OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Point Sprites

Point sprites are essentially OpenGL points rendered using a fragment shader that takes the fragment’s coordinates within the point into account when running. The coordinate within the point is available in the two-dimensional vector gl_PointCoord. This variable can be used in any number of ways. Two common uses are to use it as a texture coordinate (this is the classic origin of the term point sprite), or to use it to analytically compute color or coverage. The following are a few examples of how to use the gl_PointCoord vector to produce interesting effects in the fragment shader.