read_image() or png_read_update_info()
- after all the transformations are set (even if you don't set
- any). You may start getting rows before png_process_data()
- returns, so this is your last chance to prepare for that.
- */
- }
- /* This function is called when each row of image data is complete */
- void
- row_callback(png_structp png_ptr, png_bytep new_row,
- png_uint_32 row_num, int pass)
- {
- /* If the image is interlaced, and you turned on the interlace
- handler, this function will be called for every row in every pass.
- Some of these rows will not be changed from the previous pass.
- When the row is not changed, the new_row variable will be NULL.
- The rows and passes are called in order, so you don't really
- need the row_num and pass, but I'm supplying them because it
- may make your life easier.
- For the non-NULL rows of interlaced images, you must call
- png_progressive_combine_row() passing in the row and the
- old row. You can call this function for NULL rows (it will
- just return) and for non-interlaced images (it just does the
- memcpy for you) if it will make the code easier. Thus, you
- can just do this for all cases:
- */
- png_progressive_combine_row(png_ptr, old_row, new_row);
- /* where old_row is what was displayed for previous rows. Note
- that the first pass (pass == 0, really) will completely cover
- the old row, so the rows do not have to be initialized. After
- the first pass (and only for interlaced images), you will have
- to pass the current row, and the function will combine the
- old row and the new row.
- */
- }
- void
- end_callback(png_structp png_ptr, png_infop info)
- {
- /* This function is called after the whole image has been read,
- including any chunks after the image (up to and including
- the IEND). You will usually have the same info chunk as you
- had in the header, although some data may have been added
- to the comments and time fields.
- Most people won't do much here, perhaps setting a flag that
- marks the image as finished.
- */
- }
- IV. Writing
- Much of this is very similar to reading. However, everything of
- importance is repeated here, so you won't have to constantly look
- back up in the reading section to understand writing.
- You will want to do the I/O initialization before you get into libpng,
- so if it doesn't work, you don't have anything to undo. If you are not
- using the standard I/O functions, you will need to replace them with
- custom writing functions. See the discussion under Customizing libpng.
-
- FILE *fp = fopen(file_name, "wb");
- if (!fp)
- {
- return;
- }
- Next, png_struct and png_info need to be allocated and initialized.
- As these can be both relatively large, you may not want to store these
- on the stack, unless you have stack space to spare. Of course, you
- will want to check if they return NULL.
- png_structp png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING,
- (void *)user_error_ptr, user_error_fn, user_warning_fn);
- if (!png_ptr)
- return;
- png_infop info_ptr = png_create_info_struct(png_ptr);
- if (!info_ptr)
- {
- png_destroy_write_struct(&png_ptr, (png_infopp)NULL);
- return;
- }
- After you have these structures, you will need to set up the
- error handling. When libpng encounters an error, it expects to
- longjmp() back to your routine. Therefore, you will need to call
- setjmp and pass the jmpbuf field of your png_struct. If you
- write the file from different routines, you will need to update
- the jmpbuf field every time you enter a new routine that will
- call a png_ function. See your documentation of setjmp/longjmp
- for your compiler for more information on setjmp/longjmp. See
- the discussion on libpng error handling in the Customizing Libpng
- section below for more information on the libpng error handling.
-
- if (setjmp(png_ptr->jmpbuf))
- {
- png_destroy_write_struct(&png_ptr, &info_ptr);
- fclose(fp);
- return;
- }
- Now you need to set up the output code. The default for libpng is to
- use the C function fwrite(). If you use this, you will need to pass a
- valid FILE * in the function png_init_io(). Be sure that the file is
- opened in binary mode. Again, if you wish to handle writing data in
- another way, see the discussion on libpng I/O handling in the Customizing
- Libpng section below.
- png_init_io(png_ptr, fp);
- You now have the option of modifying how the compression library will
- run. The following functions are mainly for testing, but may be useful
- in some cases, like if you need to write PNG files extremely fast and
- are willing to give up some compression, or if you want to get the
- maximum possible compression at the expense of slower writing. If you
- have no special needs in this area, let the library do what it wants by
- not calling this function at all, as it has been tuned to deliver a good
- speed/compression ratio. The second parameter to png_set_filter() is
- the filter method, for which the only valid value is '0' (as of the
- 06/96 PNG specification. The third parameter is a flag that indicates
- which filter type(s) are to be tested for each scanline. See the
- Compression Library for details on the specific filter types.
