> On Sep 1, 2015, at 10:47 , Yay295 <yay...@gmail.com> wrote: > > 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.
yes, but that is rare. There are indeed videos that don’t play well backward, or consume lots of memory and/or CPU, but most are fine. > > >> 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. There is no *generation* on the browser side; these tables are part of the file format. David Singer Manager, Software Standards, Apple Inc.
