Name
MESA_sprite_point
Name Strings
GL_MESA_sprite_point
Contact
Brian Paul, VA Linux Systems Inc. (brianp 'at' valinux.com)
Status
Obsolete - see GL_ARB_point_sprite.
Version
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.