docs/MESA_sprite_point.spec

   1 XXX - Not complete yet!!!
   2
   3 Name
   4
   5     MESA_sprite_point
   6
   7 Name Strings
   8
   9     GL_MESA_sprite_point
  10
  11 Contact
  12
  13     Brian Paul, VA Linux Systems Inc. (brianp 'at' valinux.com)
  14
  15 Status
  16
  17     Not yet shipping.
  18
  19 Version
  20
  21     $Id: MESA_sprite_point.spec,v 1.1 2000/12/08 00:21:02 brianp Exp $
  22
  23 Number
  24
  25     ???
  26
  27 Dependencies
  28
  29     GL_EXT_point_parameters effects the definition of this extension
  30     GL_ARB_multitexture effects the definition of this extension
  31
  32 Overview
  33
  34     This extension modifies the way in which points are rendered,
  35     specifically when they're textured.  When SPRITE_POINT_MESA is enabled
  36     a point is rendered as if it were a quadrilateral with unique texture
  37     coordinates at each vertex.  This extension effectively turns points
  38     into sprites which may be rendered more easily and quickly than using
  39     conventional textured quadrilaterals.
  40
  41     When using point size > 1 or attenuated points this extension is an
  42     effective way to render many small sprite images for particle systems
  43     or other effects.
  44
  45 Issues:
  46
  47     1. How are the texture coordinates computed?
  48
  49        The lower-left corner has texture coordinate (0,0,r,q).
  50        The lower-right, (1,0,r,q).  The upper-right, (1,1,r,q).
  51        The upper-left, (0,1,r,q).
  52
  53     2. What about texgen and texture matrices?
  54
  55        Texgen and the texture matrix have no effect on the point's s and t
  56        texture coordinates.  The r and q coordinates may have been computed
  57        by texgen or the texture matrix.  Note that with a 3D texture and/or
  58        texgen that the r coordinate could be used to select a slice in the
  59        3D texture.
  60
  61     3. What about point smoothing?
  62
  63        When point smoothing is enabled, a triangle fan could be rendered
  64        to approximate a circular point.  This could be problematic to
  65        define and implement so POINT_SMOOTH is ignored when drawing sprite
  66        points.
  67
  68        Smoothed points can be approximated by using an appropriate texture
  69        images, alpha testing and blending.
  70
  71        POLYGON_SMOOTH does effect the rendering of the quadrilateral, however.
  72
  73     4. What about sprite rotation?
  74
  75        There is none.  Sprite points are always rendered as window-aligned
  76        squares.  One could define rotated texture images if desired.  A 3D
  77        texture and appropriate texture r coordinates could be used to
  78        effectively specify image rotation per point.
  79
  80     5. What about POLYGON_MODE?
  81
  82        POLYGON_MODE does not effect the rasterization of the quadrilateral.
  83
  84     6. What about POLYGON_CULL?
  85
  86        TBD.  Polygon culling is normally specified and implemented in the
  87        transformation stage of OpenGL.  However, some rasterization hardware
  88        implements it later during triangle setup.
  89
  90        Polygon culling wouldn't be useful for sprite points since the
  91        quadrilaterals are always defined in counter-clockwise order in
  92        window space.  For that reason, polygon culling should probably be
  93        ignored.
  94
  95     7. Should sprite points be alpha-attenuated if their size is below the
  96        point parameter's threshold size?
  97
  98     8. Should there be an advertisized maximum sprite point size?
  99
 100        No.  Since we're rendering the point as a quadrilateral there's no
 101        need to limit the size.
 102
 103
 104 New Procedures and Functions
 105
 106     None.
 107
 108 New Tokens
 109
 110     Accepted by the <pname> parameter of Enable, Disable, IsEnabled,
 111     GetIntegerv, GetBooleanv, GetFloatv and GetDoublev:
 112
 113         SPRITE_POINT_MESA               0x????
 114         MAX_SPRITE_POINT_SIZE_MESA      0x????   (need this?)
 115
 116 Additions to Chapter 2 of the 1.1 Specification (OpenGL Operation)
 117
 118     None
 119
 120 Additions to Chapter 3 of the 1.1 Specification (Rasterization)
 121
 122     Section ???.
 123
 124     When SPRITE_POINT_MESA is enabled points are rasterized as screen-
 125     aligned quadrilaterals.  If the four vertices of the quadrilateral
 126     are labeled A, B, C, and D, starting at the lower-left corner and moving
 127     counter-clockwise around the quadrilateral, then the vertex and
 128     texture coordinates are computed as follows:
 129
 130       vertex   window coordinate       texture coordinate
 131         A      (x-r, y-r, z, w)          (0, 0, r, q)
 132         B      (x+r, y-r, z, w)          (1, 0, r, q)
 133         C      (x+r, y+r, z, w)          (1, 1, r, q)
 134         D      (x-r, y+r, z, w)          (0, 1, r, q)
 135
 136     where x, y, z, w are the point's window coordinates, r and q are the
 137     point's 3rd and 4th texture coordinates  and r is half the point's
 138     size.  The other vertex attributes (such as the color and fog coordinate)
 139     are simply duplicated from the original point vertex.
 140
 141     Point size may either be specified with PointSize or computed
 142     according to the EXT_point_parameters extension.
 143
 144     The new texture coordinates are not effected by texgen or the texture
 145     matrix.  Note, however, that the texture r and q coordinates are passed
 146     unchanged and may have been computed with texgen and/or the texture
 147     matrix.
 148
 149     If multiple texture units are present the same texture coordinate is
 150     used for all texture units.
 151
 152     The point is then rendered as if it were a quadrilateral using the
 153     normal point sampling rules.  POLYGON_MODE does not effect the
 154     rasterization of the quadrilateral but POLYGON_SMOOTH does.
 155
 156     POINT_SMOOTH has no effect when SPRITE_POINT_MESA is enabled.
 157
 158 Additions to Chapter 4 of the 1.1 Specification (Per-Fragment Operations
 159 and the Frame Buffer)
 160
 161     None.
 162
 163 Additions to Chapter 5 of the 1.1 Specification (Special Functions)
 164
 165     None
 166
 167 Additions to Chapter 6 of the 1.1 Specification (State and State Requests)
 168
 169     None
 170
 171 Additions to the GLX Specification
 172
 173     None
 174
 175 GLX Protocol
 176
 177     TBD
 178
 179 Errors
 180
 181     None
 182
 183 New State
 184
 185     Add boolean variable SPRITE_POINT_MESA to the point attribute group.
 186
 187 Revision History
 188
 189     Version 1.0 - 4 Dec 2000
 190         Original draft.
 191
 192
 193