attention color whizes: non-typical sRGB/RGB/ProPhoto question

[Iliah]

> Try doing it with 50% grey

It is a known fact, and I already said it 2 times. Assign does not preserve colour appearance. I do not see what to argue about. Tools are just tools, one needs to know how to use those properly and how to interpret results. By the way, XYZ is also an RGB space - sort of. So, yes - one can use assign here too, ignoring adaptation.

[Peter_DL]

..., but I'm pretty sure you need to account for chromatic adaptation when moving into the connecting space regardless of rendering intent.

Look, any plain 2D CIE xy plot will show us two different points for D65 white and D50 white.
Simply two different points with different CIE xy coordinates.
/> Adobe RGB per definition is D65.
/> CIE xy / XYZ / Lab is often referred as D50 but it is finally irrelevant here
/> ProPhoto RGB per definition is D50

When we map Adobe RGB's D65 white via CIE xy / XYZ / Lab to ProPhoto RGB there two different options:
1.) RelCol will move the D65 white onto D50 white. Chromatic adaptation is assumed and needed to do so.
2.) Whereas with AbsCol, Adobe’s D65 white should be kept right where it is on the D65 point. At least some CMMs still do so, and it seems to me intuitively correct according to the term "absolute colorimetric".  No chromatic adaptation desired or needed here.  Without chromatic adaptation, D65 white is a non-neutral color in the D50 target space (actually it is even clipped).

What 3D adds to party it to illustrate that in case 2.) i.e. without chromatic adaptation from D65 to D50 white
Adobe RGB’s blue primary is out of ProPhoto RGB.

Peter

--

[sandymc]

As I recall from a previous post by Thomas Knoll on one of the Adobe forums, XYZ does not have a white point. The original color matching experiments were done with emissive monochromatic sources that were roughly red, green, and blue projected on a white screen. The actual wavelengths were chosen for convenience. According to Wikipedia, " The primaries with wavelengths 546.1 nm and 435.8 nm were chosen because they are easily reproducible monochromatic lines of a mercury vapor discharge. The 700 nm wavelength, which in 1931 was difficult to reproduce as a monochromatic beam, was chosen because the eye's perception of color is rather unchanging at this wavelength." The subjects in the experiments matched a given color that was projected on by screen by adjusting the intensities of the three sources that were projected on the screen adjacent to the color to be analyzed. Negative values were need to match some colors as explained in the Wikipedia article. The actual XYZ values were derived mathematically. The subjects in the Wright and Guild experiments presumably were adapted to some color of light, but that is not stated in the article. In any event, the XYZ stimulus values can be related to actual wavelengths of light. The primaries for the various color spaces such as AdobeRGB and ProPhotoRGB can be expressed in an absolute manner in terms of the related CIE xyY values. In CIE L*a*b and the various RGB spaces one must include a white point in order to define a color.

D50 is usually used for this purpose. The articles talk about Illuminant E, which is not a spectrum of a black body radiator, but has equal XYZ tri-stimulus values.

XYZ is indeed an absolute measure of color. In other words, you can relate a XYZ color to the wavelength of a radiating body one-to-one. the problem is, our eyes don't see absolute color, they see relative to the general ambient surroundings. The best known aspect of this adaption is the well known "red shift" or scoptic effect. Which is why we have the whole color temperature adventure. So, a color patch radiating a particular XYZ combination will appear different in color depending on what our eyes are adapted to at the time.

Now I don't like Adobe's terminology here, but as best as I can translate it in non-mathematical form, what they are saying when they talk about XYZ D50 is that the output color rendering will show a D50 XYZ color as white. So the matrixes in the raw to XYZ computation are built such that the appropriate XYZ for any given color temperature will become white in RIMM space. Put in alternate form, what it says is that when translating into an output referred space, the white reference is D50.

Sandy

[jbrembat]

Tools are just tools, one needs to know how to use those properly and how to interpret results.

True, you don't understand how to use Lindbloom calculator and get a wrong result (Adobe1998 blue is outside of ProPhoto gamut).

By the way, XYZ is also an RGB space - sort of.

I don't agree. Do you know how you can define a RGB color space?

 

So, yes - one can use assign here too, ignoring adaptation.

You can do everythink you like, this doesn't mean that your conclusions make sense.

Jacopo

[MarkM]

With these considerations, what exactly is XYZ D50 or D65?

