geometry and RGB? - AVS Forum
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post #1 of 9 Old 10-01-2005, 02:44 AM - Thread Starter
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This might seem like a silly question.

Given a color gamut and D65 reference, is RGB levels calculated as the geometric distance from the D65 point to each color's primary xy coordinate?

Given that distance, is this the reference value for RGB?

For example, if you look at wizards in colorfacts pro, you get 3 bars (R, G and B). Are the values of these bars derived from the distance of the current xy value to the gamut's primary?

Am I making any sense here?

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post #2 of 9 Old 10-01-2005, 10:07 PM
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oferlaor

See the new sticky from Bob Sorel - we covered the whole RGB% topic there ..... in depth....the short answer is RGB% depends on how & when it was calculated - but the only thing relevant is when the bars are aligned.
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post #3 of 9 Old 10-01-2005, 11:35 PM - Thread Starter
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I'm still not clear.

As I understand it, when you have a point on the CIE chart, the RGB absolute values are the geometric distance between the current point and each specific primary point (R, G or B) factored by the D65 position (if that's what we want our target to be).

Is this true or false?

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post #4 of 9 Old 10-02-2005, 12:03 AM
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False. The CIE chart is a simplification, and does not include any luminance (Y). RGB is (simplistically) a linear light representation of XYZ, and thus, must include luminance. Check Poynton's ColorFAQ for more info.
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post #5 of 9 Old 10-02-2005, 04:55 PM
 
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CIE is accurate and not a simplification, YCbCr is a way to encode video in a compressed method, it decodes into RGB eventually, so not sure what you mean there....?
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post #6 of 9 Old 10-02-2005, 07:36 PM
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Chris - There is no luminance in the CIE Cartesian coordinate system. Therefore, I would argue that it is a simplification of the concept of color. However, it was not designed to be a complete description of color from what I know (merely hue and saturation), so it's not a point I feel the need to argue. The point is that you cannot derive RGB from xy, but you can get xy out of RGB by converting through XYZ.

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Bill
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post #7 of 9 Old 10-02-2005, 11:44 PM - Thread Starter
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Ursa,

I don't mean absolute RGB (i.e., actual video data coming out of a plug).

I'm talking about relative RGB, the kind you use in colorfacts to determine which one is too high... that means I only need xy (Y stays out).

The calculation I'm talking about would like like:
B=distance from xy to blue primary factored by D65's distance to blue primary
R=distance from xy to red primary factored by D65's distance to red primary
G=distance from xy to green primary factored by D65's distance to green primary

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post #8 of 9 Old 10-03-2005, 01:03 AM
 
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Bill: I see now what you meant! :) And indeed, you are correct, CIE defines colors, not intensity or luminance.

As to Ofer's question, I would think that yes, that would be the case, but I've never seen that stated, or measured it. Why are you asking the question, is there something you're trying to do with this? I'm just thinking this through my head, and it seems that this would be the case, but it is 1 in the morning, so maybe I'll feel stupid tomorrow :).
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post #9 of 9 Old 10-03-2005, 01:05 AM
 
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Quote:
The point is that you cannot derive RGB from xy, but you can get xy out of RGB by converting through XYZ.
:D :D lol! This almost sounds like it could be a really bad pickup line. :)
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