OpenGL.GL.NV.geometry_program4

OpenGL extension NV.geometry_program4

This module customises the behaviour of the OpenGL.raw.GL.NV.geometry_program4 to provide a more Python-friendly API

Overview (from the spec)

NV_geometry_program4 defines a new type of program available to be run on the GPU, called a geometry program. Geometry programs are run on full primitives after vertices are transformed, but prior to flat shading and clipping.

A geometry program begins with a single primitive - a point, line, or triangle. Quads and polygons are allowed, but are decomposed into individual triangles prior to geometry program execution. It can read the attributes of any of the vertex in the primitive and use them to generate new primitives. A geometry program has a fixed output primitive type, either a point, a line strip, or a triangle strip. It emits vertices (using the EMIT opcode) to define the output primitive. The attributes of emitted vertices are specified by writing to the same set of result bindings (e.g., "result.position") provided for vertex programs. Additionally, a geometry program can emit multiple disconnected primitives by using the ENDPRIM opcode, which is roughly equivalent to calling End and then Begin again. The primitives emitted by the geometry program are then clipped and then processed like an equivalent OpenGL primitive specified by the application.

This extension provides four additional primitive types: lines with adjacency, line strips with adjacency, separate triangles with adjacency, and triangle strips with adjacency. Some of the vertices specified in these new primitive types are not part of the ordinary primitives. Instead, they represent neighboring vertices that are adjacent to the two line segment end points (lines/strips) or the three triangle edges (triangles/tstrips). These "adjacency" vertices can be accessed by geometry programs and used to match up the outputs of the geometry program with those of neighboring primitives.

Additionally, geometry programs allow for layered rendering, where entire three-dimensional, cube map, or array textures (EXT_texture_array) can be bound to the current framebuffer. Geometry programs can use the "result.layer" binding to select a layer or cube map face to render to. Each primitive emitted by such a geometry program is rendered to the layer taken from its provoking vertex.

Since geometry programs expect a specific input primitive type, an error will occur if the application presents primtives of a different type. For example, if an enabled geometry program expects points, an error will occur at Begin() time, if a primitive mode of TRIANGLES is specified.

The official definition of this extension is available here: http://www.opengl.org/registry/specs/NV/geometry_program4.txt