-
- /* turn on or off filtering, and/or choose specific filters */
- png_set_filter(png_ptr, 0,
- PNG_FILTER_NONE | PNG_FILTER_SUB | PNG_FILTER_PAETH);
- The png_set_compression_???() functions interface to the zlib compression
- library, and should mostly be ignored unless you really know what you are
- doing. The only generally useful call is png_set_compression_level()
- which changes how much time zlib spends on trying to compress the image
- data. See the Compression Library for details on the compression levels.
- /* set the zlib compression level */
- png_set_compression_level(png_ptr, Z_BEST_COMPRESSION);
- /* set other zlib parameters */
- png_set_compression_mem_level(png_ptr, 8);
- png_set_compression_strategy(png_ptr, Z_DEFAULT_STRATEGY);
- png_set_compression_window_bits(png_ptr, 15);
- png_set_compression_method(png_ptr, 8);
- You now need to fill in the png_info structure with all the data you
- wish to write before the actual image. Note that the only thing you
- are allowed to write after the image is the text chunks and the time
- chunk (as of PNG Specification 1.0, anyway). See png_write_end() and
- the latest PNG specification for more information on that. If you
- wish to write them before the image, fill them in now, and flag that
- data as being valid. If you want to wait until after the data, don't
- fill them until png_write_end(). For all the fields in png_info and
- their data types, see png.h. For explanations of what the fields
- contain, see the PNG specification.
- Some of the more important parts of the png_info are:
- png_set_IHDR(png_ptr, info_ptr, width, height, bit_depth, color_type,
- interlace_type, compression_type, filter_type)
- width - holds the width of the image in pixels (up to 2^31).
- height - holds the height of the image in pixels (up to 2^31).
- bit_depth - holds the bit depth of one of the image channels.
- (valid values are 1, 2, 4, 8, 16 and depend also on the
- color_type. See also significant bits (sBIT) below).
- color_type - describes which color/alpha channels are present.
- PNG_COLOR_TYPE_GRAY (bit depths 1, 2, 4, 8, 16)
- PNG_COLOR_TYPE_GRAY_ALPHA (bit depths 8, 16)
- PNG_COLOR_TYPE_PALETTE (bit depths 1, 2, 4, 8)
- PNG_COLOR_TYPE_RGB (bit_depths 8, 16)
- PNG_COLOR_TYPE_RGB_ALPHA (bit_depths 8, 16)
- PNG_COLOR_MASK_PALETTE
- PNG_COLOR_MASK_COLOR
- PNG_COLOR_MASK_ALPHA
- interlace_type - PNG_INTERLACE_TYPE_NONE or PNG_INTERLACE_TYPE_ADAM7
- compression_type - (must be PNG_COMPRESSION_TYPE_DEFAULT for PNG 1.0)
- filter_type - (must be PNG_FILTER_TYPE_DEFAULT for PNG 1.0)
- png_set_PLTE(png_ptr, info_ptr, palette, num_palette);
- palette - the palette for the file (array of png_color)
- num_palette - number of entries in the palette
- png_set_gAMA(png_ptr, info_ptr, gamma);
- gamma - the gamma the image was created at (PNG_INFO_gAMA)
- png_set_sRGB(png_ptr, info_ptr, srgb_intent);
- srgb_intent - the rendering intent (PNG_INFO_sRGB)
- The presence of the sRGB chunk means that the pixel
- data is in the sRGB color space. This chunk also
- implies specific values of gAMA and cHRM.
- Rendering intent is the CSS-1 property that has been
- defined by the International Color Consortium
- (http://www.color.org). It can be one of
- PNG_SRGB_INTENT_SATURATION, PNG_SRGB_INTENT_PERCEPTUAL,
- PNG_SRGB_INTENT_ABSOLUTE, or PNG_SRGB_INTENT_RELATIVE.
-
- png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr, srgb_intent);
- srgb_intent - the rendering intent (PNG_INFO_sRGB)
- The presence of the sRGB chunk means that the pixel
- data is in the sRGB color space. This chunk also
- causes gAMA and cHRM chunks with the specific values
- that are consistent with sRGB to be written.