It's probably not quite correct to refer to the space as XYZ. It would be more precise to speak of PCS = profile connection space. It's essentially XYZ with a standard illuminant of D50 and defined by ICC.

[digitaldog]

The ACR mapping is camera to XYZ D50 referred

I was trying to go back even a bit farther in the evolution of the transform so to speak. When one says ACR mapping is camera to XYZ, what is “camera”? Is some assumption made, are known spectral sensitivities of the camera being used? If the later, are the “colors” imaginary to us excluded?

[fdisilvestro]

Disclaimer: I'm not a color scientist, just someone who wants to learn.

I went to Bruce Lindbloom page, and find this info interesting:

Conversion from XYZ to RGB:
The explanation of the algorithm starts with this assumption:

Given an XYZ color whose components are in the nominal range [0.0, 1.0] and whose reference white is the same as that of the RGB system, the conversion to RGB is as follows...

Then in the implementation notes:

If the input XYZ color is not relative to the same reference white as the RGB system, you must first apply a chromatic adaptation transform to the XYZ color to convert it from its own reference white to the reference white of the RGB system

Bruce provides a link to a chromatic adaptation algorithm to convert from  XYZ coordinates using a reference white to another set of XYZ coordinates corresponding to another reference white

So in the case of a RGB 0,0,255 in Adobe1998 RGB, D65, we get XYZ 0.1888186; 0.075274; 0.991109 as stated by previous posters

Now if I take those values to the chromatic adaptation calculator and put those values with D65 to obtain a XYZ referred to D50 I get
XYZ 0.149225; 0.063220; 0.744839 as shown in the attached screen capture

If I go back to the color calculator and input those values in the XYZ fields and set ProphotoRGB, D50, I get RGB 87.763290; 35.925802; 240.888952; (shown in the second attachment) which are the same results that you'll get if you leave the D65 reference white with the XYZ coordinates obtained from the blue primary in AdobeRGB, just as Jacopo said.

So, following Bruce instruction on his tools, you should perform a chromatic adaptation and the result is that the blue primary of AdobeRGB 1998 is inside ProphotoRGB

[joofa]

So, following Bruce instruction on his tools, you should perform a chromatic adaptation and the result is that the blue primary of AdobeRGB 1998 is inside ProphotoRGB

I believe if you do a chromatic adaption you are comparing the gamuts of Adobe RGB (D50) and Prophoto RGB (D50). The original intent was to compare Adobe RGB (D65) and Prophoto RGB (D50). This is the mistake MarkM, Digital Dog, and you are making.

There is no need of chromatic adaption here, because the Adobe RGB (D65) blue primary is not the blue primary of of Adobe RGB (D50), which you get after chromatic adaption transformation. The new blue primary of Adobe RGB (D50) can be contained within Prophoto RGB (D50), but not the original blue primary of Adobe RGB (D65).

Joofa

[ejmartin]

Why is there no need of chromatic adaptation?   It's like saying that you don't need to white balance.  Different illuminants yield different XYZ coordinates, and trying to compare color spaces under different illuminants is as pointless as trying to compare the same color space under different illuminants.  As someone mentioned earlier, one or the other of the white points of Adobe(D65) and Adobe(50), to use your terminology, is outside the gamut of the other.  But so what?  When does it ever have an impact on color managed workflow?

[joofa]

Why is there no need of chromatic adaptation?   It's like saying that you don't need to white balance.  Different illuminants yield different XYZ coordinates, and trying to compare color spaces under different illuminants is as pointless as trying to compare the same color space under different illuminants.  As someone mentioned earlier, one or the other of the white points of Adobe(D65) and Adobe(50), to use your terminology, is outside the gamut of the other.  But so what?  When does it ever have an impact on color managed workflow?

Because to do a representation of a color with tristimulus XYZ=[0.188185   0.075274   0.991108], which incidently happens to be the blue primary of Adobe RGB (D65), in Prophoto RGB (D50), you don't need chromatic adaption. Chromatic adaption is done when there is a question of "what would have been the XYZ values if I had shone D50 on my target surface instead of D65, while having D65 tristimulus values?" That is not at all an issue here.

Sincerely,

Joofa

[Iliah]

> you don't understand how to use Lindbloom calculator

You don't understand that I do. Read some books instead of spending time reading online.

