W3C home > Mailing lists > Public > whatwg@whatwg.org > August 2008

[whatwg] Video : Slow motion, fast forward effects

From: Dave Singer <singer@apple.com>
Date: Thu, 7 Aug 2008 12:08:38 -0700
Message-ID: <p06240815c4c0f833337f@[10.0.1.8]>
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 
way).

>
>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
Apple/QuickTime
Received on Thursday, 7 August 2008 12:08:38 UTC

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