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RE: Review: Use Case & Requirements Draft

From: Davy Van Deursen <davy.vandeursen@ugent.be>
Date: Thu, 4 Dec 2008 09:12:22 +0100
To: "'Media Fragment'" <public-media-fragment@w3.org>
Message-ID: <003d01c955e8$090bc630$1b235290$@vandeursen@ugent.be>

Hi all,

please find below our review for the Use cases and requirements document
(ACTION-14). It is based on the discussion regarding RaphaŽl's review of
this document.

>-----Original Message-----
>From: public-media-fragment-request@w3.org [mailto:public-media-
>fragment-request@w3.org] On Behalf Of Silvia Pfeiffer
>Sent: Saturday, November 08, 2008 4:35 AM
>To: RaphaŽl Troncy
>Cc: Media Fragment
>Subject: Re: Review: Use Case & Requirements Draft
>
[SNIP]
>> * Section 1.1:

>>  - (scenario 2): I'm not sure we want to say that only the region of
>an
>> image should be displayed. What about saying: "Tim wants the region of
>the
>> photo highlighted and the rest grey scaled"?
>
>If we get the rest of the image in grey scale, then we have to receive
>the rest of the data. This is not a fragment then. In this case, you
>would receive all the image and do some transformation on some parts.
>That is not what media fragments would be for IMO.

IMO, the UC 'Display of media fragments' is not equal to
'Delivery/Adaptation of media fragments'. In other words, media fragment
URIs could be used by the user agent to highlight (i.e., point out, display)
the media fragment within the parent media resource. Note that no
transformation is needed on server-side. The server sends the full media
resource, the user agent displays the full media resource and interprets
fragment URIs and 'highlights' them for example with a rectangular. Further,
annotations could be displayed next to the highlighted fragments.


>> * Section 1.4:
>>  - This use case is again for me an application of 1.1, that is this
>time
>> linking for recomposing (making playlist)
>
>Recompositing poses very different challenges to an application that
>just playback. I can see where you come from - for you it is just a
>piece of content delivered to a user agent and you don't really care
>what the user agent does with it. However, I am looking at use cases
>from the user's POV. A user would not regard watching a video as the
>same use case as editing clips together. I am just weary that we might
>oversee some things if we throw these use cases together too much.

Note that there is another issue with the recomposition of media fragments.
For instance, consider scenario 3 where video fragments (possibly all having
different parent media resources) are put together. Since different parent
resources implies potentially different underlying media formats, things
become complicated if we expect a smooth playback of a mashup of media
fragments. This is because different underlying media formats require the
(re-)initialization of the decoder(s) during playback of the mashup.
Therefore, I think we should make clear that in this scenario, we do not
expect smooth transitions between the different media fragments in the
playlist, because otherwise, this use case is far from trivial to implement.



>> * Section 1.6:
>>  - (scenario 1): I find it out-of-scope. I think it is worth to let it
>in
>> the document but to say that this is where we think it is out of scope
>...
>> if everybody agrees :-)
>
>I tend to agree. However, I would like to qualify why we think it's
>out of scope. I think what should be in scope with fragments are where
>we create what we called at the F2F "croppings" of the original file,
>i.e. where we take the original file, make a selection of bytes, and
>return these to the user agent. No transformations or recoding is done
>on the original data (apart from potentially changing a file header).
>This however means that as soon as we have a codec that is adaptable,
>i.e. where a lower framerate version or a smaller image resolution can
>be created without having to re-encode the content, we may have to
>consider these use cases as being part of media fragmentation.
>
>Maybe what we can agree on is that this is potential for future work
>as such codecs evolve and become more common. It is not our general
>use case right now though.
>
>What do ppl think?

