[FFmpeg-devel] [PATCH] avfilter/vf_subtitles: pass storage size to libass

[Oneric]

On Mon, Mar 14, 2022 at 19:57:05 +0000, Soft Works wrote:
> > > ass_set_pixel_aspect() is setting settings.par and if I'm not mistaken,
> > > an existing par setting leads to the storage size setting to be ignored:
> > 
> > It’s not overridden. Only the explicit PAR is currently preferd over the
> > implicit derivation from storage and frame size. However as I stated in
> > the patch description and the comment:
> >   “some tags depend on the exact storage resolution of the video”
> >   “it actually goes farther than just the aspect ratio”
> > 
> 
> I found only one other place where storage_h is used (for determining 
> blur size) but I didn't find any other usage in the libass source code.
> That's what I'm wondering about.

Well, blur is one of the things that depend on it. If you follow the usage 
of the blur scale, you'll see it also plays a role in the projection 
matrix for 3D-transforms (what the provided samples use) and if 
ScaledBorderAndShadow is not set to "yes", it also affects some other 
scaling values.

This unfortunate dependence is a result of how SSA and then ASS 
histoically developed and required to maintain compaitibility with 
existing subtitles.