It makes much more sense to see it broken down like so :
http://www.lafcpug.org/Tutorials/basic_chroma_sample.html
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
assumptions.
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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