- png_set_sBIT(png_ptr, info_ptr, sig_bit);
- sig_bit - the number of significant bits for (PNG_INFO_sBIT)
- the gray, red, green, and blue channels, whichever
- are appropriate for the given color type (png_color_16)
- png_set_tRNS(png_ptr, info_ptr, trans, num_trans, trans_values);
- trans - array of transparent entries for palette (PNG_INFO_tRNS)
- trans_values - transparent pixel for non-paletted images (PNG_INFO_tRNS)
- num_trans - number of transparent entries (PNG_INFO_tRNS)
- png_set_hIST(png_ptr, info_ptr, hist); (PNG_INFO_hIST)
- hist - histogram of palette (array of png_color_16)
- png_set_tIME(png_ptr, info_ptr, mod_time);
- mod_time - time image was last modified (PNG_VALID_tIME)
- png_set_bKGD(png_ptr, info_ptr, background);
- background - background color (PNG_VALID_bKGD)
- png_set_text(png_ptr, info_ptr, text_ptr, num_text);
- text_ptr - array of png_text holding image comments
- text_ptr[i]->key - keyword for comment.
- text_ptr[i]->text - text comments for current keyword.
- text_ptr[i]->compression - type of compression used on "text"
- PNG_TEXT_COMPRESSION_NONE or
- PNG_TEXT_COMPRESSION_zTXt
- num_text - number of comments in text_ptr
- png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y, unit_type);
- offset_x - positive offset from the left edge of the screen
- offset_y - positive offset from the top edge of the screen
- unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
- png_set_pHYs(png_ptr, info_ptr, res_x, res_y, unit_type);
- res_x - pixels/unit physical resolution in x direction
- res_y - pixels/unit physical resolution in x direction
- unit_type - PNG_RESOLUTION_UNKOWN, PNG_RESOLUTION_METER
- In PNG files, the alpha channel in an image is the level of opacity.
- If your data is supplied as a level of transparency, you can invert the
- alpha channel before you write it, so that 0 is fully transparent and 255
- (in 8-bit or paletted images) or 65535 (in 16-bit images) is fully opaque,
- with
- png_set_invert_alpha(png_ptr);
- This must appear here instead of later with the other transformations
- because in the case of paletted images the tRNS chunk data has to
- be inverted before the tRNS chunk is written. If your image is not a
- paletted image, the tRNS data (which in such cases represents a single
- color to be rendered as transparent) won't be changed.
- A quick word about text and num_text. text is an array of png_text
- structures. num_text is the number of valid structures in the array.
- If you want, you can use max_text to hold the size of the array, but
- libpng ignores it for writing (it does use it for reading). Each
- png_text structure holds a keyword-text value, and a compression type.
- The compression types have the same valid numbers as the compression
- types of the image data. Currently, the only valid number is zero.
- However, you can store text either compressed or uncompressed, unlike
- images which always have to be compressed. So if you don't want the
- text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.
- Until text gets around 1000 bytes, it is not worth compressing it.
- After the text has been written out to the file, the compression type
- is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR,
- so that it isn't written out again at the end (in case you are calling
- png_write_end() with the same struct.
- The keywords that are given in the PNG Specification are:
- Title Short (one line) title or caption for image
- Author Name of image's creator
- Description Description of image (possibly long)
- Copyright Copyright notice
- Creation Time Time of original image creation (usually
- RFC 1123 format, see below)
- Software Software used to create the image
- Disclaimer Legal disclaimer
- Warning Warning of nature of content
- Source Device used to create the image
- Comment Miscellaneous comment; conversion from other
- image format
- The keyword-text pairs work like this. Keywords should be short
- simple descriptions of what the comment is about. Some typical
- keywords are found in the PNG specification, as is some recomendations
- on keywords. You can repeat keywords in a file. You can even write
- some text before the image and some after. For example, you may want
- to put a description of the image before the image, but leave the
- disclaimer until after, so viewers working over modem connections
- don't have to wait for the disclaimer to go over the modem before
- they start seeing the image. Finally, keywords should be full
- words, not abbreviations. Keywords and text are in the ISO 8859-1
- (Latin-1) character set (a superset of regular ASCII) and can not
- contain NUL characters, and should not contain control or other
- unprintable characters. To make the comments widely readable, stick
- with basic ASCII, and avoid machine specific character set extensions
- like the IBM-PC character set. The keyword must be present, but
- you can leave off the text string on non-compressed pairs.
