[FFmpeg-devel] [PATCH 4/8] avfilter/vf_scale: properly respect to input colorimetry

[Michael Niedermayer]

On Fri, Oct 27, 2023 at 07:04:42PM +0200, Niklas Haas wrote:
> From: Niklas Haas <git at haasn.dev>
> 
> This code, as written, skips sws_init_context if scale->in_range was not
> set, even if scale->in_frame_range is set to something. So this would
> hit the 'no sws context' fast path in scale_frame and skip color range
> conversion even where technically required. This had the effect of, in
> practice, effectively applying limited/full range conversion if and only
> if you set e.g. a nonzero yuv color matrix, which is obviously
> undesirable behavior.
> 
> Second, the assignment of out->color_range inside the branch would fail
> to properly propagate tags for any actually applied conversion, for
> example on conversion to a forced full range format.
> 
> Solve both of these problems and make the code much easier to understand
> and follow by doing the following:
> 
> 1. Always initialize sws context on get_props
> 2. Always use sws_getColorspaceDetails + sws_setColorspaceDetails to
>    fix the conversion matrices per-frame.
> 3. Rather than testing if the context exists, do the no-op test after
>    settling the above values and deciding what conversion to actually
>    perform.
> ---
>  libavfilter/vf_scale.c | 186 +++++++++++++++++------------------------
>  1 file changed, 76 insertions(+), 110 deletions(-)

This breaks
tickets/2939/lena.jpg

(color looks slightly wrong compared to reference lena image)



[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Asymptotically faster algorithms should always be preferred if you have
asymptotical amounts of data
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