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All of that also applies to '''tbd-hsr''' GPUs such as PowerVR's, which are similar to '''tbd-rast''' GPUs except for an additional optimization that they automatically perform: when a '''tbd-hsr''' GPU is about to start rasterizing the triangles in a given tile, it first identifies for each fragment which primitives may be visible at that fragment: see Section 4.4 in [http://www.imgtec.com/powervr/insider/docs/POWERVR%20Series5%20Graphics.SGX%20architecture%20guide%20for%20developers.1.0.8.External.pdf this PowerVR document]. What this means in practice is that a '''tbd-hsr''' GPU will be equally efficient regardless of the ordering of opaque primitives, whereas other types of GPUs will perform better if opaque geometry is submitted in front-to-back order. | All of that also applies to '''tbd-hsr''' GPUs such as PowerVR's, which are similar to '''tbd-rast''' GPUs except for an additional optimization that they automatically perform: when a '''tbd-hsr''' GPU is about to start rasterizing the triangles in a given tile, it first identifies for each fragment which primitives may be visible at that fragment: see Section 4.4 in [http://www.imgtec.com/powervr/insider/docs/POWERVR%20Series5%20Graphics.SGX%20architecture%20guide%20for%20developers.1.0.8.External.pdf this PowerVR document]. What this means in practice is that a '''tbd-hsr''' GPU will be equally efficient regardless of the ordering of opaque primitives, whereas other types of GPUs will perform better if opaque geometry is submitted in front-to-back order. | ||
= Notes about tile-based GPUs = | |||
Traditional OpenGL pipeline requires a lot of memory bandwidth which is very bad for power consumption. Mobile GPUs try to alleviate that by moving the frame buffer out of main memory, into high-speed on-chip memory (GMEM). This memory is very fast and power-efficient, but also very small. So the GPU will break up the framebuffer into smaller tiles and render to them using the high-speed memory one after the other. The size of tile buffers varries accross hardware, but it can be as small as 16x16 pixels tiles. To avoid overdraw, the GPU collects all the geometry, computes culling based on the output of the vertex shader and store the result in a spatial data structure for later use. When a tile is rendered, the data structre is consulted to see which triangles are relevent. | |||
= Performance implications of "deferred" = | = Performance implications of "deferred" = | ||
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While '''tbd-hsr''' GPUs optimize away overdraw regardless of the order of submission of geometry, '''tbd-rast''' GPUs don't. Therefore it remains important to sort opaque geometry in front-to-back order. | While '''tbd-hsr''' GPUs optimize away overdraw regardless of the order of submission of geometry, '''tbd-rast''' GPUs don't. Therefore it remains important to sort opaque geometry in front-to-back order. | ||
== Incremental frame updates == | |||
While only rerendering the part of the framebuffer that has changed since the last swap look like an intersting performance trick, it turns out to cause bad performance problems on tile-based GPUs. This is because if the previous tile buffer is not cleared, the GPU must restore its state which requires a bandwidth cost that can be higher than just redrawing the entire frame. | |||
Some vendors provide with extensions (like QCOM_tiled_rendering) to address this use-case. Such extensions make it possible to minimize the amount of tiles that need to be restored. | |||
= Misc mobile GPU performance topics = | = Misc mobile GPU performance topics = | ||