- Compressed pairs must have a text string, as only the text string
- is compressed anyway, so the compression would be meaningless.
- PNG supports modification time via the png_time structure. Two
- conversion routines are proved, png_convert_from_time_t() for
- time_t and png_convert_from_struct_tm() for struct tm. The
- time_t routine uses gmtime(). You don't have to use either of
- these, but if you wish to fill in the png_time structure directly,
- you should provide the time in universal time (GMT) if possible
- instead of your local time. Note that the year number is the full
- year (ie 1996, rather than 96 - PNG is year 2000 compliant!), and
- that months start with 1.
- If you want to store the time of the original image creation, you should
- use a plain tEXt chunk with the "Creation Time" keyword. This is
- necessary because the "creation time" of a PNG image is somewhat vague,
- depending on whether you mean the PNG file, the time the image was
- created in a non-PNG format, a still photo from which the image was
- scanned, or possibly the subject matter itself. In order to facilitate
- machine-readable dates, it is recommended that the "Creation Time"
- tEXt chunk use RFC 1123 format dates (eg 22 May 1997 18:07:10 GMT"),
- although this isn't a requirement. Unlike the tIME chunk, the
- "Creation Time" tEXt chunk is not expected to be automatically changed
- by the software. To facilitate the use of RFC 1123 dates, a function
- png_convert_to_rfc1123(png_timep) is provided to convert from PNG
- time to an RFC 1123 format string.
- You are now ready to write all the file information up to the actual
- image data. You do this with a call to png_write_info().
- png_write_info(png_ptr, info_ptr);
- After you've written the file information, you can set up the library
- to handle any special transformations of the image data. The various
- ways to transform the data will be described in the order that they
- should occur. This is important, as some of these change the color
- type and/or bit depth of the data, and some others only work on
- certain color types and bit depths. Even though each transformation
- checks to see if it has data that it can do somthing with, you should
- make sure to only enable a transformation if it will be valid for the
- data. For example, don't swap red and blue on grayscale data.
- PNG files store RGB pixels packed into 3 bytes. This code tells
- the library to expect input data with 4 bytes per pixel
- png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);
- where the 0 is the value that will be put in the 4th byte, and the
- location is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending
- upon whether the filler byte is stored XRGB or RGBX.
- PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
- they can, resulting in, for example, 8 pixels per byte for 1 bit files.
- If the data is supplied at 1 pixel per byte, use this code, which will
- correctly pack the pixels into a single byte:
- png_set_packing(png_ptr);
- PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your
- data is of another bit depth, you can write an sBIT chunk into the
- file so that decoders can get the original data if desired.
-
- /* Set the true bit depth of the image data */
- if (color_type & PNG_COLOR_MASK_COLOR)
- {
- sig_bit.red = true_bit_depth;
- sig_bit.green = true_bit_depth;
- sig_bit.blue = true_bit_depth;
- }
- else
- {
- sig_bit.gray = true_bit_depth;
- }
- if (color_type & PNG_COLOR_MASK_ALPHA)
- {
- sig_bit.alpha = true_bit_depth;
- }
- png_set_sBIT(png_ptr, info_ptr, &sig_bit);
- If the data is stored in the row buffer in a bit depth other than
- one supported by PNG (ie 3 bit data in the range 0-7 for a 4-bit PNG),
- this will scale the values to appear to be the correct bit depth as
- is required by PNG.
- png_set_shift(png_ptr, &sig_bit);
- PNG files store 16 bit pixels in network byte order (big-endian,
- ie. most significant bits first). This code would be used if they are
- supplied the other way (little-endian, ie. least significant bits
- first, eg. the way PCs store them):
- if (bit_depth > 8)
- png_set_swap(png_ptr);
- If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
- need to change the order the pixels are packed into bytes, you can use:
- if (bit_depth < 8)
- png_set_packswap(png_ptr);
- PNG files store 3 color pixels in red, green, blue order. This code
- would be used if they are supplied as blue, green, red:
- png_set_bgr(png_ptr);
- PNG files describe monochrome as black being zero and white being
- one. This code would be used if the pixels are supplied with this reversed
- (black being one and white being zero):
- png_set_invert_mono(png_ptr);
- It is possible to have libpng flush any pending output, either manually,
- or automatically after a certain number of lines have been written. To
- flush the output stream a single time call:
- png_write_flush(png_ptr);
- and to have libpng flush the output stream periodically after a certain
- number of scanlines have been written, call:
- png_set_flush(png_ptr, nrows);
- Note that the distance between rows is from the last time png_write_flush()
- was called, or the first row of the image if it has never been called.
