On Tue, Sep 1, 2015 at 11:30 AM, David Singer <sin...@apple.com> wrote:

> > On Sep 1, 2015, at 4:03 , Robert O'Callahan <rob...@ocallahan.org>
> wrote:
> >> On Tue, Sep 1, 2015 at 8:02 PM, Kevin Marks <kevinma...@gmail.com>
> wrote:
> >> QuickTime supports full variable speed playback and has done for well
> over
> >> a decade. With bidirectionally predicted frames you need a fair few
> buffers
> >> anyway, so generalising to full variable wait is easier than posters
> above
> >> claim - you need to work a GOP at a time, but memory buffering isn't the
> >> big issue these days.
> >
> > "GOP”?
> Group of Pictures.  Video-speak for the run between random access points.
> > How about a hard but realistic (IMHO) case: 4K video (4096 x 2160), 25
> fps,
> > keyframe every 10s. Storing all those frames takes 250 x 4096 x 2160 x 2
> > bytes = 4.32 GiB. Reading back those frames would kill performance so
> that
> > all has to stay in VRAM. I respectfully deny that in such a case, memory
> > buffering "isn't a big issue”.
> well, 10s is a pretty long random access interval.

There's no way to know the distance between keyframes though. The video
could technically have only one keyframe and still work as a video.

> >> What QuickTime got right was having a ToC approach to video so being
> able
> >> to seek rapidly was possible without thrashing , whereas the stream
> >> oriented approaches we are stuck with no wean knowing which bit of the
> file
> >> to read to get the previous GOP is the hard part.
> >
> > I don't understand. Can you explain this in more detail?
> The movie file structure (and hence MP4) has a table-of-contents approach
> to file structure; each frame has its timestamps, file location, size, and
> keyframe-nature stored in compact tables in the head of the file. This
> makes trick modes and so on easier; you’re not reading the actual video to
> seek for a keyframe, and so on.

I suppose the browser could generate this data the first time it reads
through the video. It would use a lot less memory. Though that sounds like
a problem for the browsers to solve, not the standard.

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