[FFmpeg-user] Transform dvdsub to png

[Bouke / Videotoolshed]

> On 21 Jul 2024, at 23:59, Thomas Ledoux <tledouxfr at gmail.com> wrote:
> 
> Hi Nicolas,
> 
> Thanks a lot for the hint, that allows me to make great progress.
> Using an overlay I'm indeed able to extract the subtitles
> I use :
> ffmpeg -f dvdvideo -title 1 -i dvd.iso -copyts -f lavfi -i
> color=size=pal:rate=25:color=black at 0.0,format=rgba -filter_complex
> "[0:i:0x20][1:v]overlay[v]" -map "[v]" -f image2 -frame_pts true -c:s png
> title_01_fre_32_%d.png
> 
> I put the subtitle stream first so that the length of the "video" is driven
> by it, and I overlay a black transparent video. I end up having thousands
> of images (25 per second, of course) but they are all repetitions and only
> changing when the subtitle changes. So I wonder now how to only have as
> many images as there are different subtitles. Should I try a decimation ?
> This seems a lot of processing...


I totally don’t get this.
For images, you can do timeline editing (enable between yadda), but there is open source software to rip subtitles to images and do OCR, so you end up with an SRT.

DVD subs are horrible. They have only 4 colours, one being transparant, so you have one level of anti-alias (and with DVD SP generated ones, that’s not even on the highest contrast.)

SRT can give you the best image quality possible, since it’s metadata.

Bouke


> All the best
>  Thomas
> 
> Le dim. 21 juil. 2024 à 13:57, Nicolas George <george at nsup.org> a écrit :
> 
>> Thomas Ledoux (12024-07-21):
>>> Indeed, PNG is not supposed to be used for subtitles. But in this case,
>>> subtitles are images
>>> 
>>> so they could be transform in PNG. Am I forgetting a filter to make it
>>> react as if it was a video ?
>> 
>> You need to overlay the subtitles on a dummy video.
>> 
>> Regards,
>> 
>> --
>>  Nicolas George
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