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. 188 189 190 191
