[Bug 18501] New: position:0%, position:50% and position:100% are all weird/broken

https://www.w3.org/Bugs/Public/show_bug.cgi?id=18501

           Summary: position:0%, position:50% and position:100% are all
                    weird/broken
           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


The concept is defined in
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-video-element.html#text-track-cue-text-position
as

"A number giving the position of the text of the cue within each line, to be
interpreted as a percentage of the video, as defined by the writing direction."

However, in http://dev.w3.org/html5/webvtt/#webvtt-cue-text-rendering-rules it
influences the maximum size is non-obvious ways:

* For align:middle, both position:0% and position:100% results in a maximum
size 0.
* For align:start, position:100% results in a maximum size 0.
* For align:end, position:0% results in a maximum size 0.

The default position:50% also interacts badly with align:start and align:end,
where (for left-to-right horizontal text), align:start by default makes the
text occupy the right half of the viewport, while align:end makes the text
occupy the left half of the viewport.

Evidence of this being less than obvious is
http://www.delphiki.com/html5/playr/examples/dw_trailer_low.vtt where the
author clearly assumed that position:0% would do something useful, perhaps
left-aligning?

It seems to me that what authors needs the ability to specify is:

1. The width of the boxes to fit the cue text into (0-100%)
2. The alignment of cue text within those boxes (start/middle/end)
3. The alignment of those boxes within the viewport if they are not 100% wide.

We have 1 (size:50%) and 2 (align:end) but 3 is not easy. The solution I would
find most intuitive is to redefine position to be more similar to e.g. CSS
background-position, such that 0% means "all the way to the left" and 100%
means "all the way to the right". That way, size:50% position:100% would result
in the right half of the viewport being used. The default position should
remain 50%, but would have no effect unless width < 100%.

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Received on Wednesday, 8 August 2012 13:07:32 UTC