But if you like on-line, "in the CIE XYZ color space, the tristimulus values are not the S, M, and L responses of the human eye, but rather a set of tristimulus values called X, Y, and Z, which are roughly red, green and blue, respectively (note that the X,Y,Z values are not physically observed red, green, blue colors. Rather, they may be thought of as 'derived' parameters from the red, green, blue colors)."

> and get a wrong result

No. The result is correct given certain pre-requisites.

[fdisilvestro]

I believe if you do a chromatic adaption you are comparing the gamuts of Adobe RGB (D50) and Prophoto RGB (D50). The original intent was to compare Adobe RGB (D65) and Prophoto RGB (D50). This is the mistake MarkM, Digital Dog, and you are making.

I may be making a mistake, I was just following the instructions on the tools in the Bruce Lindbloom site.

When does it ever have an impact on color managed workflow?

I'd like to know if I could lose some deep saturated visible blues while in ProphotoRGB that I could have achieved in AdobeRGB 1998

[MarkM]

This is the mistake MarkM, Digital Dog, and you are making.

Don't stop there Joofa. It's also the mistake Bruce Lindbloom is making in his implementation notes and the mistake ICC is making in their specification.

there is a question of "what would have been the XYZ values if I had shone D50 on my target surface instead of D65, while having D65 tristimulus values?"

This is wrong too, but in a kind of subtle way. Chromatic Adaptations are NOT correcting for differences in the XYZ numbers arising from different spectral power distributions reflecting off a surface. They are account for the change in retinal response that happens when our eyes adapt to color changes. If you want the gory details I'll refer you to Wyszecki and Stiles, 5.12 ( starting on page 429 in the 2nd edition). This is why you make the round trip trough cone response (ρ, γ, β).

[Iliah]

> It's also the mistake Bruce Lindbloom is making in his implementation notes and the mistake ICC is making in their specification.

The important thing to note is that XYZ as a PCS is often perceived differently from an absolute, device-independent XYZ.

> I'll refer you to Wyszecki and Stiles

Yes. Absolutely yes. But it is far from being enough.

[digitaldog]

Why is there no need of chromatic adaptation? 

Because it provides the results he wishes despite the fact that no one would use such a transformation, as so many here have pointed out.

Don't stop there Joofa. It's also the mistake Bruce Lindbloom is making in his implementation notes and the mistake ICC is making in their specification.

Exactly, but again, using the tools properly when they don’t produce the results you wish to express is moot here. It explains why so many simple gamut viewers tell us the correct answers (and why Joofa refused to even look at them). If you are provided buttons or options in a tool, use them incorrectly but they produce the results you wish, that’s all that counts apparently. I’m actually surprised so many of us are still trying to explain this to him.

[digitaldog]

> and get a wrong result

No. The result is correct given certain pre-requisites.

I would agree that the results based on the tool setting is correct (assuming Bruce has done all his math correctly which is pretty darn likely). But the results in this case are far from real world and are totally theoretical based on settings that make little sense.

[joofa]

Don't stop there Joofa. It's also the mistake Bruce Lindbloom is making in his implementation notes and the mistake ICC is making in their specification.

Please don't be melodramatic.

Joofa

[MarkM]

That's fine Joofa but it doesn't prove you point. Unless your point is that you can plot in 3D. I'm sure a lot of people here can do that. ( I can too, see: Blog post and and another)

CIE XYZ space only guarantees that colors with the same coordinates will match when viewed under the same conditions. When you plot various XYZ values in that space taken under different assumptions, like illuminant, you can't expect to be able to compare them. Plotting them on the same graph becomes meaningless. 

[joofa]

That's fine Joofa but it doesn't prove you point. Unless your point is that you can plot in 3D. I'm sure a lot of people here can do that. ( I can too, see: Blog post and and another)

Are we in some sort of plotting competition here?

CIE XYZ space only guarantees that colors with the same coordinates will match when viewed under the same conditions. When you plot various XYZ values in that space taken under different assumptions, like illuminant, you can't expect to be able to compare them. Plotting them on the same graph becomes meaningless. 

I mentioned before also and I repeat it again. XYZ space does not have a varying notion of white point, because you purposely restrict it. Other RGB spaces, which are schematically no different than RGB, are allowed to have varying white points.

Joofa

[joofa]

When you plot various XYZ values in that space taken under different assumptions, like illuminant, you can't expect to be able to compare them. Plotting them on the same graph becomes meaningless. 

Nope. Not at all.

Joofa