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Author Topic: Color Gamut RGB Cube  (Read 33980 times)
crames
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« Reply #40 on: October 25, 2009, 01:37:10 PM »
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Quote from: JeremyLangford
Isn't brightness usually measured by the amount if white there mixed into the hue?

I think you're describing purity or chroma; the amount of white mixed with a spectral hue is a measure of purity.

Brightness is an absolute perception of a color's intensity.

Maybe a good book about color will help. "The Reproduction of Colour" by Hunt is an excellent resource, expensive but can probably be found in a local library.

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JeremyLangford
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« Reply #41 on: October 25, 2009, 02:45:34 PM »
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Im confused because this quote from handprint:

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A final observation is that the white point is not located on the luminosity function. This simply demonstrates that white is not the same as bright. The perception of white is a form of color sensation, whereas the perception of bright is a unique intensity sensation. The cone excitation space implies that a "bright" stimulus produces more than two times the cone excitation of a "white" surface, and therefore visual "white" always has a lower luminosity than visual "bright" under the same viewing conditions.

doesn't seem to match up with this HSV color model. In the HSV model, the value or brightness goes up when the color starts containing more white and less black. So I've always thought of color as a variety of hues that have a brightness determined by the shade of gray from black to white present in the color, and a saturation level determined by the amount of that shade of gray compared to the amount of the actual hue present in the color. Is this view wrong?

« Last Edit: October 25, 2009, 02:47:58 PM by JeremyLangford » Logged
crames
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« Reply #42 on: October 25, 2009, 08:30:51 PM »
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Quote from: JeremyLangford
Im confused because this quote from handprint:

doesn't seem to match up with this HSV color model. In the HSV model, the value or brightness goes up when the color starts containing more white and less black. So I've always thought of color as a variety of hues that have a brightness determined by the shade of gray from black to white present in the color, and a saturation level determined by the amount of that shade of gray compared to the amount of the actual hue present in the color. Is this view wrong?

You're mixing pieces of models from computer graphics with models of color perception.

The HSV model is a device-dependent RGB model from computer graphics that has little to do with perception-based color models like CIELAB, CIELUV, or color appearance models like CIECAM02. It's not clear to me what the "V" or value in HSV is supposed to correspond to: lightness, brightness, Munsell Value, or (probably) something else.

CIELAB in cylindrical coordinates, which is Lightness, Chroma, and hue, is similar to your HSV model. Note that the white-gray-black axis in CIELAB is Lightness, not Brightness. There is a difference between Lightness and Brightness, (and saturation and Chroma) and you should be careful to differentiate the two while delving into Handprint.

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JeremyLangford
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« Reply #43 on: November 04, 2009, 10:11:24 PM »
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Why are there extratraspectral hues mixed by Red and Violet between 620 nm and 445 nm?


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crames
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« Reply #44 on: November 05, 2009, 07:36:12 AM »
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Quote from: JeremyLangford
Why are there extratraspectral hues mixed by Red and Violet between 620 nm and 445 nm?


Those are the purple hues that we can see, but don't exist anywhere as a monochromatic color on the spectrum/spectral locus. Those hues can only be produced by mixing red and violet.
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JeremyLangford
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« Reply #45 on: November 05, 2009, 04:29:20 PM »
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Those are the purple hues that we can see, but don't exist anywhere as a monochromatic color on the spectrum/spectral locus. Those hues can only be produced by mixing red and violet.

So is the reason they're different than other mixed colors because they make up an edge that is past monochromatic colors in the cone excitation space? Or is this completely wrong?

I thought I was beginning to understand the spectral locus as the monochromatic wavelengths curving around in a 3d space but the extraspectral colors are confusing me because they look like they're outside the spectral locus.
« Last Edit: November 05, 2009, 06:49:02 PM by JeremyLangford » Logged
crames
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« Reply #46 on: November 05, 2009, 07:38:50 PM »
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So is the reason they're different than other mixed colors because they make up an edge that is past monochromatic colors in the cone excitation space? Or is this completely wrong?

I thought I was beginning to understand the spectral locus as the monochromatic wavelengths curving around in a 3d space but the extraspectral colors are confusing me because they look like they're outside the spectral locus.

