Subject: [MMUSIC] Image attribute and sar parameter

From: "Ingemar Johansson S" <[email-masked]>
Date: Mon, 23 Mar 2009 19:38:08 +0100

Hi fellow MMUSIC'ers

As a follow up of the discussion on the image attribute draft during the MMUSIC session. After the session I became less certain what the problem with the sar parameter is, whether it is my comment that non-square pixels is no issue in certain standards fora or if I have missed some important facts. Please read the example below and feel free to comment.  

We setup a session between Alice and Bob, Alice is the offeror of the session.
The offer (from Alice) contains the image attribute below
 
======
a=imageattr:97 
  send [sar=[1.0-1.3],x=400:16:800],y=[320:16:640],par=[1.2-1.3]]
  recv [sar=1.1,x=800,y=600]
======

First we consider the recv direction:
The offeror (Alice) explicitly states that she wish to receive the screen resolution 800x600, however she also indicates that the screen does not use square pixels, the sar value=1.1 means that Bob must (preferrably) compensate for this 
So.. If Bob's video camera produces square pixels, and wish to satisty Alice's sar requirement, the image processing algorithm must rescale a 880x600 pixel image (880=800*1.1) to 800x600 pixels (could be done other ways).

... and now the send direction:
Alice indicates that she can (in the image processing algorithms) rescale the image for sample aspect ratios in the range 1.0 to 1.3. She can also provide with a number of different image sizes (in pixels) ranging from 400x320 to 800x640. Bob inspects the offered sar and image sizes and responds with the modified image attribute
======
a=imageattr:97 
  recv [sar=1.15,x=464,y=384]
  send [sar=1.1,x=800,y=600]
======
Alice will in order to satisfy Bob's request, need to rescale the image from her video camera from 534x384 (534=464*1.15) to 464x384.

That said, neither part is required to rescale like this (sar MAY be ignored), the consequence will of course be a distorted image.

Regards
Ingemar

******************************************* 
Ingemar Johansson 
Senior Research Engineer, IETF "nethead" 
EAB/TVP - Multimedia Technologies 
Ericsson Research Ericsson AB 
Box 920 S-971 28 Luleå, Sweden 
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