[FFmpeg-devel] [PATCH] libavcodec/ccaption_dec.c: Fixed indentation overwriting text in the cea608 caption decoder.

[Aman Karmani]

On Mon, Jan 25, 2021 at 3:16 PM Levi Dooley <i.am.stickfigure at gmail.com>
wrote:

> There was an assumption in the existing code that indentation would not
> occur more than once on the same row.
> This was a bad assumption. There are examples of 608 streams which call
> handle_pac multiple times on the same row with different indentation.
> As the code was before this change, the new indentation would overwrite
> existing text with spaces.
> These changes make indentation skip over columns instead. Text gets cleared
> with spaces on handle_edm.
> Instead of relying on the null character, trailing spaces are trimmed off
> the end of a row.
> This is necessary so that a null character doesn't end up between two
> words.
>
> Signed-off-by: Levi Dooley <i.am.stickfigure at gmail.com>
>
> Here's a link to a sample file that will reproduce this issue.
>
> https://snapstream-dev-test-public.s3.us-east-1.amazonaws.com/ffmpeg-caption-issue/cleveland-clip.ts
>
> The issue can be reproduced by running the following command:
>
> > ffmpeg -f lavfi -i "movie=cleveland-clip.ts[out0+subcc]" -map s
> > cleveland-clip.ass
>
>
> I've gone ahead and ran this command both before and after my code changes.
> The following output files demonstrate that there are some clear cases of
> missing words or sentences in the beforepatch file, and it is entirely
> fixed by this patch in the afterpatch file.
>
> Before this patch:
>
> https://snapstream-dev-test-public.s3.us-east-1.amazonaws.com/ffmpeg-caption-issue/cleveland-clip-beforepatch.ass
>
> After this patch:
>
> https://snapstream-dev-test-public.s3.us-east-1.amazonaws.com/ffmpeg-caption-issue/cleveland-clip-afterpatch.ass
>
> And here is the full sample video in case anyone wants to play around with
> a larger example with many more caption errors. The above video sample
> "cleveland-clip.ts" is just a 60 second clip of the following.
>
> https://snapstream-dev-test-public.s3.us-east-1.amazonaws.com/ffmpeg-caption-issue/The%20Cleveland%20Show%20-%20%28Brown%20Magic%29-2018-12-14-0.ts
>
> The full patch file is attached to this email.


Patch looks reasonable to me. Thanks for sharing the sample and
before/after output.

Will commit in a couple days if there are no additional comments.

Aman


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