- So if you write 50 lines, and then png_set_flush 25, it will flush the
- output on the next scanline, and every 25 lines thereafter, unless
- png_write_flush()ls is called before 25 more lines have been written.
- If nrows is too small (less than about 10 lines for a 640 pixel wide
- RGB image) the image compression may decrease noticably (although this
- may be acceptable for real-time applications). Infrequent flushing will
- only degrade the compression performance by a few percent over images
- that do not use flushing.
- That's it for the transformations. Now you can write the image data.
- The simplest way to do this is in one function call. If have the
- whole image in memory, you can just call png_write_image() and libpng
- will write the image. You will need to pass in an array of pointers to
- each row. This function automatically handles interlacing, so you don't
- need to call png_set_interlace_handling() or call this function multiple
- times, or any of that other stuff necessary with png_write_rows().
- png_write_image(png_ptr, row_pointers);
- where row_pointers is:
- png_bytef *row_pointers[height];
- You can point to void or char or whatever you use for pixels.
- If you can't want to write the whole image at once, you can
- use png_write_rows() instead. If the file is not interlaced,
- this is simple:
- png_write_rows(png_ptr, row_pointers, number_of_rows);
- row_pointers is the same as in the png_write_image() call.
- If you are just writing one row at a time, you can do this with
- row_pointers:
- png_bytep row_pointer = row;
- png_write_row(png_ptr, &row_pointer);
- When the file is interlaced, things can get a good deal more
- complicated. The only currently (as of 6/96 -- PNG Specification
- version 1.0) defined interlacing scheme for PNG files is a
- compilcated interlace scheme, known as Adam7, that breaks down an
- image into seven smaller images of varying size. libpng will build
- these images for you, or you can do them yourself. If you want to
- build them yourself, see the PNG specification for details of which
- pixels to write when.
- If you don't want libpng to handle the interlacing details, just
- use png_set_interlace_handling() and call png_write_rows() the
- correct number of times to write all seven sub-images.
- If you want libpng to build the sub-images, call this before you start
- writing any rows:
- number_of_passes = png_set_interlace_handling(png_ptr);
- This will return the number of passes needed. Currently, this
- is seven, but may change if another interlace type is added.
- Then write the complete image number_of_passes times.
- png_write_rows(png_ptr, row_pointers, number_of_rows);
- As some of these rows are not used, and thus return immediately,
- you may want to read about interlacing in the PNG specification,
- and only update the rows that are actually used.
- After you are finished writing the image, you should finish writing
- the file. If you are interested in writing comments or time, you should
- pass an appropriately filled png_info pointer. If you are not interested,
- you can pass NULL.
- png_write_end(png_ptr, info_ptr);
- When you are done, you can free all memory used by libpng like this:
- png_destroy_write_struct(&png_ptr, &info_ptr);
- You must free any data you allocated for info_ptr, such as comments,
- palette, or histogram, before the call to png_destroy_write_struct();
- For a more compact example of writing a PNG image, see the file example.c.
- V. Modifying/Customizing libpng:
- There are two issues here. The first is changing how libpng does
- standard things like memory allocation, input/output, and error handling.
- The second deals with more complicated things like adding new chunks,
- adding new transformations, and generally changing how libpng works.
- All of the memory allocation, input/output, and error handling in libpng
- goes through callbacks which are user setable. The default routines are
- in pngmem.c, pngrio.c, pngwio.c, and pngerror.c respectively. To change
- these functions, call the approprate png_set_???_fn() function.
- Memory allocation is done through the functions png_large_malloc(),
- png_malloc(), png_realloc(), png_large_free(), and png_free(). These
- currently just call the standard C functions. The large functions must
- handle exactly 64K, but they don't have to handle more than that. If
- your pointers can't access more then 64K at a time, you will want to set
- MAXSEG_64K in zlib.h. Since it is unlikely that the method of handling
- memory allocation on a platform will change between applications, these
- functions must be modified in the library at compile time.
