[whatwg] <track> / WebVTT issues

Silvia Pfeiffer silviapfeiffer1 at gmail.com
Thu Sep 29 02:18:05 PDT 2011

On Wed, Sep 21, 2011 at 7:15 PM, Philip Jägenstedt <philipj at opera.com> wrote:
> == Scrolling captions ==
> The WebVTT layout algorithm tries to not move cues around once they've been
> displayed and to never obscure other cues. This means that for cues that
> overlap in time, the rendering will often be out of order, with the earliest
> cue at the bottom. This is quite contrary to the (mainly US?) style of
> (live) scrolling captions, where cues are always in order and scroll to
> bring new captions into view. (I am not suggesting any specific change.)

I have a particular issue with this since text grows from the top down
and not from the bottom up. If you have ever seen live captions (even
in their recorded state), you will notice the scrolling feature (also
called "roll-up" captions), e.g.
http://www.youtube.com/watch?v=F_q-RRXw-vY . This example also shows
how captions may move location if they try to avoid other text on

Also note that YouTube is experimenting with richer captions, see
http://www.youtube.com/watch?v=0xTURXWoJ6A (check the different
caption tracks) . These are representing some of the features that the
US TV standard CEA608/708 captions support, so we need to make sure
they are also supported by browsers, otherwise we get a lower quality
result with captions on the Web that we get with captions on TV.

> == Scaling up and down ==
> Scaling the font size with the video will not be optimal for either small
> screens (text will be too small) or very large screens (text will be too
> big). Do we change the default rendering in some way, or do we let users
> override the font size? If users can override it, do we care that this may
> break the intended layout of the author?

We definitely need to give users the possibility to scale up the text
size. YouTube does that already (just press + or - on the video player
as you are watching with captions on). YouTube also allows users to
change the font color and the background color on captions (see the
caption settings menu) because some people may have different types of
color blindness to deal with.

YouTube deals with the increased front size by wrapping lines that go
outside the determined rendering box width. This also happens when we
get long caption texts, so isn't really anything new that we have to
deal with.

What happens with the new lines that are created by wrapping should,
however, be defined better than what we have right now. In other
existing caption formats, there is the concept of an "anchor". The box
into which the caption text is rendered is "anchored" to the video by
choosing a point inside the one-line caption cue box and a point on
the video viewport and anchoring that point. The box then grows around
that point in equal parts. For example, if the box is anchored at its
top middle point and assuming horizontally rendered text, the box will
grow down from that point. If it's anchored at the bottom middle
point, the box will grow up (even if the text is wrapped down and
grows down - i.e. the first line will be moved up before the second
line is rendered).


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