[FFmpeg-user] Transform dvdsub to png

[Thomas Ledoux]

 Hi all,

with the introduction of the new dvdvideo decoder, we are able to access
successfully any video and transcode it. Moreover, we can extract subtitles
in the native image-based subdvd format.

The command
ffmpeg -y -hide_banner -f dvdvideo -title 1 -i dvd.iso -vn -an -map
0:i:0x22 -c:s dvdsub title_01_fre_32.vob
allows the extraction of the subtitle with the id 0x20 in dvdsub format.

If I add the image2 demuxer, I'm able to extract each RLE frames with the
PTS in the filename
ffmpeg -y -hide_banner -f dvdvideo -title 1 -i dvd.iso -vn -an -copyts -map
0:i:0x20 -c:s dvdsub -f image2 -frame_pts true title_01_fre_32_%d.rle

However, if I ask for a image format like png, with a command like this

ffmpeg -y -hide_banner -f dvdvideo -title 1 -i dvd.iso -vn -an -copyts -map
0:i:0x20 -f image2 -frame_pts true title_01_fre_32_%d.png
I got
[sost#0:0 @ 000002187d8e73c0] Automatic encoder selection failed Default
encoder for format image2 (codec none) is probably disabled. Please choose
an encoder manually.
[sost#0:0 @ 000002187d8e73c0] Error selecting an encoder

If I explicitly ask for png with
ffmpeg -y -hide_banner -f dvdvideo -title 1 -i dvd.iso -vn -an -copyts -map
0:i:0x20 -c:s png -f image2 -frame_pts true title_01_fre_32_%d.png

[sost#0:0 @ 0000021eb03cc640] Invalid encoder type 'png'
[sost#0:0 @ 0000021eb03cc640] Error selecting an encoder

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 ?

Thanks for any insight
  Thomas