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[whatwg] HTML5 video: frame accuracy / SMPTE

From: Kevin Marks <kevinmarks@gmail.com>
Date: Tue, 11 Jan 2011 15:46:36 -0800
Message-ID: <AANLkTimur4AgGvhhCFn3RtWYNuUaDHg5fsw6WJHyqsL0@mail.gmail.com>
These goals are orthogonal though - stepping between frames is valuable
whether they are regularly spaced or not.

Timecode is a representation that comes from the legacy video world that
does assume a uniform frame rate

On Tue, Jan 11, 2011 at 2:40 PM, Rob Coenen <coenen.rob at gmail.com> 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.
>>
>>
>
Received on Tuesday, 11 January 2011 15:46:36 UTC

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