Understanding Color Sampling

Hi Barry,
I've always thought that the three numbers separated by colons (eg 4:2:2 ) stood for y Cb Cr (luma, channel blue, channel red) and
that channel green was calculated mathematically by the difference of the other two channels.
And going on my understanding, prores 4:4:4:4 would then be y, Cb, Cr, Cg.
edit: or more likely 4:4:4:4: is y, Cb, Cr, alpha
Is there any truth to that or am I laboring under a misconception?

Thanks

J Davis
writer / director
jdMAX.com
 
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J Davis said:
Hi Barry,
I've always thought that the three numbers separated by colons (eg 4:2:2 ) stood for y Cb Cr (luma, channel blue, channel red) and
that channel green was calculated mathematically by the difference of the other two channels.
And going on my understanding, prores 4:4:4:4 would then be y, Cb, Cr, Cg.
edit: or more likely 4:4:4:4: is y, Cb, Cr, alpha
Is there any truth to that or am I laboring under a misconception?
Click to expand...

Not quite... the whole YCbCr (component) concept is simply the method of storing RGB as a form of color differences - as you said. However, you can still have 4:4:4 YCbCr (I don't know a format that allows this, but it's theoretically possible). True, this discards some color information, as any color conversion does, but it still maintains the same resolution, assuming it was 4:4:4.

Color Subsampling refers to that ratio you're mentioning, and this is where we often see 4:2:0, 4:1:1, 3:1:1, 4:2:2, etc... It's usually used in conjunction with color difference formats, such as YCbCr or YUV (which are similar systems, though use different values for their RGB conversions if I'm not mistaken), hence the confusion here.

Of course, then we have bit depth, but that's another story!

Final Note: YCbCr is not the same as YPbPr, though in modern times, the terms are used interchangeably. Confusing, isn't it?
 
I just had confirmation from Barry (via PM) that the numbers do indeed refer to the Y (luma), Cb (blue), Cr (red) channels. I didn't hear back from about what the fourth digit in prores4444 represents. My bet is that it is alpha.

Hopefully he will chime in here with the correct info

edit:
I would also be interested in knowing if the nomenclature can only stand for y Cb Cr or whether it can also refer to YUV as written above by RX782
 
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J Davis said:
I just had confirmation from Barry (via PM) that the numbers do indeed refer to the Y (luma), Cb (blue), Cr (red) channels. I didn't hear back from about what the fourth digit in prores4444 represents. My bet is that it is alpha.

Hopefully he will chime in here with the correct info

edit:
I would also be interested in knowing if the nomenclature can only stand for y Cb Cr or whether it can also refer to YUV as written above by RX782
Click to expand...
On a strictly technical level, they are two different systems, but again, the only difference being the coefficients used to convert RGB to R/B difference signals; however, yes, the terms are often used interchangeably.
 
HPX 3000 AVC-Intra codec

HPX 3000 AVC-Intra codec

Ok-- so, in Barry's article -- which is great btw -- he mentions that DVCPRO-HD uses 4:2:2 color sampling, but Panasonic markets their HPX 3000 camera as providing 4:2:2 using AVC-Intra codec. Is this true, or should I be using DVCPRO HD for green screen shoots instead?
 
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AVC-Intra (at 100mbps) and DVCPRO-HD are both 4:2:2. However, AVC-Intra100 is better in pretty much all ways than DVCPRO-HD, so always use it whenever you can.

However, AVC-Intra50 is 4:2:0, so I would definitely use DVCPRO-HD instead of AVC-Intra50.
 
If XDCAM 422 were thrown into the comparison, how would you rank AVC-Intra50, XDCAM 422 and DVCPRO-HD in order of your preference?
 
Loaded question. XDCAM 422 is only available as a long-GoP codec on a disc, vs. solid-state intraframe codecs. If Sony would make 422 available on SxS, we'd be talking.

XDCAM 422's pretty good. I'd probably rank it between DVCPRO-HD and AVC-Intra100.
 
Barry_Green said:
Loaded question. XDCAM 422 is only available as a long-GoP codec on a disc, vs. solid-state intraframe codecs. If Sony would make 422 available on SxS, we'd be talking.

XDCAM 422's pretty good. I'd probably rank it between DVCPRO-HD and AVC-Intra100.
Click to expand...
I believe what Canon is calling "MPEG-2 HD @ 50 Mbit/s with 4:2:2" pretty much amounts to XDCAM 422, the latter is just shorter to write. Or maybe Canon decided not to license Sony's file structure and have only IMX files. Anyway, when I asked the question I meant this "new codec" from Canon as well.

So, you would put it below AVC-I 100. What about AVC-I 50?
 
Of course it's below AVCI-100, it's an 8-bit codec, and long-GoP at that. But I'm not all that thrilled with AVC-I 50. I don't think it looks as good as DVCPRO-HD. Of course, at half the bitrate, it doesn't really have to look as good in order to justify its existence, but no, AVC-I50 isn't as good as DVCPRO-HD and I wouldn't expect it to look as good as XDCAM 422 either.
 
Thanks Barry!

Last question -- and I'll be on my merry way – is it correct to assume that 4:4:4 sampling won't show a visible difference raw over 4:2:2, but only AFTER color correction, or have I completely missed the point?
 
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