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On macOS, Adobe Media Encoder has options for hardware-accelerated rendering utilizing OpenCL, which seems to speed up encoding time.
On the other hand, according to the ffmpeg wiki, ffmpeg's hardware acceleration "typically generate output of significantly lower quality than good software encoders". Is this true of both Media Encoder and ffmpeg, or does Media Encoder somehow better utilize hardware for encoding?
Media Encoder and ffmpeg should generate the same quality if given the same encoding parameters.
GPU encoders are provided by GPU vendors. I haven't heard of any third-party implementations. The possible differences lie in the wrappers i.e. the code which initializes the encoder parameters, and written by the interfacing app. There, I would expect ffmpeg to offer more fine-grained choices although I haven't look at AME's offerings as of late.
Speaking from a computer programming background, MPEG and H.264 encoding do not lend themselves to GPU hardware. The encoding requires a lot of conditional logic and "if statements" where a CPU is superior. The dedicated encoder chip in your camera does not work the same as the GPU in your computer; both are intended for video work, but specialize on different types of tasks. Note a GPU works fine for decoding, which is an easier task.
My personal experience is that few decent GPU encoders exist, and the best ones have slightly higher performance but noticeably poorer quality than CPU encoders. Overall, I cannot recommend OpenCL or CUDA based encoders. But - my experience is from a few years ago. Software may have improved. Side-by-side quality comparison with your particular footage is always best.
EDIT: Looks like GPUs do have dedicated encoder circuits these days. Support is available in select software on Linux and Windows. See here and here. Adobe Media Encoder is not listed. If it uses OpenCL, then it's using the main part of the GPU and not the encoder circuit, so above comments on quality still stand.