Pretty much. Take magenta as an example of an extraspectral color. There is no single pure wavelength of light that looks magenta - there is no magenta on the spectral locus. You can only get that color by mixing at least two different wavelengths from opposite ends of the spectrum, such as red and blue. The extraspectral colors are different because you can't match them with a single pure spectral color (or pure spectral color plus an amount of white light), like you can with the "non-extraspectral" ones.
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JeremyLangford
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« Reply #47 on: November 05, 2009, 08:59:17 PM »
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Quote from: crames
Pretty much. Take magenta as an example of an extraspectral color. There is no single pure wavelength of light that looks magenta - there is no magenta on the spectral locus. You can only get that color by mixing at least two different wavelengths from opposite ends of the spectrum, such as red and blue. The extraspectral colors are different because you can't match them with a single pure spectral color (or pure spectral color plus an amount of white light), like you can with the "non-extraspectral" ones.

So all the other colors that are not monochromatic are mixtures of monochramitic colors and grayscale colors, except for the extraspectral colors?
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crames
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« Reply #48 on: November 06, 2009, 05:47:04 AM »
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Quote from: JeremyLangford
So all the other colors that are not monochromatic are mixtures of monochramitic colors and grayscale colors, except for the extraspectral colors?

Yes, that is the concept of "Dominant Wavelength" .
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JeremyLangford
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« Reply #49 on: November 17, 2009, 10:23:37 AM »
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So when you take extraspectral colors into consideration, this doesn't show every color we can see right?



But this does?

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crames
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« Reply #50 on: November 17, 2009, 10:04:40 PM »
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Quote from: JeremyLangford
So when you take extraspectral colors into consideration, this doesn't show every color we can see right?



But this does?


The first one - the spectrum - shows the visible frequencies of light and their approximate hues at each frequency. It doesn't show the hues like magenta that you can only get by mixing.

The second one - the spectrum locus - shows not only a similar spectrum along the curved edge, but also all the less saturated colors and mixtures that can be made. The straight line between 380 and 700 is called the purple line, along which you can see magenta and the other extraspectral hues.

The spectrum locus doesn't show "every color we can see" because it leaves out the lightness dimension. Actually, I think the spectrum locus doesn't define a gamut of what can be seen, but rather what can be "physically realized." In fact, there are colors outside the spectrum locus that could be seen if there were a way to stimulate only one or two of the LMS cone types without stimulating the others. It's possible to do it by using adaptation - stare at a bright magenta for a while to suppress the L and S cones, then look at a spectral green and it should look "super-saturated," or more saturated than a pure spectral green.
« Last Edit: November 17, 2009, 10:07:03 PM by crames » Logged

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JeremyLangford
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« Reply #51 on: November 18, 2009, 11:53:00 AM »
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Quote from: crames
The first one - the spectrum - shows the visible frequencies of light and their approximate hues at each frequency. It doesn't show the hues like magenta that you can only get by mixing.

The second one - the spectrum locus - shows not only a similar spectrum along the curved edge, but also all the less saturated colors and mixtures that can be made. The straight line between 380 and 700 is called the purple line, along which you can see magenta and the other extraspectral hues.

The spectrum locus doesn't show "every color we can see" because it leaves out the lightness dimension. Actually, I think the spectrum locus doesn't define a gamut of what can be seen, but rather what can be "physically realized." In fact, there are colors outside the spectrum locus that could be seen if there were a way to stimulate only one or two of the LMS cone types without stimulating the others. It's possible to do it by using adaptation - stare at a bright magenta for a while to suppress the L and S cones, then look at a spectral green and it should look "super-saturated," or more saturated than a pure spectral green.

Is Magenta kind of like white, in that white has no actual wavelength but is seen as a mixture of all wavelengths?
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crames
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« Reply #52 on: November 18, 2009, 05:18:21 PM »
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Quote from: JeremyLangford
Is Magenta kind of like white, in that white has no actual wavelength but is seen as a mixture of all wavelengths?

Yes, you can say that you need at least two wavelengths to make either magenta or white. You can make white from two complimentary dominant wavelengths, you don't need all wavelengths to make white. This is shown in the following diagram from page 179 of Wyszecki & Stiles:
[attachment=18022:chromaticitydiag.png]
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JeremyLangford
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« Reply #53 on: November 19, 2009, 08:26:07 PM »
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Does a glass prism or a rainbow show the extraspectral hues?
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crames
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« Reply #54 on: November 20, 2009, 06:01:25 AM »
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Quote from: JeremyLangford
Does a glass prism or a rainbow show the extraspectral hues?

