[whatwg] HTML5 video: frame accuracy / SMPTE

[David Singer]

OK, but it does seem kinda a tautology if you say "I want to use a time-expression that represents fractions of seconds as frame numbers, and it's not very accurate if there aren't very many frames/second..." !

On Jan 11, 2011, at 23:40 , Rob Coenen wrote:

> Hi David- that is b/c in an ideal world I'd want to seek to a time expressed as a SMPTE timecode (think web apps that let users step x frames back, seek y frames forward etc.). In order to convert SMPTE to the floating point value for video.seekTime I need to know the frame rate.
> 
> -Rob
> 
> On Tue, Jan 11, 2011 at 10:35 PM, David Singer <singer at apple.com> wrote:
> why does the frame rate make any difference on the accuracy of seeking to a time?  Imagine a video that runs at 1 frame every 10 seconds, and I seek to 25 seconds.  I would expect to see 5 seconds of the third frame, 10 seconds of the 4th, and so on.
> 
> On Jan 11, 2011, at 18:54 , Rob Coenen wrote:
> 
> > just a follow up question in relation to SMPTE / frame accurate playback: As
> > far as I can tell there is nothing specified in the HTML5 specs that will
> > allow us to determine the actual frame rate (FPS) of a movie? In order to do
> > proper time-code calculations it's essential to know both the video.duration
> > and video.fps - and all I can find in the specs is video.duration, nothing
> > in video.fps
> >
> > -Rob
> >
> >
> > On Mon, Jan 10, 2011 at 9:32 PM, Kevin Marks <kevinmarks at gmail.com> wrote:
> >
> >> If you really want to test timecode, you need to get into SMPTE drop-frame
> >> timecode too (possibly the single most annoying standards decision of. all
> >> time was choosing 30000/1001 as the framerate of NTSC video)
> >>
> >> Eric, can you make BipBop movie for this? - Like the ones used in this
> >> demo:
> >>
> >>
> >> http://developer.apple.com/library/mac/#documentation/NetworkingInternet/Conceptual/StreamingMediaGuide/UsingHTTPLiveStreaming/UsingHTTPLiveStreaming.html
> >>
> >> http://devimages.apple.com/iphone/samples/bipbopgear3.html
> >>
> >>
> >> On Mon, Jan 10, 2011 at 11:18 AM, Rob Coenen <coenen.rob at gmail.com> wrote:
> >>
> >>> Thanks for the update.
> >>> I have been testing with WebKit nightly / 75294 on MacOSX 10.6.6 / 13"
> >>> Macbook Pro, Core Duo.
> >>>
> >>> Here's a test movie that I created a while back. Nevermind the video
> >>> quality- the burned-in timecodes are 100% correct, I have verified this by
> >>> exploring each single frame by hand.
> >>>
> >>>
> >>> http://www.massive-interactive.nl/html5_video/transcoded_03_30_TC_sec_ReviewTest.mp4
> >>>
> >>> Please let me know once you guys have downloaded the file, I like to
> >>> remove
> >>> it from my el-cheapo hosting account ASAP.
> >>>
> >>> thanks,
> >>>
> >>> Rob
> >>>
> >>>
> >>> On Mon, Jan 10, 2011 at 2:54 PM, Eric Carlson <eric.carlson at apple.com
> >>>> wrote:
> >>>
> >>>>
> >>>> On Jan 9, 2011, at 11:14 AM, Rob Coenen wrote:
> >>>>
> >>>> I have written a simple test using a H264 video with burned-in timecode
> >>>> (every frame is visually marked with the actual SMPTE timecode)
> >>>> Webkit is unable to seek to the correct timecode using 'currentTime',
> >>> it's
> >>>> always a whole bunch of frames off from the requested position. I reckon
> >>> it
> >>>> simply seeks to the nearest keyframe?
> >>>>
> >>>>  WebKit's HTMLMediaElement implementation uses different media engines
> >>> on
> >>>> different platforms (eg. QuickTime, QTKit, GStreamer, etc). Each media
> >>>> engine has somewhat different playback characteristics so it is
> >>> impossible
> >>>> to say what you are experiencing without more information. Please file a
> >>> bug
> >>>> report at https://bugs.webkit.org/ with your test page and video file,
> >>> and
> >>>> someone will look into it.
> >>>>
> >>>> eric
> >>>>
> >>>>
> >>>
> >>
> >>
> 
> David Singer
> Multimedia and Software Standards, Apple Inc.
> 
> 

David Singer
Multimedia and Software Standards, Apple Inc.