Re: WebVTT Text Positioning

On Wed, Jun 26, 2013 at 10:11 AM, Silvia Pfeiffer
<silviapfeiffer1@gmail.com> wrote:
> On Wed, Jun 26, 2013 at 12:53 AM, David Ronca <dronca@netflix.com> wrote:
>>> Is the conversion spec not clear enough on this?
>>
>> Not really.  Trying to reconcile what you had previously said, "Line height
>> is set by CSS automatically based on font size".
>>
>> with c/s 1.4:
>>
>> The line height is defined as 5.33% of the video height, because CEA-608
>> requires 15 character rows to be rendered on 80% of the video rendering
>> area’s height.
>> The font size is defined from the line height by dividing it by 1.3; this is
>> to make sure there is enough space for the characters in a line.
>>
>>
>> and c/s 1.5:
>>
>> The actual height and width of the caption window depends, however, on the
>> actual font size.
>>
>>
>> So, according to 1.4, font size is line height /1.3, and line height is
>> defined as 5.33% of video.  This makes sense, and suggests that the renderer
>> will select the appropriate line height and font size based on video window
>> size.  However, your comment and 1.5 seem to suggest the inverse, that the
>> dimensions depend on font size.
>>
>> Is there is an obvious point that I am missing?
>
> So, the way I'd look at it is:
>
> 1. Line height is set to 0.0533vh by the WebVTT spec (hmm..  I think
> this should be 5.33vh actually, looking at the CSS spec)

Actually, I got this wrong (this only applies to the new region spec
that I just included into the spec).

Without regions, the line height is calculated by the browser from the
font size. By default, font is set to '5vh sans-serif'. Browsers make
individual decisions on what the best line height is for that (see
https://www.w3.org/Bugs/Public/show_bug.cgi?id=19273).



> 2. You override this with CSS for CEA-608 if necessary (in this case,
> it probably isn't because becase 5.33vh is exactly what you want)

The CEA-608 spec fixes the different line heights that browsers choose to be
  line-height: 5.33vh;


> 3. You can now also change the font size in the application (e.g. if
> older people want to increase the font size) - then the line height
> should be changed accordingly

If you change the font size, you should adapt the line-height setting
accordingly in your application.

HTH,
Silvia.

Received on Wednesday, 26 June 2013 14:23:09 UTC