Fixed-function (computer graphics)

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In computer graphics, fixed-function is a term primarily used to describe 3D graphics APIs and GPUs designed prior to the advent of programmable shaders. The term is also used to describe APIs and graphics pipelines that do not allow users to change its underlying processing techniques, hence the word 'fixed'. Fixed-function can also refer to graphics processing techniques that employ non-programmable dedicated hardware, like the use of ROPs to rasterize an image.^{[citation needed]}

The first known appearance of fixed-function hardware came with the IBM 8514.^[1]

Historically fixed-function APIs consisted of a set of function entry points that would approximately or directly map to dedicated logic for their named purpose in GPUs designed to support them. As shader based GPUs and APIs evolved, fixed-function APIs were implemented by graphics driver engineers using the more general purpose shading architecture. This approach served as a segue that would continue providing the fixed-function API abstraction most developers were experienced with while allowing further development and enhancements of the newer shader-based architectures.

OpenGL, OpenGL ES and DirectX (Direct3D) are all 3D graphics APIs that went through the transition from the fixed-function programming model to the shader-based programming model.^[2] Below is a table of when the transition from fixed-function to shaders was made:

Fixed function APIs tend to be a simpler programming abstraction with a series of well-defined and specifically named graphics pipeline stages. Shader-based APIs treat graphics data (vertices and pixels / texels) generically and allow a great deal of flexibility in how this data is modulated. More sophisticated rendering techniques are possible using a shader-based API.