[vlc-devel] [PATCH] cc: Only reset line position if rollup mode changed

Devin Heitmueller dheitmueller at ltnglobal.com
Mon Jul 23 14:56:27 CEST 2018

Hello Francois,

> I can't really figure how it should behave on RU2, RU3, RU4,
> as the spec only mentions differences in hw decoder and clearing of rollups.

I think perhaps you’ve misunderstood the nature of this patch.  The issue here is that the rollup code is sent very frequently, in many cases in the middle of a line being rendered.  This is common in particular in live captioned content (where an operator is typing captions in real-time, as opposed to cases where the captions are prepared in advance).  We want to ensure that if we receive the rollup code we don’t reset the line position if we’re already in the same mode.

> I can't be sure if the command should be ignored, or only committed on
> another one.

IIRC, the changing of the rollup shouldn’t actually clear the line.  I believe the WGBH reference stream actually demonstrates that case where the rollup is changed to some other value and it isn’t supposed to clear the existing captions.  That said, I haven’t really tested this use case and while this patch improves the situation considerably (the captions go from “completely unwatchable” to “pretty good”) there may be other improvements/fixes which are warranted.

I would encourage you to accept this patch as-is, and we can look at further improvements which can be made to the quality of the implementation.

> How does behave the 708 decoder with the same stream ?

I’ve just sent you a dropbox link to a short sample I used to debug/fix the issue.  It includes a 708 stream and you are welcome to try it out for yourself.  If it’s helpful, I can screencast the rendering with a “known good” application which will allow you to compare the behavior to VLC's decoder.


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