ASS subtitles render bug: issue with Ordered Chapters in MKVs

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ASS subtitles render bug: issue with Ordered Chapters in MKVs

Post by marcgii on Sat Nov 10, 2018 2:34 am

So the over the years I've occasionally experienced a bug when when viewing a .mkv with ASS subtitles. Basically, a subtitle line will fail the render if the video playing normally.  However if you skip back a couple of seconds, the subtitle will render as expected.  But if you skip back far enough, the subtitle will again fail to render.  Any idea of a fix?   I could also post a link to a video torrent if you want an example.  Or I could just upload the video to my google drive and PM the link.  

So after messing around for sometime, I discovered that the bug seems to be related to whether the "EditionFlagOrdered" is enabled in the .mkv file. The issue disappears if I disable the flag.  However, disabling the flag causes external video segmentation support to stop working, so that's not a solution.

Info on the flag:
https://www.matroska.org/technical/specs/chapters/index.html


Here are some other things I initially looked at:
1.  Bug occurs when using both the MPC subtitle renderer and DirectVOBSub renderer
2.  I've replicated the issue on several computers
3.  Bug doesn't occur when using VLC (for the particular case I'm looking at).
4.  Using the official MPC-HC release made no difference.
5.  Changing any DirectVobsub performance settings made no difference.'
6.  I tried using ASS filter, which made the issue worse.  With it enabled I couldn't get the subtitle to render by skipping back and forth.
7.  Going back to older MPC versions didn't seem to help either.  At around 1.7.6, the subtitles don't render correctly at all.

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Re: ASS subtitles render bug: issue with Ordered Chapters in MKVs

Post by Admin on Sat Nov 10, 2018 10:39 pm

Most likely a bug/limitation of the splitter.

Can you provide a sample file?

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Re: ASS subtitles render bug: issue with Ordered Chapters in MKVs

Post by marcgii on Sun Nov 11, 2018 2:41 am

I PM'd you a link to a sample video.  It appears the bug only affects subtitles that occur as soon as a new chapter starts.  It occurs several times in this video.  I also noticed the bug doesn't occur when using chapter skip.  

At 5:10, the first line dialog doesn't render.  At the very next chapter, 5:44, the text on the sign doesn't render.

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Re: ASS subtitles render bug: issue with Ordered Chapters in MKVs

Post by Admin on Sun Nov 11, 2018 4:19 pm

I can reproduce.

My advice would be to report it here directly:
https://github.com/Nevcairiel/LAVFilters/issues

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Re: ASS subtitles render bug: issue with Ordered Chapters in MKVs

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