Not Programmable

OpenGL does not provide a programming language. Its function may be controlled by turning operations on or off or specifying parameters to operations, but the rendering algorithms are essentially fixed. One reason for this decision is that, for performance reasons, graphics hardware is usually designed to apply certain operations in a specific order; replacing these operations with arbitrary algorithms is usually infeasible. Programmability would conflict with keeping the API close to the hardware and thus with the goal of maximum performance.

• The Graphics Pipeline and Per-Fragment Operations