[FFmpeg-user] dvdsub scaling 'fatness' work around?

[Devin Heitmueller]

On Wed, Jul 23, 2025 at 8:00 PM Mark Filipak <markfilipak.imdb at gmail.com> wrote:
>
> dvdsubs are RLE (run length encoded) bitmaps.
>
> It appears that when dvdsubs from SD are encoded into full-HD, the run
> lengths are 720-to-1920 scaled and the lines (for NTSC DVDs) are
> 480-to-1080 scaled. That's great. However, the character bodies in the
> resulting dvdsubs are very fat and overrun each other. I presume the
> overruns are because the spaces between letters are not also scaled. The
> resulting subtitles are still somewhat readable but difficult.
>
> Does anyone know of a work around?
>
> Ideal would be scaling without 'fatness' by keeping letter stoke widths
> unchanged but scaling the heights and widths in addition to position --
> yes, that will be tricky. Also ideal would be control over the palette.

If I had to hazard a guess, it would probably be the issue is the DVD
subtitle images are expected to be rendered with a 4:3 aspect ratio.
Scaling them from 720x480 to 1920x1080 causes the pixel aspect ratio
to change, and thus they appear to be horizontally stretched.

I'm not sure what your pipeline looks like, but I would assume you
would need to change your scaling of the DVD subs to be scaled to
preserve the appropriate aspect ratio.

Devin

-- 
Devin Heitmueller, Senior Software Engineer
LTN Global Communications
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