OpenGL.GL.ARB.tessellation_shader

OpenGL extension ARB.tessellation_shader

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

Overview (from the spec)

This extension introduces new tessellation stages and two new shader types to the OpenGL primitive processing pipeline. These pipeline stages operate on a new basic primitive type, called a patch. A patch consists of a fixed-size collection of vertices, each with per-vertex attributes, plus a number of associated per-patch attributes. Tessellation control shaders transform an input patch specified by the application, computing per-vertex and per-patch attributes for a new output patch. A fixed-function tessellation primitive generator subdivides the patch, and tessellation evaluation shaders are used to compute the position and attributes of each vertex produced by the tessellator.

When tessellation is active, it begins by running the optional tessellation control shader. This shader consumes an 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 shader invocations are performed -- one per output patch vertex. Each tessellation control shader invocation writes all the attributes of its corresponding output patch vertex. A tessellation control shader may also read the per-vertex outputs of other tessellation control shader invocations, as well as read and write shared per-patch outputs. The tessellation control shader invocations for a single patch effectively run as a group. A built-in barrier() function is provided to allow synchronization points where no shader invocation will continue until all shader invocations have reached the barrier.

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" splits a quad-shaped patch into a set of line strips running across the patch horizontally. 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 shader is run to compute its position and other attributes of the vertex, using its (u,v) or (u,v,w) coordinate. When computing final vertex attributes, the tessellation evaluation shader can also read the attributes of any of the vertices of the patch written by the tessellation control shader. Tessellation evaluation shader 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 shader. The primitives generated by the tessellator are passed further down the OpenGL pipeline, where they can be used as inputs to geometry shaders, transform feedback, and the rasterizer.

The tessellation control and evaluation shaders are both optional. If neither shader type is present, the tessellation stage has no effect. If no tessellation control shader is present, the input patch provided by the application is passed directly to the tessellation primitive generator, and a set of default tessellation level parameters is used to control primitive generation. In this extension, patches may not be passed beyond the tessellation evaluation shader, and an error is generated if an application provides patches and the current program object contains no tessellation evaluation shader.

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