[opendtv] Re: (No Date: Tue, 28 Sep 2004 10:30:37 -0400

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Tue, 9 Nov 2004 09:49:41 -0400

At 12:38 PM -0500 11/8/04, John Shutt wrote:
>And I have argued that this is how MPEG 2 should compress an image.  DCT the
>edges first, lowering the high frequency information from the edges towards
>the middle as necessary to meet your target compression rate.
>Very similar to the old resolution specs of pickup tube cameras, where the
>highest resolution was specified in Zone one, a circle the diameter of the
>screen height, with lower resolution in Zone Two, a circle the diameter of
>the screen width, and the lowest resolution in Zone Three, the four corners
>not covered by Zones One and Two.

Three problems.

First, if we are talking about SDTV, every sample is critical. The 
typical display covers an area that is roughly equivalent of foveal 
vision. Thus there is no need for directed eye movement, as the 
entire display will have the same apparent sharpness, without the 
need to track moving objects.

Second, for HDTV - or any form of moving imagery on a big screen - 
you cannot control what the viewer eye-tracks. If they don;t look at 
what has encoded to be sharp, they will see compression artifacts. 
And critical info may very well come in from the edge of the screen.

Third, the DCT does not provide the tools to soften the samples 
within a block - all it can do is quantize away coefficients making 
them more alike. What you would see is blocking artifacts and 
quantization errors near the edge of the screen, not "soft focus." 
You COULD, however,  pre-filter the imagery to be softer near the 
edges, then compress it.

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