Once your application issues a patch, the tessellation control shader will be called (if one is bound) and is responsible for completing the following actions:
• Generate the tessellation output patch vertices that are passed to the tessellation evaluation shader, as well as update any per-vertex, or per-patch attribute values as necessary.
• Specify the tessellation level factors that control the operation of the primitive generator. These are special tessellation control shader variables called
gl_TessLevelOuter, and are implicitly declared in your tessellation control shader.
We’ll discuss each of these actions in turn.