[Tig] LUTs for Video File Codecs - carrots

Bob Friesenhahn bfriesen
Wed May 3 16:00:03 BST 2006

Thanks to oktobor for supporting the TIG
> This is something of a long established fallacy - that DNR assists in lossy 
> compression schemes. It should always be borne in mind that sophisticated 
> lossy CODECs rely upon psychovisual effects to mask their losses, including 
> picture noise. If you spend time removing normal levels of noise from your 
> programme prior to compression, the most likely result will be to make your 
> CODECs compression artifacts MORE visible.

Quite true.  In order to help, noise reduction can not rely on 
filtering at the per-frame level.  It needs to consider multiple 
frames, and in fact it may be useful to add more codec-friendly 
"noise" in order to avoid the appearance of banding, flashing, 
blocking, and other artifacts.

I often observe banding on my HDTV.  At first I thought that it must 
be the TV's fault, but when I see high-grade transfers without any 
banding it causes me to realize that the banding must be due to the 
program content, and is quite visible on a TV supporting a high 
contrast ratio.  Some of this banding may have been caused by 
simplistic per-frame filtering combined with the limitations of an 
8-bit path.

There was a very interesting presentation at the HPA Tech Retreat 
regarding an algorithm to add the appearance of film grain back into 
the picture.  The approach used was to characterise the existing film 
grain, use advanced filtering to remove it prior to compression, and 
then add the film grain back in on the decoding end.  Only a few bits 
are necessary to describe the grain to the decoder.  The end result is 
much better since the original image is better retained.  The 
unfortunate thing about this approach is that decoders would need to 
support the algorithm.

Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

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