[webvtt] Cue box's size would be incorrectly changed when setting 'align:start' or 'align:end' (#458)

alastor0325 has just created a new issue for https://github.com/w3c/webvtt:

== Cue box's size would be incorrectly changed when setting 'align:start' or 'align:end' ==
In current spec, [`ComputedPosition`](https://www.w3.org/TR/webvtt1/#cue-computed-position) only takes the text alignment `left` and `right` into account, but it should consider the text alignment `start` and `end` as well. Otherwise, it would result in incorrect cue box size.

For example,
If user set `align:start`, the `ComputedPosition` would be `50`, which is its default value.  Assume the text is LTR, so the [`computed position alignment`](https://www.w3.org/TR/webvtt1/#cue-computed-position-alignment) would be `line-left`. That makes the cue box's size become `50` according to the spec [7.2.2](https://www.w3.org/TR/webvtt1/#ref-for-cue-computed-position-2). 

As a result, the cue box's left side would be aligned to the center of the video rendering area. However, I think the correct behavior is to make the cue box's size to `100` and align the cue box's left side to the left side of video rendering area, like what we've done in [`align_start-ref.html`](http://w3c-test.org/webvtt/rendering/cues-with-video/processing-model/align_start-ref.html) or [`align_end-ref.html`](http://w3c-test.org/webvtt/rendering/cues-with-video/processing-model/align_end-ref.html).

I think the simple solution is that to check `computed position alignment` in the algorithm for `computed position`. If `computed position alignment` is `line-left`, then return `0`; if `computed position alignment` is `line-right`, then return `100`.


Please view or discuss this issue at https://github.com/w3c/webvtt/issues/458 using your GitHub account

Received on Saturday, 15 June 2019 00:55:22 UTC