[Bug 17483] New: WebVTT: Don't overlap cues that don't fit

https://www.w3.org/Bugs/Public/show_bug.cgi?id=17483

           Summary: WebVTT: Don't overlap cues that don't fit
           Product: TextTracks CG
           Version: unspecified
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: WebVTT
        AssignedTo: ian@hixie.ch
        ReportedBy: philipj@opera.com
         QAContact: dave.null@w3.org
                CC: mike@w3.org, public-texttracks@w3.org


http://dev.w3.org/html5/webvtt/#webvtt-cue-text-rendering-rules

In step 10.13.15 of the rendering algorithm the cue boxes are moved back to the
default position before trying the other direction, even the second time that
point in the algorithm is reached. The net result is that if the cue did not
fit at all, it's moved back to its default position where it will overlap some
other cue.

The first part of the fix is to switch the order of steps 10.13.15 and 10.13.16
so that when "giving up" the cue boxes are left wherever they ended up.

However, this is not enough to avoid overlap in all cases, since the condition
in 10.13.12 is to check if any part of the cue box is outside the video
rendering area, which means that the cues may still overlap. There are a two
alternatives that we can see:

1. In step 10.13.16, if the cue is still overlapping any other cue, remove it
from the output.

2. Modify 10.13.12 to have a different condition depending on switched flag. if
switched is false, step until any part of the cue box is outside the video
rendering area. If switched is true, step until all of the cue box is outside
the video rendering area.

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Received on Wednesday, 13 June 2012 15:36:43 UTC