XXX - Not complete yet!!! Name MESA_sprite_point Name Strings GL_MESA_sprite_point Contact Brian Paul, VA Linux Systems Inc. (brianp 'at' valinux.com) Status Not yet shipping. Version $Id: MESA_sprite_point.spec,v 1.1 2000/12/08 00:21:02 brianp Exp $ Number ??? Dependencies GL_EXT_point_parameters effects the definition of this extension GL_ARB_multitexture effects the definition of this extension Overview This extension modifies the way in which points are rendered, specifically when they're textured. When SPRITE_POINT_MESA is enabled a point is rendered as if it were a quadrilateral with unique texture coordinates at each vertex. This extension effectively turns points into sprites which may be rendered more easily and quickly than using conventional textured quadrilaterals. When using point size > 1 or attenuated points this extension is an effective way to render many small sprite images for particle systems or other effects. Issues: 1. How are the texture coordinates computed? The lower-left corner has texture coordinate (0,0,r,q). The lower-right, (1,0,r,q). The upper-right, (1,1,r,q). The upper-left, (0,1,r,q). 2. What about texgen and texture matrices? Texgen and the texture matrix have no effect on the point's s and t texture coordinates. The r and q coordinates may have been computed by texgen or the texture matrix. Note that with a 3D texture and/or texgen that the r coordinate could be used to select a slice in the 3D texture. 3. What about point smoothing? When point smoothing is enabled, a triangle fan could be rendered to approximate a circular point. This could be problematic to define and implement so POINT_SMOOTH is ignored when drawing sprite points. Smoothed points can be approximated by using an appropriate texture images, alpha testing and blending. POLYGON_SMOOTH does effect the rendering of the quadrilateral, however. 4. What about sprite rotation? There is none. Sprite points are always rendered as window-aligned squares. One could define rotated texture images if desired. A 3D texture and appropriate texture r coordinates could be used to effectively specify image rotation per point. 5. What about POLYGON_MODE? POLYGON_MODE does not effect the rasterization of the quadrilateral. 6. What about POLYGON_CULL? TBD. Polygon culling is normally specified and implemented in the transformation stage of OpenGL. However, some rasterization hardware implements it later during triangle setup. Polygon culling wouldn't be useful for sprite points since the quadrilaterals are always defined in counter-clockwise order in window space. For that reason, polygon culling should probably be ignored. 7. Should sprite points be alpha-attenuated if their size is below the point parameter's threshold size? 8. Should there be an advertisized maximum sprite point size? No. Since we're rendering the point as a quadrilateral there's no need to limit the size. New Procedures and Functions None. New Tokens Accepted by the parameter of Enable, Disable, IsEnabled, GetIntegerv, GetBooleanv, GetFloatv and GetDoublev: SPRITE_POINT_MESA 0x???? MAX_SPRITE_POINT_SIZE_MESA 0x???? (need this?) Additions to Chapter 2 of the 1.1 Specification (OpenGL Operation) None Additions to Chapter 3 of the 1.1 Specification (Rasterization) Section ???. When SPRITE_POINT_MESA is enabled points are rasterized as screen- aligned quadrilaterals. If the four vertices of the quadrilateral are labeled A, B, C, and D, starting at the lower-left corner and moving counter-clockwise around the quadrilateral, then the vertex and texture coordinates are computed as follows: vertex window coordinate texture coordinate A (x-r, y-r, z, w) (0, 0, r, q) B (x+r, y-r, z, w) (1, 0, r, q) C (x+r, y+r, z, w) (1, 1, r, q) D (x-r, y+r, z, w) (0, 1, r, q) where x, y, z, w are the point's window coordinates, r and q are the point's 3rd and 4th texture coordinates and r is half the point's size. The other vertex attributes (such as the color and fog coordinate) are simply duplicated from the original point vertex. Point size may either be specified with PointSize or computed according to the EXT_point_parameters extension. The new texture coordinates are not effected by texgen or the texture matrix. Note, however, that the texture r and q coordinates are passed unchanged and may have been computed with texgen and/or the texture matrix. If multiple texture units are present the same texture coordinate is used for all texture units. The point is then rendered as if it were a quadrilateral using the normal point sampling rules. POLYGON_MODE does not effect the rasterization of the quadrilateral but POLYGON_SMOOTH does. POINT_SMOOTH has no effect when SPRITE_POINT_MESA is enabled. Additions to Chapter 4 of the 1.1 Specification (Per-Fragment Operations and the Frame Buffer) None. Additions to Chapter 5 of the 1.1 Specification (Special Functions) None Additions to Chapter 6 of the 1.1 Specification (State and State Requests) None Additions to the GLX Specification None GLX Protocol TBD Errors None New State Add boolean variable SPRITE_POINT_MESA to the point attribute group. Revision History Version 1.0 - 4 Dec 2000 Original draft.