qball 1.9 KB

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  1. .TH QBALL 2
  2. .SH NAME
  3. qball \- 3-d rotation controller
  4. .SH SYNOPSIS
  5. .B
  6. #include <draw.h>
  7. .br
  8. .B
  9. #include <geometry.h>
  10. .PP
  11. .B
  12. void qball(Rectangle r, Mouse *mousep,
  13. .br
  14. .B
  15. Quaternion *orientation,
  16. .br
  17. .B
  18. void (*redraw)(void), Quaternion *ap)
  19. .SH DESCRIPTION
  20. .I Qball
  21. is an interactive controller that allows arbitrary 3-space rotations to be specified with
  22. the mouse. Imagine a sphere with its center at the midpoint of rectangle
  23. .IR r ,
  24. and diameter the smaller of
  25. .IR r 's
  26. dimensions. Dragging from one point on the sphere to another specifies the endpoints of a
  27. great-circle arc. (Mouse points outside the sphere are projected to the nearest point
  28. on the sphere.) The axis of rotation is normal to the plane of the arc, and the
  29. angle of rotation is twice the angle of the arc.
  30. .PP
  31. Argument
  32. .I mousep
  33. is a pointer to the mouse event that triggered the interaction. It should
  34. have some button set.
  35. .I Qball
  36. will read more events into
  37. .IR mousep ,
  38. and return when no buttons are down.
  39. .PP
  40. While
  41. .I qball
  42. is reading mouse events, it calls out to the caller-supplied routine
  43. .IR redraw ,
  44. which is expected to update the screen to reflect the changing orientation.
  45. Argument
  46. .I orientation
  47. is the orientation that
  48. .I redraw
  49. should examine, represented as a unit Quaternion (see
  50. .IR quaternion (2)).
  51. The caller may set it to any orientation.
  52. It will be updated before each call to
  53. .I redraw
  54. (and on return) by multiplying by the rotation specified with the mouse.
  55. .PP
  56. It is possible to restrict
  57. .I qball's
  58. attention to rotations about a particular axis.
  59. If
  60. .I ap
  61. is null, the rotation is unconstrained.
  62. Otherwise, the rotation will be about the same axis as
  63. .IR *ap .
  64. This is accomplished by projecting points on the sphere to
  65. the nearest point also on the plane through the sphere's center
  66. and normal to the axis.
  67. .SH SOURCE
  68. .B /sys/src/libgeometry/qball.c
  69. .SH SEE ALSO
  70. .IR quaternion (2)
  71. .br
  72. Ken Shoemake,
  73. ``Animating Rotation with Quaternion Curves'',
  74. .I
  75. SIGGRAPH '85 Conference Proceedings.