Geometry shader
Geometry shader (abbreviation GS) is a shader program, normally executed on the graphics processing unit.
Function
A geometry shader can generate new graphics primitives, such as pixels, lines, and triangles, from those primitives that were sent to the beginning of the graphics pipeline.
Geometry shader programs are executed after vertex shaders. They take as input a whole primitive, possibly with adjacency information. For example, when operating on triangles, the three vertices are the geometry shader's input. The shader can then emit zero or more primitives, which are rasterized and their fragments ultimately passed to a pixel shader.
Typical uses of a geometry shader include point sprite generation, geometry tessellation, shadow volume extrusion, and single pass rendering to a cube map.
Programming
Geometry shaders are programmed in the following languages: Assembly, Cg and HLSL. GLSL does not yet support geometry shaders, but it is expected that the next version of OpenGL will support. Although OpenGL does not support Geometry Shaders in its core functionality extensions are now available te enable GLSL as programming language for Geometry Shaders.