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manpage: document how hardware decoding might cause quality loss

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DOCS/man/options.rst
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@ -629,6 +629,42 @@ Video
codecs. See ``--hwdec-codecs`` to enable hardware decoding for more
codecs.
.. admonition:: Quality reduction with hardware decoding
Normally, hardware decoding does not reduce video quality (at least for
the codecs h264 and HEVC). However, due to restrictions in video output
APIs, there can be some loss, or blatantly incorrect results.
In some cases, RGB conversion is forced, which means the RGB conversion
is performed by the hardware decoding API, instead of the OpenGL code
used by ``--vo=opengl``. This means certain obscure colorspaces may
not display correctly, and not certain filtering (such as debanding)
can not be applied in an ideal way.
``vdpau`` forces RGB conversion. Currently, it does not treat certain
colorspaces like BT.2020 correctly. This is mostly a mpv-specific
restriction. This does not apply if the ``vdpauprb`` filter is used.
``vaapi`` is safe if the ``vaapi-egl`` backend is indicated in the logs.
If ``vaapi-glx`` is indicated, and the video colorspace is either BT.601
or BT.709, a forced but correct RGB conversion is performed. Otherwise,
the result will be incorrect.
``d3d11va`` is usually safe. If the ``EGL_KHR_stream path`` is indicated
in the logs, it is lossless and full OpenGL filtering is done. If
``ID3D11VideoProcessor path`` is indicated, and the video colorspace is
either BT.601 or BT.709, a forced but correct RGB conversion is
performed. Otherwise, the result will be incorrect.
``dxva2`` is not safe. It uses either BT.601 or BT.709 for forced RGB
conversion, and which colorspace is used is driver-dependent. This can
give incorrect results even with completely ordinary video sources.
All other methods, in particular the copy-back methods (like
``dxva2-copy`` etc.) are either fully safe, or not worse than software
decoding. In particular, ``auto-copy`` will only select safe modes
(although potentially slower than other methods).
``--hwdec-preload=<api>``
This is useful for the ``opengl`` and ``opengl-cb`` VOs for creating the
hardware decoding OpenGL interop context, but without actually enabling