[Tig] HDSR RGB vs YUV

glenn chan glennchan at gmail.com
Sun Dec 14 22:49:56 GMT 2008


There is absolutely a visible difference between 4:4:4 and (the various
flavours of) 4:2:2, though most of the time you won't run into the
situations where you can see a difference.  You see the difference when
dealing with highly-saturated colors.  Because the real world tends to have
very few highly-saturated colors, we usually do not run into problems.  But
there are times when graphic artists like to put red text on a black
background, or the set may contain red LEDs, lasers, lights, etc.  In these
situations, the use of chroma subsampling causes visibly noticeable
artifacts as a red object on a black background will have a slightly blurry
edge.
More information here:
http://www.glennchan.info/articles/technical/chroma/chroma1.htm
The short gist of it is that chroma subsampling can cause major differences
in the luminance of the displayed pixels, for two different reasons.

*These effects may be difficult to spot on a CRT.  This is partly because
the CRT's 'pixels' are 'drawn' with a blurry spot, so its pixel equivalent
has blurry edges (unlike a LCD, which has well defined edges for its
pixels).  And I believe it is partly due to the way human vision works- we
don't notice it when color is blurry (except where there is no luminance
transition).

At very high bitrates, chroma subsampling is a terrible method for reducing
bandwidth.  It is better to go with higher DCT compression (or wavelet, etc.
etc.).  Chroma subsampling (done well) also suffers from generation loss if
the image needs to be converted to 4:4:4 for signal processing and back.

2- In practice, I don't think that these issues are a big deal.  The show
Lazytown (a greenscreen / virtual set show) no longer does its keying from
4:4:4 DPXes, but switched over to Avid's DNxHD codec (4:2:2 with DCT
compression on top of that) for workflow reasons (this is according to the
Avid website).  While I do not believe that their compression is the most
efficient codec around, it is certainly good enough for practical purposes
and can definitely be useful.  Not all compression is not evil.


Glenn Chan
www.colormancer.com
Toronto, Canada



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