swresample: pass input layout in mpv's order via custom channel order - #18034
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Dudemanguy
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LGTM
Cleans up things nicely. Doing custom layouts specifically even when they possibly match an FFmpeg standard layout is a bit specific, but I don't really see it as a too big negative. We will anyways have to handle custom channel layouts in chmap_avchannel at some point.
Since 9ef614d this helper has been a no-op. fudge_layout_conversion() returned the modified uint64_t channel-layout bitmask, but the new AVChannelLayout path builds out_layout from the unfudged out_lavc and feeds that to swr. Even when it was wired up, libswresample handles the BL/BR <-> SL/SR mapping if needed. SDL/SDR is not currently handled, but I would rather patch the swr/rematrix.c for those too, instead of having custom glue here. Note that SDL/SDR are not that common in-practice.
Drop the swr_set_channel_mapping() + transpose_order() + reorder_in[] plumbing. Instead, build the swr input channel layout directly from the mpv chmap as AV_CHANNEL_ORDER_CUSTOM, so the labels in the layout already match the positions of the channels in the input data. For all unknown map, we return all UNSPEC, so the swr expands to default layout for this channel number.
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