fixedfunction

Fixed-function, in the context of computer graphics, refers to a traditional rendering pipeline in which most operations are performed by hardware-implemented stages with fixed functionality rather than by programmable code. In a fixed-function pipeline, vertex processing, transformation and lighting, texturing, rasterization, and pixel processing are carried out by dedicated circuits that perform a predefined set of operations. Developers control rendering primarily through a limited set of state changes and function calls rather than by writing custom shaders.

Historically, fixed-function pipelines were standard in early OpenGL and Direct3D implementations. OpenGL supports instruments such as matrix stacks (model-view and projection matrices), glRotate/glTranslate/glScale, lighting models (often per-vertex Phong lighting), and texturing operations controlled by state calls like glLight, glMaterial, glTexEnv, and glTexCoord. The programmer’s role was to configure these states to achieve visual effects, with much of the shading determined by the hardware’s fixed algorithms.

Limitations of fixed-function pipelines include inflexibility and a lack of pixel-accurate or physically based shading capabilities. Complex effects such as advanced post-processing, per-pixel lighting beyond the built-in model, and custom material models are difficult or impossible to implement without resorting to newer programmable techniques. As hardware evolved, consumer graphics APIs introduced programmable pipelines.

Transition to programmable pipelines began in the late 1990s and early 2000s with vertex and fragment shaders, platforms such as GLSL and HLSL, and APIs that emphasized programmability. Today, fixed-function functionality is largely obsolete for new applications, though compatibility modes on modern GPUs retain limited support for legacy software and certain embedded systems may retain some fixed-function elements for simplicity.