- Input/Output in libpng is done through png_read() and png_write(),
- which currently just call fread() and fwrite(). The FILE * is stored in
- png_struct and is initialized via png_init_io(). If you wish to change
- the method of I/O, the library supplies callbacks that you can set
- through the function png_set_read_fn() and png_set_write_fn() at run
- time, instead of calling the png_init_io() function. These functions
- also provide a void pointer that can be retrieved via the function
- png_get_io_ptr(). For example:
- png_set_read_fn(png_structp png_ptr, voidp io_ptr,
- png_rw_ptr read_data_fn)
- png_set_write_fn(png_structp png_ptr, voidp io_ptr,
- png_rw_ptr write_data_fn, png_flush_ptr output_flush_fn);
- voidp io_ptr = png_get_io_ptr(png_ptr);
- The replacement I/O functions should have prototypes as follows:
- void user_read_data(png_structp png_ptr, png_bytep data,
- png_uint_32 length);
- void user_write_data(png_structp png_ptr, png_bytep data,
- png_uint_32 length);
- void user_flush_data(png_structp png_ptr);
- Supplying NULL for the read, write, or flush functions sets them back
- to using the default C stream functions. It is an error to read from
- a write stream, and vice versa.
- Error handling in libpng is done through png_error() and png_warning().
- Errors handled through png_error() are fatal, meaning that png_error()
- should never return to its caller. Currently, this is handled via
- setjmp() and longjmp(), but you could change this to do things like
- exit() if you should wish. On non-fatal errors, png_warning() is called
- to print a warning message, and then control returns to the calling code.
- By default png_error() and png_warning() print a message on stderr via
- fprintf() unless the library is compiled with PNG_NO_STDIO defined. If
- you wish to change the behavior of the error functions, you will need to
- set up your own message callbacks. These functions are normally supplied
- at the time that the png_struct is created. It is also possible to change
- these functions after png_create_???_struct() has been called by calling:
- png_set_error_fn(png_structp png_ptr, png_voidp error_ptr,
- png_error_ptr error_fn, png_error_ptr warning_fn);
- png_voidp error_ptr = png_get_error_ptr(png_ptr);
- If NULL is supplied for either error_fn or warning_fn, then the libpng
- default function will be used, calling fprintf() and/or longjmp() if a
- problem is encountered. The replacement error functions should have
- parameters as follows:
- void user_error_fn(png_structp png_ptr, png_const_charp error_msg);
- void user_warning_fn(png_structp png_ptr, png_const_charp warning_msg);
- The motivation behind using setjmp() and longjmp() is the C++ throw and
- catch exception handling methods. This makes the code much easier to write,
- as there is no need to check every return code of every function call.
- However, there are some uncertainties about the status of local variables
- after a longjmp, so the user may want to be careful about doing anything after
- setjmp returns non-zero besides returning itself. Consult your compiler
- documentation for more details.
- If you need to read or write custom chunks, you will need to get deeper
- into the libpng code, as a mechanism has not yet been supplied for user
- callbacks with custom chunks. First, read the PNG specification, and have
- a first level of understanding of how it works. Pay particular attention
- to the sections that describe chunk names, and look at how other chunks
- were designed, so you can do things similarly. Second, check out the
- sections of libpng that read and write chunks. Try to find a chunk that
- is similar to yours and copy off of it. More details can be found in the
- comments inside the code. A way of handling unknown chunks in a generic
- method, potentially via callback functions, would be best.
- If you wish to write your own transformation for the data, look through
- the part of the code that does the transformations, and check out some of
- the simpler ones to get an idea of how they work. Try to find a similar
- transformation to the one you want to add and copy off of it. More details
- can be found in the comments inside the code itself.
- Configuring for 16 bit platforms:
- You may need to change the png_large_malloc() and png_large_free()
- routines in pngmem.c, as these are requred to allocate 64K, although
- there is already support for many of the common DOS compilers. Also,
- you will want to look into zconf.h to tell zlib (and thus libpng) that
- it cannot allocate more then 64K at a time. Even if you can, the memory
- won't be accessable. So limit zlib and libpng to 64K by defining MAXSEG_64K.
