[whatwg] Video : Slow motion, fast forward effects

Simon Fraser smfr at mac.com
Thu Aug 7 15:52:17 PDT 2008

On Aug 7, 2008, at 12:23 PM, Charles Iliya Krempeaux wrote:

> Hello,
> On Thu, Aug 7, 2008 at 12:11 PM, Jonas Sicking <jonas at sicking.cc>  
> wrote:
> Dave Singer wrote:
> 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).
> Honestly, this seems useless enough that the spec should just say  
> that when playback is less than 0 sound should be turned off. I'd  
> hate to see engineers working on this just because "the spec says it  
> should work that way".
> I don't think turning sound off is a good idea.
> This feature would be used to implement "scrubing".  Like what you  
> see in Non-Linear Editors... for making movies, etc.  (I.e.,  
> grabbing the "position handle" of the player, and moving it forwards  
> and backwards through the video, and varying speeds, to find what  
> you are looking for.)
> In those types of applications, the audio is on.  And it is  
> important for usability, for the video editor to hear the sound.

But those applications are not naïvely playing the sound at the  
current playback rate; they are grabbing little snippets of audio  
samples from the current playhead position, and playing them at normal  
rate. I don't think the spec should go into this much detail about  
what happens to audio when rate != 1.


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