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Re: Range request and transcoding.

From: Yves Lafon <ylafon@w3.org>
Date: Wed, 1 Jul 2009 06:01:01 -0400 (EDT)
To: "Martin J. Dürst" <duerst@it.aoyama.ac.jp>
cc: www-tag@w3.org, public-media-fragment@w3.org
Message-ID: <alpine.DEB.1.10.0907010554390.12670@wnl.j3.bet>
On Wed, 1 Jul 2009, "Martin J. Dürst" wrote:

> Just some random comments:
>
> - Whether transcoding is needed or not will depend on whether the server has 
> prestored a certain piece of the big picture or not. Looks like it's purely 
> server-internal.
Unless the big pictures is made of tiles and served with a format using 
those tiles, what's on the server side is completely opaque. In the 
example I gave, I used jpeg to avoid the tiles aspect.

> - Content-Range seems to be hard-coded to "bytes", no other unit is usable. 
> (see RFC 2616, 14.16).

Julian already answered :)

> - For a jpeg image, because of the lossiness of the coding and the numeric 
> inaccuracy of the transforms used, it may not be possible to reconstruct the 
> bit-by-bit exact equivalent image from covering parts.

Yes, that's the point. Is the bit-by-bit exact reconstruction really 
mandatory/needed/desirable?
I can imagine something like
GET /earthmap.jpg HTTP/1.1
Host: www.example.com
->
HTTP/1.1 200 OK
Content-Type: image/jpeg
ETag: foo

GET /earthmap.jpg HTTP/1.1
Host: www.example.com
Range: pixel-crop xywh=5160,5120,32,24 (this is entirely made up)
->
HTTP/1.1 206 Partial Content
Content-Type: image/jpeg
ETag: W/foo
Content-Range: pixel-crop=5132,5100,64,64

Note the weak etag here to signal the possible loss of bit-by-bit 
equivalence during transcoding.


> On 2009/06/30 23:26, Yves Lafon wrote:
>> All,
>> The Media Fragment WG is working on... defining how to identify and
>> server media fragments, the primary use case is of course video, but it
>> is not limited to it [1]. There is a aso a clear relation to the TAG WD
>> "Usage Patterns For Client-Side URI parameters" [2], as hash parameters
>> are used to generate range requests to send back to the clients only the
>> desired sub-part of the resource representation.
>> 
>> There are different axis defined, the most obvious and easy to handle is
>> the time axis for video and audio, as in most cases the sub-part
>> extraction can be done in the compressed domain.
>> 
>> The geometrical axis (like asking for a clipped version of a picture or
>> a video) is the origin of this email, as the WG did not reach a
>> conclusion on "Is it legal to do transcoding when serving a fragment".
>> 
>> ie: the World Map is defined by http://www.example.com/earthmap.jpg and
>> is a 10000x10000 picture.
>> Small country M is defined by the following URI:
>> http://www.example.com/earthmap.jpg#xywh=pixel:5160,5120,32,24
>> 
>> Suppose that we have a way to construct a ranged request out of that,
>> serving only that 32x34 jpg file requires transcoding, while being
>> served as a partial response.
>> 
>> Input needed :)
>> Thanks,
>> 
>> [1] http://www.w3.org/TR/2009/WD-media-frags-reqs-20090430/
>> [2] http://www.w3.org/TR/2009/WD-hash-in-uri-20090415/
>> 
>
>

-- 
Baroula que barouleras, au tiéu toujou t'entourneras.

         ~~Yves
Received on Wednesday, 1 July 2009 10:01:12 GMT

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