AFAIK the only technical reason why there are no 4K images at the moment is, that the 4K resolution requires different memory layout (framebuffer is larger...) and hence a tweak to the kernel and other associated things are required which could break stuff and is a bigger development effort. That is a separate issue to 3D or video acceleration. A lot of people seem to mix up those two anyways. We have a ARM Mali GPU for 3D acceleration, which is a memory-to-memory accelerator (reads screen content from memory and writes changes back) for mostly OpenGL stuff, this requires a binary driver based on sources from ARM, and additional DRM compatible display driver stack. Then we have a Video Engine that can accelerate video decoding and encoding for various formats and has been reverse engineered for Linux as http://linux-sunxi.org/Cedrus. The official binary driver for the Video engine is CedarX from Allwinner... you can use Cedrus already with mplayer for example to get accelerated video under Linux... For KODI there is work in progress since KODI requires a certain interface for video acceleration to work: http://forum.kodi.tv/showthread.php?tid=254202
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