No, prisms and rainbows both display spectrums, purple and magenta are non-spectral = extra-spectral hues.
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« Reply #55 on: November 21, 2009, 05:46:16 PM »
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This might be a really stupid question, but, can you take the spectrum locus and connect the wavelengths to make a cage around every single color possibility like this?


If (0,0,0) was black and all the colors made a white point somewhere, then wouldn't this be a good representation of all colors that can be seen by humans in a way that looks like a normal 3d color space like this?









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crames
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« Reply #56 on: November 22, 2009, 08:45:10 PM »
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Quote from: JeremyLangford
This might be a really stupid question, but, can you take the spectrum locus and connect the wavelengths to make a cage around every single color possibility like this?
...

If (0,0,0) was black and all the colors made a white point somewhere, then wouldn't this be a good representation of all colors that can be seen by humans in a way that looks like a normal 3d color space like this?

That should work.

The "cone excitation space" you are showing is not as intuitive as other color spaces, because it is far from perceptually uniform, and isn't shaped like "normal" color spaces.
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JeremyLangford
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« Reply #57 on: November 24, 2009, 10:28:11 AM »
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As Im still reading through handprint, I'm to the part where the "cone excitation space" is converted in the "chromaticity plane" by taking away luminance or dividing each of the three cone types by the sum of all three. I am trying to understand exactly what is happening here but I'm really confused. Are we turning a 3d graph into a 2d graph? How does this diagram show brightness?


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crames
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« Reply #58 on: November 29, 2009, 11:47:34 AM »
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Quote from: JeremyLangford
As Im still reading through handprint, I'm to the part where the "cone excitation space" is converted in the "chromaticity plane" by taking away luminance or dividing each of the three cone types by the sum of all three. I am trying to understand exactly what is happening here but I'm really confused. Are we turning a 3d graph into a 2d graph? How does this diagram show brightness?

Chromaticity shows the proportion that each component (L,M,S or X,Y,Z etc., depending on color space) contributes to the whole stimulus. You can think of it like a percentage.

Here's an example using XYZ. Let's say you're looking at someone's face, and the X,Y,Z of the skin in a certain light is 182, 38, 28. The total is 182+38+28 = 248. The chromaticity of the skin would be:

x = 182/248 = 0.7339 (or 73.4%)
y = 38/248 = 0.1532 (or 15.3%)
z = 28/248 = 0.1129 (or 11.3%)

Then a cloud passes across the sun and the light is cut in half. The X,Y,Z are also cut in half, to 91, 19, 14 respectively. The total now is 124, and the chromaticity is:
x = 91/124 = 0.7339
y = 19/124 = 0.1532
z = 14/124 = 0.1129

So the chromaticity doesn't change. It is independent of exposure. Chromaticity represents that part of the color that doesn't change with lightness/brightness: the combination of hue and saturation.

If you plot chromaticities x,y,z in the 3-dimensional X,Y,Z space, they all lie on a triangular plane. The standard CIE chromaticity diagram only plots the two dimensions x and y, as z is implied (since x + y + z = 1). Brightness or lightness is not shown on the chromaticity diagram.

I will add that, unlike a lot of the concepts in this thread, chromaticity is actually pretty relevant for photographers. When you change exposure in the camera, chromaticity does not change in the raw file. But when you adjust exposure in your image editor, chromaticity is not necessarily preserved. Some editing tools change the hue or saturation while you change lightness. Luminosity Blend Mode doesn't preserve chromaticity. Some color spaces don't preserve chromaticy, either, such as in CIELAB when editing the lightness channel. When chromaticity isn't preserved, you get those often unwanted changes in saturation and hue that make images look less realistic.
« Last Edit: November 29, 2009, 11:49:02 AM by crames » Logged

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JeremyLangford
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« Reply #59 on: December 01, 2009, 12:58:41 AM »
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Let me back up a little bit. I understand that this diagram represents the hues that our brain interprets from single wavelengths of light.



I also understand that Magenta is a mix of both ends of the spectrum. Where do black and white come into play with these other colors? Is white simply a mixture of complementary hues and/or all hues? Is white considered an extra-spectral color like Magenta or is it in a completely separate category? Is black simply what we see in space when light is absent and is white just the color of light that is sent to us from the sun?
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