docs/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     $Id: MESA_sprite_point.spec,v 1.2 2003/09/19 14:58:21 brianp Exp $
  20
  21 Number
  22
  23     ???
  24
  25 Dependencies
  26
  27     GL_EXT_point_parameters effects the definition of this extension
  28     GL_ARB_multitexture effects the definition of this extension
  29
  30 Overview
  31
  32     This extension modifies the way in which points are rendered,
  33     specifically when they're textured.  When SPRITE_POINT_MESA is enabled
  34     a point is rendered as if it were a quadrilateral with unique texture
  35     coordinates at each vertex.  This extension effectively turns points
  36     into sprites which may be rendered more easily and quickly than using
  37     conventional textured quadrilaterals.
  38
  39     When using point size > 1 or attenuated points this extension is an
  40     effective way to render many small sprite images for particle systems
  41     or other effects.
  42
  43 Issues:
  44
  45     1. How are the texture coordinates computed?
  46
  47        The lower-left corner has texture coordinate (0,0,r,q).
  48        The lower-right, (1,0,r,q).  The upper-right, (1,1,r,q).
  49        The upper-left, (0,1,r,q).
  50
  51     2. What about texgen and texture matrices?
  52
  53        Texgen and the texture matrix have no effect on the point's s and t
  54        texture coordinates.  The r and q coordinates may have been computed
  55        by texgen or the texture matrix.  Note that with a 3D texture and/or
  56        texgen that the r coordinate could be used to select a slice in the
  57        3D texture.
  58
  59     3. What about point smoothing?
  60
  61        When point smoothing is enabled, a triangle fan could be rendered
  62        to approximate a circular point.  This could be problematic to
  63        define and implement so POINT_SMOOTH is ignored when drawing sprite
  64        points.
  65
  66        Smoothed points can be approximated by using an appropriate texture
  67        images, alpha testing and blending.
  68
  69        POLYGON_SMOOTH does effect the rendering of the quadrilateral, however.
  70
  71     4. What about sprite rotation?
  72
  73        There is none.  Sprite points are always rendered as window-aligned
  74        squares.  One could define rotated texture images if desired.  A 3D
  75        texture and appropriate texture r coordinates could be used to
  76        effectively specify image rotation per point.
  77
  78     5. What about POLYGON_MODE?
  79
  80        POLYGON_MODE does not effect the rasterization of the quadrilateral.
  81
  82     6. What about POLYGON_CULL?
  83
  84        TBD.  Polygon culling is normally specified and implemented in the
  85        transformation stage of OpenGL.  However, some rasterization hardware
  86        implements it later during triangle setup.
  87
  88        Polygon culling wouldn't be useful for sprite points since the
  89        quadrilaterals are always defined in counter-clockwise order in
  90        window space.  For that reason, polygon culling should probably be
  91        ignored.
  92
  93     7. Should sprite points be alpha-attenuated if their size is below the
  94        point parameter's threshold size?
  95
  96     8. Should there be an advertisized maximum sprite point size?
  97
  98        No.  Since we're rendering the point as a quadrilateral there's no
  99        need to limit the size.
 100
 101
 102 New Procedures and Functions
 103
 104     None.
 105
 106 New Tokens
 107
 108     Accepted by the <pname> parameter of Enable, Disable, IsEnabled,
 109     GetIntegerv, GetBooleanv, GetFloatv and GetDoublev:
 110
 111         SPRITE_POINT_MESA               0x????
 112         MAX_SPRITE_POINT_SIZE_MESA      0x????   (need this?)
 113
 114 Additions to Chapter 2 of the 1.1 Specification (OpenGL Operation)
 115
 116     None
 117
 118 Additions to Chapter 3 of the 1.1 Specification (Rasterization)
 119
 120     Section ???.
 121
 122     When SPRITE_POINT_MESA is enabled points are rasterized as screen-
 123     aligned quadrilaterals.  If the four vertices of the quadrilateral
 124     are labeled A, B, C, and D, starting at the lower-left corner and moving
 125     counter-clockwise around the quadrilateral, then the vertex and
 126     texture coordinates are computed as follows:
 127
 128       vertex   window coordinate       texture coordinate
 129         A      (x-r, y-r, z, w)          (0, 0, r, q)
 130         B      (x+r, y-r, z, w)          (1, 0, r, q)
 131         C      (x+r, y+r, z, w)          (1, 1, r, q)
 132         D      (x-r, y+r, z, w)          (0, 1, r, q)
 133
 134     where x, y, z, w are the point's window coordinates, r and q are the
 135     point's 3rd and 4th texture coordinates  and r is half the point's
 136     size.  The other vertex attributes (such as the color and fog coordinate)
 137     are simply duplicated from the original point vertex.
 138
 139     Point size may either be specified with PointSize or computed
 140     according to the EXT_point_parameters extension.
 141
 142     The new texture coordinates are not effected by texgen or the texture
 143     matrix.  Note, however, that the texture r and q coordinates are passed
 144     unchanged and may have been computed with texgen and/or the texture
 145     matrix.
 146
 147     If multiple texture units are present the same texture coordinate is
 148     used for all texture units.
 149
 150     The point is then rendered as if it were a quadrilateral using the
 151     normal point sampling rules.  POLYGON_MODE does not effect the
 152     rasterization of the quadrilateral but POLYGON_SMOOTH does.
 153
 154     POINT_SMOOTH has no effect when SPRITE_POINT_MESA is enabled.
 155
 156 Additions to Chapter 4 of the 1.1 Specification (Per-Fragment Operations
 157 and the Frame Buffer)
 158
 159     None.
 160
 161 Additions to Chapter 5 of the 1.1 Specification (Special Functions)
 162
 163     None
 164
 165 Additions to Chapter 6 of the 1.1 Specification (State and State Requests)
 166
 167     None
 168
 169 Additions to the GLX Specification
 170
 171     None
 172
 173 GLX Protocol
 174
 175     TBD
 176
 177 Errors
 178
 179     None
 180
 181 New State
 182
 183     Add boolean variable SPRITE_POINT_MESA to the point attribute group.
 184
 185 Revision History
 186
 187     Version 1.0 - 4 Dec 2000
 188         Original draft.
 189
 190
 191