- Configuring for DOS:
- For DOS users which only have access to the lower 640K, you will
- have to limit zlib's memory usage via a png_set_compression_mem_level()
- call. See zlib.h or zconf.h in the zlib library for more information.
- Configuring for Medium Model:
- Libpng's support for medium model has been tested on most of the popular
- compilers. Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets
- defined, and FAR gets defined to far in pngconf.h, and you should be
- all set. Everything in the library (except for zlib's structure) is
- expecting far data. You must use the typedefs with the p or pp on
- the end for pointers (or at least look at them and be careful). Make
- note that the row's of data are defined as png_bytepp which is a
- unsigned char far * far *.
- Configuring for gui/windowing platforms:
- You will need to write new error and warning functions that use the GUI
- interface, as described previously, and set them to be the error and
- warning functions at the time that png_create_???_struct() is called,
- in order to have them available during the structure initialization.
- They can be changed later via png_set_error_fn(). On some compliers,
- you may also have to change the memory allocators (png_malloc, etc.).
- Configuring for compiler xxx:
- All includes for libpng are in pngconf.h. If you need to add/change/delete
- an include, this is the place to do it. The includes that are not
- needed outside libpng are protected by the PNG_INTERNAL definition,
- which is only defined for those routines inside libpng itself. The
- files in libpng proper only include png.h, which includes pngconf.h.
- Configuring zlib:
- There are special functions to configure the compression. Perhaps the
- most useful one changes the compression level, which currently uses
- input compression values in the range 0 - 9. The library normally
- uses the default compression level (Z_DEFAULT_COMPRESSION = 6). Tests
- have shown that for a large majority of images, compression values in
- the range 3-6 compress nearly as well as higher levels, and do so much
- faster. For online applications it may be desirable to have maximum speed
- (Z_BEST_SPEED = 1). With versions of zlib after v0.99, you can also
- specify no compression (Z_NO_COMPRESSION = 0), but this would create
- files larger than just storing the raw bitmap. You can specify the
- compression level by calling:
- png_set_compression_level(png_ptr, level);
- Another useful one is to reduce the memory level used by the library.
- The memory level defaults to 8, but it can be lowered if you are
- short on memory (running DOS, for example, where you only have 640K).
- png_set_compression_mem_level(png_ptr, level);
- The other functions are for configuring zlib. They are not recommended
- for normal use and may result in writing an invalid PNG file. See
- zlib.h for more information on what these mean.
- png_set_compression_strategy(png_ptr, strategy);
- png_set_compression_window_bits(png_ptr, window_bits);
- png_set_compression_method(png_ptr, method);
- Controlling row filtering:
- If you want to control whether libpng uses filtering or not, which
- filters are used, and how it goes about picking row filters, you
- can call one of these functions. The selection and configuration
- of row filters can have a significant impact on the size and
- encoding speed and a somewhat lesser impact on the decoding speed
- of an image. Filtering is enabled by default for RGB and grayscale
- images (with and without alpha), and for 8-bit paletted images, but
- not for paletted images with bit depths less than 8 bits/pixel.
- The 'method' parameter sets the main filtering method, which is
- currently only '0' in the PNG 1.0 specification. The 'filters'
- parameter sets which filter(s), if any, should be used for each
- scanline. Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS
- to turn filtering on and off, respectively.
- Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB,
- PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise
- ORed together '|' to specify one or more filters to use. These
- filters are described in more detail in the PNG specification. If
- you intend to change the filter type during the course of writing
- the image, you should start with flags set for all of the filters
- you intend to use so that libpng can initialize its internal
- structures appropriately for all of the filter types.
- filters = PNG_FILTER_NONE | PNG_FILTER_SUB | PNG_FILTER_UP;
- png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE, filters);
- It is also possible to influence how libpng chooses from among the
- available filters. This is done in two ways - by telling it how
- important it is to keep the same filter for successive rows, and
- by telling it the relative computational costs of the filters.
