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RE: Transcoding (was: Action 53 - Check whether ffmpeg can be used from clipping and cropping)

From: Davy Van Deursen <Davy.VanDeursen@ugent.be>
Date: Thu, 26 Mar 2009 08:10:24 +0100
To: "'Jack Jansen'" <Jack.Jansen@cwi.nl>
Cc: "'Media Fragment'" <public-media-fragment@w3.org>
Message-ID: <D3A7A47A9ECD47EB80F0B9B950B3C284@elis.UGent.be>

>From: public-media-fragment-request@w3.org
[mailto:public-media-fragment->request@w3.org] On Behalf Of Jack Jansen
>Sent: woensdag 25 maart 2009 21:42
>To: Davy Van Deursen
>Cc: 'Media Fragment'
>Subject: Transcoding (was: Action 53 - Check whether ffmpeg can be used
>from clipping and cropping)
>
>
>>On  25-Mar-2009, at 17:14 , Davy Van Deursen wrote:
>>
>>
>>Hi Jack,
>>
>>very interesting! However, other issues/thoughts I can imagine:
>>- spatial cropping is obtained by transcoding the media files, but (lossy)
>>transcoding is not allowed to extract media fragments
>>- is there any transcoding involved in case of temporal cropping? In other
>>words, does ffmpeg look for the closest previous intra coded frame (i.e.,
>>no transcoding) or does it create an intra coded frame from the current
>>frame (i.e., transcoding)?
>
>Interesting... Apparently we have a different view the "no transcoding"
>statement.
>
>I had interpreted it as "we will not standardise any type of fragmenting
>that we think cannot be implemented without using transcoding".
>You seem to interpret it as "an implementation is not allowed to do any
>transcoding".
Interesting, I like your interpretation more than mine :-). But how do other
people interpret this one?

>
>I could go both ways on this. There's definitely advantages to disallowing
>transcoding, because it means we can guarantee lossless recombination.
>OTOH, using transcoding as a last-ditch approach, especially client-side,
>is convenient for end-users (or, probably, end-user-application
>programmers).
>
>As a thought experiment, let's assume a boundary case: a 1-hour video that
>has an I-frame at the beginning, an I-frame at the end and only B-frames in
>between.
>
>If I ask for a 10-second fragment in the middle, what would the original
>server send to a caching server? Always the full video?
>What would a media API deliver to the end-user application? The full video?
>The requested 10 seconds, with synthetic I-frames at both ends and original
>B-frames in between?
>What would an application like curl or wget store on the users' local disk?

I agree that in such boundary cases, it is likely to involve transcoding
techniques. Note that, in this context, spatial cropping of a video is a
boundary case for most of the coding formats ;-).

Best regards,

Davy

-- 
Davy Van Deursen

Ghent University - IBBT
Department of Electronics and Information Systems Multimedia Lab
URL: http://multimedialab.elis.ugent.be
Received on Thursday, 26 March 2009 07:12:35 GMT

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