Texture mapping unit

A texture mapping unit (TMU) is a component in modern graphics processing units (GPUs), historically it was a separate physical processor. A TMU is able to rotate, resize, and distort a bitmap image (performing texture sampling), to be placed onto an arbitrary plane of a given 3D model as a texture. This process is called texture mapping. In modern graphics cards it is implemented as a discrete stage in a graphics pipeline, whereas when first introduced it was implemented as a separate processor, e.g. as seen on the Voodoo2 graphics card.

The TMU came about due to the compute demands of sampling and transforming a flat image (as the texture map) to the correct angle and perspective it would need to be in 3D space. The compute operation is a large matrix multiply, which CPUs of the time (early Pentiums) could not cope with at acceptable performance.

Today, TMUs are part of the shader pipeline and decoupled from the Render Output Pipelines (ROPs). For example, in AMD's Cypress GPU, each shader pipeline (of which there are 20) has four TMUs, giving the GPU 80 TMUs. This is done by chip designers to closely couple shaders and the texture engines they will be working with.

3D scenes are generally composed of two things: 3D geometry, and the textures that cover that geometry. Texture units in a video card take a texture and 'map' it to a piece of geometry. That is, they wrap the texture around the geometry and produce textured pixels which can then be written to the screen. Textures can be an actual image, a lightmap, or even normal maps for advanced surface lighting effects.

To render a 3D scene, textures are mapped over the top of polygon meshes. This is called texture mapping and is accomplished by texture mapping units (TMUs) on the videocard. Texture fill rate is a measure of the speed with which a particular card can perform texture mapping.

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