[PD] Colour in Digital Video

vade doktorp at mac.com
Fri Dec 21 20:27:34 CET 2007

It makes much more sense to see it broken down like so :


or http://adamwilt.com/pix-sampling.html and http://adamwilt.com/DV-FAQ-tech.html#colorSampling

These color-spaces use different sampling cadences, and as mentioned  
before leverage the fact that eyes are more sensitive to luma changes  
that chroma changes. Using different sampling cadences is essentially  
a form of compression ( less bandwidth required for a frame), typical  
Y cr'cb' or commonly known YUV video is the norm for broadcast  
masters, and use what is called 4:2:2 sampling cadence (the  
terminology is quite.... lacking), which means for every 4 luma  
samples (or every 4 pixels, you have 4 luma samples,) but only 2 color  
samples. Note that YUV does not have to have reduced sampling  
cadences, 4:4:4 (one luma, one cr' one cb' sampler per pixel) is  
perfectly valid, but not very common.

These reduced candences can significantly effect any attempts to do  
good chromakeying or color work, why after effects and other  
applications that are not strictly editors typically work at 4:4:4  
RGB. Anyway, Its definitely an interesting to know, and can  
significantly help make footage you are working with look better.

So while yes, it may be boring on the surface, if you plan on making  
any ART with video, and it wont be in black and white, its good to  
know about ;)

P.S., there was some discussion concerning the difference of human  
genetics not being accounted for. The older broadcast formats for NTSC/ 
PAL and SECAM are not ... final, per se. There are variants of all of  
these, and the YUV components can vary quite differently depending on  
what version of NTSC/PAL etc you want to use in what nation/geography.  
The 609 spec for SD was supposed to fix this (at least for NTSC), and  
the new HDTV rec 709 actually is a combination of the various most  
used components of PAL and NTSC, so conversion is simpler. So it used  
to be more diverse, and complicated, not its simpler but makes more  

Anyway, that last part was way off topic.

On Dec 21, 2007, at 11:57 AM, Mathieu Bouchard wrote:

> On Sat, 22 Dec 2007, hard off wrote:
>> hi andrew.  would love to see some patches that demonstrate what  
>> you're talking about, it's all a bit over my head.
> Try to zoom into a part of a GEM image that is in the YUV  
> colourspace or that formerly was (e.g. digitising a TV signal or  
> taking input from most webcams). If the image is as sharp as it can  
> be tuned to be, you will be able to see that the colouring of the  
> pixels is blurry compared to their intensity. The blur is either  
> horizontal or both horizontal and vertical. For example a pure red  
> diagonal bar over pure green background will usually show some  
> pixels that are a different shade of red or a different shade of  
> green. If it doesn't, it means that you picked two shades that have  
> the same brightness, or that the bar is positioned exactly on  
> colouring pixel boundaries. The different shades appear because the  
> colouring pixels are twice bigger (or more) than the brightness  
> pixels, and the boundary of the bar you are filming is being better  
> represented by brightness than by colour.
> About the non-linearity of vision... This is something else, and a  
> good start into that, is to look at gamma correction. Gamma  
> correction is actually correcting the monitor, which doesn't output  
> light proportional to its electric input, and has to be compensated.  
> I mention gamma because the gamma formula is both simple and non- 
> linear, so it's a good starting point about non-linearity, but it  
> doesn't actually address the non-linearity of vision. You could  
> perhaps look at the HSV colour objects that Claude was talking  
> about, and look at how the conversions are made (just some  
> floatboxes and one conversion object). It is a common example of non- 
> linear mapping from RGB, but it should also be noted that it's not  
> linear relative to vision, no matter how superficially it may look  
> like it's closer to one's understanding of colours. Look also at  
> when you crossfade two colours, even if you tune your gamma  
> correctly, how often the average of two colours doesn't feel like  
> it's halfway between the two colours.
> _ _ __ ___ _____ ________ _____________ _____________________ ...
> | Mathieu Bouchard - tél:+1.514.383.3801, Montréal QC  
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