Re: [FFmpeg-devel] [PATCH 1/8] swscale: fix sws_setColorspaceDetails after sws_init_context

[Niklas Haas <ffmpeg@haasn.xyz>]

On Fri, 27 Oct 2023 23:42:41 +0200 Michael Niedermayer <michael@niedermayer.cc> wrote:
> On Fri, Oct 27, 2023 at 07:04:39PM +0200, Niklas Haas wrote:
> > From: Niklas Haas <git@haasn.dev>
> > 
> > More commonly, this fixes the case of sws_setColorspaceDetails after
> > sws_getContext, since the latter implies sws_init_context.
> > 
> > The problem here is that sws_init_context sets up the range conversion
> > and fast path tables based on the values of srcRange/dstRange at init
> > time. This may result in locking in a "wrong" path (either using
> > unscaled fast path when range conversion later required, or using
> > scaled slow path when range conversion becomes no longer required).
> > 
> > There are two way outs:
> > 
> > 1. Always initialize range conversion and unscaled converters, even if
> >    they will be unused, and extend the runtime check.
> > 2. Re-do initialization if the values change after
> >    sws_setColorspaceDetails.
> > 
> > I opted for approach 1 because it was simpler and easier to reason
> > about.
> > ---
> >  libswscale/swscale.c | 2 +-
> >  libswscale/utils.c   | 5 +----
> >  2 files changed, 2 insertions(+), 5 deletions(-)
> > 
> > diff --git a/libswscale/swscale.c b/libswscale/swscale.c
> > index 90e5b299ab..46ba68fe6a 100644
> > --- a/libswscale/swscale.c
> > +++ b/libswscale/swscale.c
> > @@ -1016,7 +1016,7 @@ static int scale_internal(SwsContext *c,
> >      reset_ptr(src2, c->srcFormat);
> >      reset_ptr((void*)dst2, c->dstFormat);
> >  
> > -    if (c->convert_unscaled) {
> > +    if (c->convert_unscaled && !c->lumConvertRange && !c->chrConvertRange) {
> >          int offset  = srcSliceY_internal;
> >          int slice_h = srcSliceH;
> >  
> > diff --git a/libswscale/utils.c b/libswscale/utils.c
> > index e1ad685972..455955e907 100644
> > --- a/libswscale/utils.c
> > +++ b/libswscale/utils.c
> > @@ -1728,9 +1728,7 @@ static av_cold int sws_init_single_context(SwsContext *c, SwsFilter *srcFilter,
> >      }
> >  
> >      /* unscaled special cases */
> > -    if (unscaled && !usesHFilter && !usesVFilter &&
> > -        (c->srcRange == c->dstRange || isAnyRGB(dstFormat) ||
> > -         isFloat(srcFormat) || isFloat(dstFormat))){
> > +    if (unscaled && !usesHFilter && !usesVFilter) {
> >          ff_get_unscaled_swscale(c);
> >  
> >          if (c->convert_unscaled) {
> > @@ -1738,7 +1736,6 @@ static av_cold int sws_init_single_context(SwsContext *c, SwsFilter *srcFilter,
> >                  av_log(c, AV_LOG_INFO,
> >                         "using unscaled %s -> %s special converter\n",
> >                         av_get_pix_fmt_name(srcFormat), av_get_pix_fmt_name(dstFormat));
> > -            return 0;
> >          }
> >      }
> 
> the av_log() message will be wrong if this path is unused

Indeed. Perhaps the only way to fix that would be to instead try
approach 2 and go through full reinit if any params change after
setColorspaceDetails.

> also this ties convert_unscaled to never do range conversion, if thats
> intended, i guess thats ok. Otherwise that maybe is a restriction
> we dont want to add.

In the original code, c->convert_unscaled is set behind a `c->srcRange
== c->dstRange` check, so how is that a change in behavior?
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