[FFmpeg-devel] [PATCH 1/2] fftools/cmdutils: make the default swscale flags identical for simple and complex filter graph

[Michael Niedermayer]

On Sun, Aug 01, 2021 at 04:08:17PM +0200, Nicolas George wrote:
> Linjie Fu (12021-08-01):
> > Checked swscale, it uses bicubic by default.
> > Scale filter sets the flags to bilinear if sws_scale_opt is not initialized
> > by sws_dict.
> > 
> > We should use the same default flags for both simple and complex filter
> > graphs, either from swscale, or from scale filter.
> > If prefer to keep it identical with swscale, we may modify the flags in the
> > scale filter.
> 
> Thanks. I also think that unless there is a good reason, we should rely
> on a single default, from the lowest level. So:
> 
> - remove flags="bilinear" from vf_scale;
> 
> - remove setting defaults in ffmpeg.
> 
> Unless somebody can find a good reason for different defaults.
> 
> Michael, it is your commit, 8155bbc944c314fc202d31b0e4a3c77b8efc8dde,
> that introduced it. Any comments?
> 
> Also, the current code in libswscale looks nonsensical:
> 
>         if (dstW < srcW && dstH < srcH)
>             flags |= SWS_BICUBIC;
>         else if (dstW > srcW && dstH > srcH)
>             flags |= SWS_BICUBIC;
>         else
>             flags |= SWS_BICUBIC;
> 
> It was originally 6b3ff6f91a535d6383f41ca7bdf760165dcb6015 with some
> logic, but it was cancelled by the merge commit
> 5710f55e490d0c3cf7348b3ca91c8c0fe4274be2.
> 

> "The default is left at bicubic until someone has compared the scalers
> properly speed and quality wise."
> 
> But eight years later nobody did.

In case someone tests this, it should test
1. human subjective quality in addition to some objective quality
2. scaling + compression
3. various material, noisy and less crisp material may favor
   different scalers than super flawless crisp images

thx

[...]

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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

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