[whatwg] Proposal for a MediaSource API that allows sending media data to a HTMLMediaElement

[Aaron Colwell]

Hi Mark,

comments inline...

On Thu, Aug 11, 2011 at 9:46 AM, Mark Watson <watsonm at netflix.com> wrote:

> I think it would be good if the API recognized the fact that the media data
> may becoming from several different original files/streams (e.g. different
> bitrates) as the player adapts to network or other conditions.
>

I agree. I intend to document this when I spec out the format of the byte
stream that is passed into this API. Initially I'm focusing on WebM which
requires this type of functionality if the Vorbis initialization data ever
needs to change during playback. My intuition says that Ogg & MP4 will
require similar solutions.


>
> The different files may have different initialization information (Info and
> Tracks in WebM, Movie Box in mp4 etc.), which could be provided either in
> the first append call for each stream or with a separate API call. But
> subsequently you need to know which initialization information is relevant
> for each appended block. An integer streamId in the append call would be
> sufficient - the absolute value has no meaning - it would just associate
> data from the same stream across calls.
>

Since I'm using WebM for the byte stream I don't need to add explicit
streamIds to the API or data. StreamIDs are already in the byte stream. Ogg
bitstream serial numbers, and MP4 track numbers should serve the same
purpose.


>
> The alternatives are:
> (a) to require that all streams have the same or compatible initialization
> information or
> (b) to pass the initialization information every time you change streams
>
> (a) has the disadvantage of constraining encoding, and making adding new
> streams more dependent on the details of how the existing streams were
> encoded/packaged
> (b) is ok, except that it is nice for the player to know "this data is from
> the same stream you were playing a while ago" - it can re-use some
> previously established state - rather than every stream change being 'out of
> the blue'.
>

I'm leaning toward (b) right now. Any time a change in stream parameters is
needed new INFO & TRACKS elements will be appended before the media data
from the new source. This is similar to how Ogg chaining works. I don't
think we need unique IDs for marking this state. The media engine can look
at the new codec config data and see if it matches anything it has seen
before. If so then it can simply reuse whatever resources it see fit.
Another thing to note is that just because we append this data every time a
stream switch occurs, it doesn't mean we have to transfer that data across
the network each time. JavaScript can cache this data and simply append it
when necessary.


>
> A separate comment is that practically we have found it very useful for the
> media player to know the maximum resolution, frame rate and codec
> level/profile that will be used, which may be different from the resolution
> and codec/level/profile of the first stream.
>
>
I agree that this info is useful, but it isn't clear to me that this API
needs to support that. Existing APIs like
canPlayType()<http://www.w3.org/TR/html5/video.html#dom-navigator-canplaytype>
could
be used to determine whether specific codec parameters are supported. Other
DOM APIs could be used to determine max screen size. This could all be used
to prune the candidate streams sent to the MediaSource API.


Aaron