docs/specs/OLD/MESA_sprite_point.spec

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