[PD] Colour in Digital Video

Mathieu Bouchard matju at artengine.ca
Fri Dec 21 08:57:25 CET 2007

On Wed, 12 Dec 2007, Andrew Brouse wrote:

> To further complicate things, our response to brightness and colour is not
> linear and so those perceptual curves have to be taken into account during
> that mapping into video signals (chroma and luma).

Oh and also all those "absolutely perceptual" systems are just discarding 
the genetic diversity of the universe (as well as the non-genetic 
diversity!). One man's metamers is another one's yuck. ;)

> Most video digitisation systems also use 'chroma sub-sampling'

Yes, we know about it, but I don't think that it's so relevant to colours 
unless using the YUV image type in GEM or PDP. (In GridFlow, YUV images 
are not subsampled, and also, Claude is so far only talking about 
individual colours, as float triplets)

> This uses that fact that we are more sensitive to differences in 
> brightness than variations in colour.

But this is not counting the fact that we could be standing close enough 
to the screen to actually see that much chroma resolution. No explanation 
of subsampling ever talks about that.

> The YCbCr (corresponds the YUV colour space of our perception) video 
> representation uses this technique which makes it more compact while 
> giving approximately equal quality to an larger RGB representation.

In order to say that, you have to define quality to correspond to some 
idea of how one should look at a screen in order to justify the 
subsampling. But people don't watch a screen from the same distance.

> Anyone working with colour video systems should know and read Charles 
> Poynton's writing on this subject - he is the acknowledged expert. If 
> you only have one book on your shelf it should be his "Digital Video and 
> HDTV".

Hey. Colours are nice, but art in general is a lot more important. If I'd 
have only one book on my shelf it should be something else than a treatise 
of colorimetry.

> [you can also be amazed, amused or angered by the way he vacillates
> between 'color' and 'colour' in his writing...]

None of the above...

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| Mathieu Bouchard - tél:+1.514.383.3801, Montréal QC Canada

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