Commit f49a1b2e authored by Committed by Fiona Glaser
OpenCL support is compiled in by default, but must be enabled at runtime by an --opencl command line flag. Compiling OpenCL support requires perl. To avoid the perl requirement use: configure --disable-opencl. When enabled, the lookahead thread is mostly off-loaded to an OpenCL capable GPU device. Lowres intra cost prediction, lowres motion search (including subpel) and bidir cost predictions are all done on the GPU. MB-tree and final slice decisions are still done by the CPU. Presets which do not use a threaded lookahead will not use OpenCL at all (superfast, ultrafast). Because of data dependencies, the GPU must use an iterative motion search which performs more total work than the CPU would do, so this is not work efficient or power efficient. But if there are spare GPU cycles to spare, it can often speed up the encode. Output quality when OpenCL lookahead is enabled is often very slightly worse in quality than the CPU quality (because of the same data dependencies). x264 must compile its OpenCL kernels for your device before running them, and in order to avoid doing this every run it caches the compiled kernel binary in a file named x264_lookahead.clbin (--opencl-clbin FNAME to override). The cache file will be ignored if the device, driver, or OpenCL source are changed. x264 will use the first GPU device which supports the required cl_image features required by its kernels. Most modern discrete GPUs and all AMD integrated GPUs will work. Intel integrated GPUs (up to IvyBridge) do not support those necessary features. Use --opencl-device N to specify a number of capable GPUs to skip during device detection. Switchable graphics environments (e.g. AMD Enduro) are currently not supported, as some have bugs in their OpenCL drivers that cause output to be silently incorrect. Developed by MulticoreWare with support from AMD and Telestream.
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