We think that this is indeed out-of-scope. I always call these two kind of
adaptations 'structural' and 'semantic' adaptations. Structural adaptations
(such as frame rate scaling, resolution scaling, bit rate reduction) do not
change the semantics of the media resource. They only lower the quality of
the media resource along a particular axis (i.e., temporal, spatial, SNR,
colour, ...). Semantic adaptations do change the semantics of the media
resource by cropping along a particular axis (i.e., temporal, spatial,
track, ...), but they do not influence the quality of the resulting content.
IMO, only semantic adaptations result in 'fragments' of the original media
resource. Structural adaptations result in (lower quality) 'versions' of the
original media resource. Furthermore, I think structural adaptation
parameters do not belong in a URI scheme. These kind of adaptations are
typically taken on server-side or in network adaptation nodes based on the
usage environment of the user agent (e.g., network conditions, end-user
device characteristics, ...). 


>>  - (scenario 2): I find this scenario also really borderline / out-of-
>scope.
>>  As it has been pointed out during the face to face meeting in Cannes,
>the
>> interactivity seems to be the most important aspect in the map use
>cases
>> (reflecting by zooming in/out, panning over, etc.) and I guess we
>don't want
>> that in our URI scheme. Do we?
>
>I included this because we had a map use case. I'd be quite happy to
>decide this to be out of scope, but wanted to give the proponents of
>that use case a chance to speak up.

Only the panning is in-scope I think. We should keep a use case such that,
similar to the temporal (scen. 1) and track axis (scen. 3 & 4), we have a
use case regarding adapting a media resource to obtain a media fragment
along the spatial axis.

>> * Section 2:
>>
>> I have hard time to understand what do you mean with these technology
>> requirements. I understand the need for enabling other Web
>technologies to
>> satisfy their use cases but I'm not sure this is strong enough to make
>a
>> next headline. Actually, I can easily see all the subsections merged
>with
>> the existing use cases, see below. Therefore, I would suggest to
>remove the
>> section 2.
>
>So, section 2 looks at media fragments from a technology POV, not from
>a user POV. Yes, most of these technology rquirements are expressed in
>the use cases above. However, they are not expressed as such. It is a
>different dimension of description of the needs that we have.
>
>I have tried to write an introduction to this section. I believe it is
>important to explicitly spell out these dimensions to make people from
>these areas that have a large interest in media fragment uris
>understand explicitly that they are being catered for.

I tend to agree with RaphaŽl. I still cannot see why for example sect. 2.1
(which introduces named fragments) is not a part of sect. 1.5 (i.e.,
Annotating media fragments). What do other people think regarding Sect. 2? 

Further, the term 'side-conditions' does not reflect the covered
requirements because they are more than 'side'-requirements IMO. Therefore,
we propose to change the titles of sect. 1 and 3 in 'Functional requirements
(Application Use Cases)' and 'Non-functional requirements'. 

>> * Section 3.8:
>>  - I find this requirement very strong and I feel we are still
>discussing
>> the issue. Perhaps we can phrase that as: "we should avoid to decode
>and
>> recompress media resource" ?
>
>I'd like to have this discussion and come to a clear conclusion,
>because it will make things a lot more complicated if we allow
>recompression. Davy and I have discussed this thoroughly. Can ppl
>express their opinions on this? In fact, is anyone for allowing
>recompression (i.e. transcoding) in the media fragment URI addressing
>process?

I think we cannot make the statement that transcoding is not allowed to
obtain media fragments. Of course, it is preferable to avoid transcoding.
Therefore, I propose to remove sect. 3.8 and add to sect. 3.9 that one
aspect of minimizing the impact on existing infrastructure is to avoid
transcoding of media resources to obtain media fragments. Further on sect.
3.9, I agree that we should minimize the impact on existing infrastructure,
but this may not be an ultimate goal if this results in too much loss of
functionality (e.g., consider specialized video caches).

Best regards,

Erik & Davy

-- 
Davy Van Deursen

Ghent University - IBBT
Department of Electronics and Information Systems Multimedia Lab
URL: http://multimedialab.elis.ugent.be
Received on Thursday, 4 December 2008 08:13:13 GMT

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