- double weights[3] = {1.5, 1.3, 1.1},
- costs[PNG_FILTER_VALUE_LAST] = {1.0, 1.3, 1.3, 1.5, 1.7};
- png_set_filter_selection(png_ptr, PNG_FILTER_SELECTION_WEIGHTED,
- 3, weights, costs);
- The weights are multiplying factors which indicate to libpng that row
- should be the same for successive rows unless another row filter is that
- many times better than the previous filter. In the above example, if
- the previous 3 filters were SUB, SUB, NONE, the SUB filter could have a
- "sum of absolute differences" 1.5 x 1.3 times higher than other filters
- and still be chosen, while the NONE filter could have a sum 1.1 times
- higher than other filters and still be chosen. Unspecified weights are
- taken to be 1.0, and the specified weights should probably be declining
- like those above in order to emphasize recent filters over older filters.
- The filter costs specify for each filter type a relative decoding cost
- to be considered when selecting row filters. This means that filters
- with higher costs are less likely to be chosen over filters with lower
- costs, unless their "sum of absolute differences" is that much smaller.
- The costs do not necessarily reflect the exact computational speeds of
- the various filters, since this would unduely influence the final image
- size.
- Note that the numbers above were invented purely for this example and
- are given only to help explain the function usage. Little testing has
- been done to find optimum values for either the costs or the weights.
- Removing unwanted object code:
- There are a bunch of #define's in pngconf.h that control what parts of
- libpng are compiled. All the defines end in _SUPPORTED. If you are
- never going to use an ability, you can change the #define to #undef
- before recompiling libpng and save yourself code and data space.
- You can also turn them off en masse with a compiler directive that
- defines PNG_READ[or WRITE]_NOT_FULLY_SUPPORTED, or
- PNG_READ[or WRITE]_ANCILLARY_CHUNKS_NOT_SUPPORTED, or all four,
- along with directives to turn on any of the capabilities that you do
- want. The NOT_FULLY_SUPPORTED directives disable the extra
- transformations but still leave the library fully capable of reading
- and writing PNG files with all known public chunks [except for sPLT].
- Use of the PNG_READ[or WRITE]_ANCILLARY_CHUNKS_NOT_SUPPORTED directive
- produces a library that is incapable of reading or writing ancillary chunks.
- All the reading and writing specific code are in separate files, so the
- linker should only grab the files it needs. However, if you want to
- make sure, or if you are building a stand alone library, all the
- reading files start with pngr and all the writing files start with
- pngw. The files that don't match either (like png.c, pngtrans.c, etc.)
- are used for both reading and writing, and always need to be included.
- The progressive reader is in pngpread.c
- If you are creating or distributing a dynamically linked library (a .so
- or DLL file), you should not remove or disable any parts of the library,
- as this will cause applications linked with different versions of the
- library to fail if they call functions not available in your library.
- The size of the library itself should not be an issue, because only
- those sections which are actually used will be loaded into memory.
- Changes to Libpng from version 0.88
- It should be noted that versions of libpng later than 0.96 are not
- distributed by the original libpng author, Guy Schalnat, nor by
- Andreas Dilger, who had taken over from Guy during 1996 and 1997, and
- distributed versions 0.89 through 0.96, but rather by another member
- of the original PNG Group, Glenn Randers-Pehrson. Guy and Andreas are
- still alive and well, but they have moved on to other things.
- The old libpng functions png_read_init(), png_write_init(),
- png_info_init(), png_read_destroy(), and png_write_destory() have been
- moved to PNG_INTERNAL in version 0.95 to discourage their use. The
- preferred method of creating and initializing the libpng structures is
- via the png_create_read_struct(), png_create_write_struct(), and
- png_create_info_struct() because they isolate the size of the structures
- from the application, allow version error checking, and also allow the
- use of custom error handling routines during the initialization, which
- the old functions do not. The functions png_read_destroy() and
- png_write_destroy() do not actually free the memory that libpng
- allocated for these structs, but just reset the data structures, so they
- can be used instead of png_destroy_read_struct() and
- png_destroy_write_struct() if you feel there is too much system overhead
- allocating and freeing the png_struct for each image read.
- Setting the error callbacks via png_set_message_fn() before
- png_read_init() as was suggested in libpng-0.88 is no longer supported
- because this caused applications which do not use custom error functions
- to fail if the png_ptr was not initialized to zero. It is still possible
- to set the error callbacks AFTER png_read_init(), or to change them with
- png_set_error_fn(), which is essentially the same function, but with a
- new name to force compilation errors with applications that try to use
- the old method.
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