OpenGL.GL.NV.tessellation_program5

OpenGL extension NV.tessellation_program5

This module customises the behaviour of the OpenGL.raw.GL.NV.tessellation_program5 to provide a more Python-friendly API

Overview (from the spec)

This extension, in conjunction with the ARB_tessellation_shader extension, introduces a new tessellation stage to the OpenGL primitive processing pipeline. The ARB_tessellation_shader extension provides programmable shading functionality using the OpenGL Shading Language as its base; this extension provides assembly programmable shaders building on the family of assembly programmability extensions including ARB_vertex_program, ARB_fragment_program, NV_gpu_program4, and NV_geometry_program4.

This extension adds a new basic primitive type, called a patch, which consists of an array of vertices plus some associated per-patch state. It also adds two new assembly program types: a tessellation control program that transforms a patch into a new patch and a tessellation evaluation program that computes the position and attributes of each vertex produced by the tesselator.

When tessellation is active, it begins by running the optional tessellation control program, if enabled. This program consumes a variable-size input patch and produces a new fixed-size output patch. The output patch consists of an array of vertices, and a set of per-patch attributes. The per-patch attributes include tessellation levels that control how finely the patch will be tessellated. For each patch processed, multiple tessellation control program invocations are performed -- one per output patch vertex. Each tessellation control program invocation writes all the attributes of its corresponding output patch vertex. A tessellation control program may also read the per-vertex outputs of other tessellation control program invocations, as well as read and write shared per-patch outputs. The tessellation control program invocations for a single patch effectively run as a group. The GL automatically synchronizes threads to ensure that when executing a given instruction, all previous instructions have completed for all program invocations in the group.

The tessellation primitive generator then decomposes a patch into a new set of primitives using the tessellation levels to determine how finely tessellated the output should be. The primitive generator begins with either a triangle or a quad, and splits each outer edge of the primitive into a number of segments approximately equal to the corresponding element of the outer tessellation level array. The interior of the primitive is tessellated according to elements of the inner tessellation level array. The primitive generator has three modes: TRIANGLES and QUADS split a triangular or quad-shaped patch into a set of triangles that cover the original patch; ISOLINES_NV splits a quad-shaped patch into a set of line strips spanning the patch. Each vertex generated by the tessellation primitive generator is assigned a (u,v) or (u,v,w) coordinate indicating its relative location in the subdivided triangle or quad.

For each vertex produced by the tessellation primitive generator, the tessellation evaluation program is run to compute its position and other attributes of the vertex, using its (u,v) or (u,v,w) coordinate. When computing the final vertex attributes, the tessellation evaluation program can also read the attributes of any of the vertices of the patch written by the tessellation control program. Tessellation evaluation program invocations are completely independent, although all invocations for a single patch share the same collection of input vertices and per-patch attributes.

The tessellator operates on vertices after they have been transformed by a vertex program or fixed-function vertex processing. The primitives generated by the tessellator are passed further down the OpenGL pipeline, where they can be used as inputs to geometry programs, transform feedback, and the rasterizer.

The tessellation control and evaluation programs are both optional. If neither program type is present, the tessellation stage has no effect. If no tessellation control program is present, the input patch provided by the application is passed directly to the tessellation primitive generator, and a set of fixed tessellation level parameters (specified via the PatchParameterfv function) is used to control primitive generation. If no tessellation evaluation program is present, the output patch produced by the tessellation control program is passed as a patch to subsequent pipeline stages, where it can be consumed by geometry programs, transform feedback, or the rasterizer.

The official definition of this extension is available here: http://www.opengl.org/registry/specs/NV/tessellation_program5.txt