Release: GL Tools, v20071125

Point sprites are an exciting feature supported by OpenGL version 1.5 and later. Although OpenGL has always supported texture mapped points, prior to version 1.5 this meant a single texture coordinate applied to an entire point. Large textured points were simply large versions of a single filtered texel. With point sprites you can place a 2D textured image anywhere onscreen by drawing a single 3D point.

Probably the most common application of point sprites is for particle systems. A large number of particles moving onscreen can be represented as points to produce a number of visual effects. However, representing these points as small overlapped 2D images can produce dramatic streaming animated filaments. For example, screensaver on the Macintosh powered by just such a particle effect.

Before point sprites, achieving this type of effect was a matter of drawing a large number of textured quads onscreen. This could be accomplished either by performing a costly rotation to each individual quad to make sure that it faced the camera, or by drawing all particles in a 2D orthographic projection. Point sprites allow you to render a perfectly aligned textured 2D square by sending down a single 3D vertex. At one-quarter the bandwidth of sending down four vertices for a quad, and no client-side matrix monkey business to keep the 3D quad aligned with the camera, point sprites are a potent and efficient feature of OpenGL.

USING POINTS : Point sprites are very easy to use. Simply bind to a 2D texture, enable GL_POINT_SPRITE, set the texture environment target GL_POINT_SPRITE’s GL_COORD_REPLACEparameter to true, and send down 3D points: glBindTexture(GL_TEXTURE_2D, objectID); glEnable(GL_POINT_SPRITE); glTexEnvi(GL_POINT_SPRITE, GL_COORD_REPLACE, GL_TRUE); glBegin(GL_POINTS); glEnd();

One serious limitation to the use of point sprites is that their size is limited by the range of aliased point sizes You can quickly determine this implementation-dependent range with the following two lines of code: GLfloat fSizes[2]; GLGetFloatfv(GL_ALIASED_POINT_SIZE_RANGE, fSizes);

Following this, the array fSizeswill contain the minimum and maximum sizes supported for point sprites, and regular aliased points.

TEXTURE APPLICATION : Point sprites obey all other 2D texturing rules. The texture environment can be set to GL_DECAL, GL_REPLACE, GL_MODULATE, and so on. They can also be mipmapped and multitextured. There are a number of ways, however, to get texture coordinates applied to the corners of the points. If GL_COORD_REPLACEis set to false, as shown here glTexEnvi(GL_POINT_SPRITE, GL_COORD_REPLACE, GL_FALSE); then a single texture coordinate is specified with the vertex and applied to the entire point, resulting in one big texel! Setting this value to GL_TRUE, however, causes OpenGL to interpolate the texture coordinates across the face of the point. All of this assumes, of course, that your point size is greater than 1.0!

POINT PARAMETERS : A number of features of point sprites (and points in general actually) can be fine-tuned with the function glPointParameter. Setting the GL_POINT_SPRITE_COORD_ORIGINparameter to GL_LOWER_LEFTplaces the origin of the texture coordinate system at the lower-left corner of the point: glPointParameteri(GL_POINT_SPRITE_COORD_ORIGIN, GL_LOWER_LEFT);

... as is so often the case :) I'll see what I can whip up :)