[whatwg] Video : Slow motion, fast forward effects

Dave Singer singer at apple.com
Thu Aug 7 12:08:38 PDT 2008

At 20:10  +1200 7/08/08, Chris Double wrote:
>On Thu, Aug 7, 2008 at 6:20 PM, Ian Hickson <ian at hixie.ch> wrote:
>>  On Thu, 7 Aug 2008, Biju Gm at il wrote:
>>>  On Thu, Aug 7, 2008 at 1:49 AM, Ian Hickson <ian at hixie.ch> wrote:
>>>  > playbackRate is the right way to do it, but maybe Firefox doesn't yet
>>>  > support it.
>>>  So can I assume HTML5 spec also allow playbackRate  to be negative value.
>>>  ie to support go backward at various speed....
>>  Yes.
>Would you expect the audio to be played backwards too?

I think that's extra credit and optional.  As you say, even with 
audio coded in independent frames you have to flip the samples, which 
is a pain.  For audio with forward dependencies, correct decoding 
means decoding forwards and then flipping whole chunks of timeline 
(though AAC doesn't suffer too badly if you don't do this, by the 

>Given that codecs are often highly optimized for forward playback the
>cost in memory can be excessive to go the other way. Could there be a
>possibility to say 'reverse playback not supported'?

Indeed, on I/P/B video, showing all the frames is also painful.

I think that the spec. should say that degraded playback (e.g. I 
frames only) or no playback (non-reversed audio) may occur...
David Singer

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