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

[Andreas Rheinhardt]

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
> 
This patch breaks FATE [1]; this needs to be analyzed. If the changes
are ok, the ref files need to be updated in the same patch.

- Andreas

[1]:
https://patchwork.ffmpeg.org/project/ffmpeg/patch/20210126215003.178259-1-levi@snapstream.com/