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Old 04-17-2009, 07:05 AM   #11
redgum
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For those who need further explanation of StormDaves answer please read on, it's interesting and substantiates the use of green over blue...
Quote:
This is Color space before...well, color space (at least in video)

The problem occured when all there was in video was "Luma" or black and white (which we will call Y)

Now, you invent color TV process (well invent a way to add it to the signal), slow down the signal to 29.97 and well...you have a problem......

How do you keep broadcasting to the Black and white TVs?

By adding a Chroma Red (Cr) and a Chroma Blue (Cb) channel.
Where will we put the Green? 60% goes in the Y channlel, and 20% go in the U and V Channels respectfully. This gives us YCrCb (but easier to think of as YUV).

Beautiful! Think about how cool that solution is. Now we have one signal that has both BW and Color....and the color TVs just use the added color signal. Any color signal is ignored by BW tv.

Oops. The problem is our tape/broadcast formats can't handle all the samples. How can we lower it?

Well, the eye sees Black and White better than color.
Lets take 4 Luma Samples for every 2 Chroma samples

This gives us 4 Y (Luma + some green) samples for every 2 UV (Chroma) samples.

Samples? Sure.
NTSC has 720 horizontal samples (and 480 or 486 lines, depending on format).

We'll Sample the 720 on every pixel for luma (yielding 720 luma samples...don't forget the green is in there too.)
But for each color channel, we'll half that...c'mon, nobody will notice. So, we'll just take 360 samples (1/2 as many as the luma).

This yields the ratio for YUV of:
4:2:2 or
720 Luma (+60% green), 360 Chroma red (+ 20% green), 360 Chorma Blue (+20% green.)

Much of the color can be interpolated back...since we've only lost half.

AH, I hear what you're saying. You want a really cheap digital format. Right. Because if it's digital it can be smaller/cheaper/faster.

There's no way to get that data on that little tape. I bet we could compress it (a la JPEG) and throw away even more color. After all, nobody really complained when we threw away half. Let's do it again, so we only take 1/4 of the samples.

So, 4:1:1
720 Luma (+60% green), 180 Chroma red (+ 20% green), 180 Chorma Blue (+20% green.)

It gets interpolated back to 720...but it doesn't matter. Most of the color is gone.

And for green (or chroma) keying, since so much of the other channels is gone (and being interpolated)....realistically a so-so key in 4:2:2 space...is a ****** key in 4:1:1 space.
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Old 04-18-2009, 11:44 AM   #12
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yet strangely like the bumblebee blue gives me a better result than green.
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Old 04-19-2009, 04:27 AM   #13
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oh but if you get a chance to do one on the reflectmedia system,

it's the bomb. no fuss no muss
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Old 04-21-2009, 09:40 AM   #14
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so whats the verdict?? Blue or Green LOL

I guess by my brief scanning of the topic, it depends on the colorspace and codec.

so i someone can summarise it, it would be great
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Old 04-21-2009, 10:03 AM   #15
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Green, almost in all scenarios.
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Old 04-21-2009, 10:10 AM   #16
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Blue is best in my experience.
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Old 07-28-2009, 04:52 AM   #17
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I will give it a try soon!
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Old 07-28-2009, 07:56 AM   #18
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California = Green
London = Blue

Must be the sunlight, what should we use in Asia :)

p.s. Nice color space story btw...
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Old 07-29-2009, 12:48 AM   #19
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The bottom line is you can chromakey on just about any colour. Green and Blue are chosen primarily because there is less chance of those two colours appearing in the human body.
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Old 07-29-2009, 12:57 AM   #20
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Red pops up on occasion, but usually this is only for special cases or keying objects, not people.
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