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diff --git a/docs/MESA_sprite_point.spec b/docs/MESA_sprite_point.spec new file mode 100644 index 0000000000..86c98047d1 --- /dev/null +++ b/docs/MESA_sprite_point.spec @@ -0,0 +1,193 @@ +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 <pname> 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. + + + |