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Author Topic: A Matter of two "ProPhoto" profiles . .  (Read 1528 times)
xpatUSA
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« on: May 26, 2013, 08:51:22 AM »
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For Sigma RAW file conversion, I use Sigma's Photo Pro (SPP) software. For digging into a less-than-perfect RAW file, I use Dave Coffin's DCraw with it's many command-line development options. While SPP embeds a bog-standard Kodak ROMM profile (D50 white point, 1.8 gamma) in its output files, DCraw embeds a so-called "ProPhoto D65" with a D65 white point and a default gamma of 1.92 !!

While purists might howl, am I right in thinking it makes no difference, provided that the image is viewed in a color-managed environment and that, of course, the color data is correct for the profile?

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best regards,

Ted
digitaldog
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« Reply #1 on: May 26, 2013, 12:11:42 PM »
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Visually there should be no differences. But if one converts, there should be a difference which you should test! Altering ProPhoto RGB in any way, and not calling it something different is to be frowned upon!
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Andrew Rodney
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xpatUSA
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« Reply #2 on: May 26, 2013, 02:48:36 PM »
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Visually there should be no differences. But if one converts, there should be a difference which you should test! Altering ProPhoto RGB in any way, and not calling it something different is to be frowned upon!

Thanks Andrew,

I didn't get " . . if one converts . .", sorry. I do have an image conversion utility, would that be how to test?

Have to agree about the "ProPhoto" which is not, hence the quote marks in the title. However, Mr Coffin seems a law unto himself and indeed nobody complains much. I read somewhere that his sparsely commented code starts with a transformation to sRGB and everything follows from there . .
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Ted
digitaldog
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« Reply #3 on: May 26, 2013, 03:50:13 PM »
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I didn't get " . . if one converts . .", sorry. I do have an image conversion utility, would that be how to test?

ProPhoto RGB version A to output color space versus ProPhoto RGB version B to output color space. If they have differing white point definitions, the resulting output should be different.

You don't need a utility to test the differences, only Photoshop:

http://digitaldog.net/files/Apply_Image.pdf

If you wanted actual deltaE differences, you'd need a 3rd party utility like ColorThink Pro:

http://digitaldog.net/files/How_to_compare_deltaE_of_ColorLists.pdf
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Andrew Rodney
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xpatUSA
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« Reply #4 on: May 26, 2013, 04:35:29 PM »
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Interesting. I'll mebbe give it a shot. I only have Elements 6, so no fancy functions available there, but I could make a couple of TIFF's one from each converter, SPP and DCraw and compare them in ColorThink V2. L*a*b* 3D gamut is quite good for playing "spot the white point"  Cheesy

Thanks again,
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Ted
xpatUSA
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« Reply #5 on: May 27, 2013, 11:57:00 AM »
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Interesting. I'll mebbe give it a shot. I only have Elements 6, so no fancy functions available there, but I could make a couple of TIFF's one from each converter, SPP and DCraw and compare them in ColorThink V2. L*a*b* 3D gamut is quite good for playing "spot the white point"  Cheesy

In the end it was easier to convert a TIFF from the "ProPhoto D65" to ProPhoto proper (D50). Opened in a color-managed application there was little difference, which confirms my thought in the OP:



Out of interest, the different white points and gammas are clearly shown below, in 3D Lab space:



Also a hue shift appeared, for which I have no explanation:



D65 is green, D50 is red.





« Last Edit: May 27, 2013, 03:35:01 PM by xpatUSA » Logged

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Ted
FranciscoDisilvestro
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« Reply #6 on: May 27, 2013, 11:55:24 PM »
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The effect of chromatic adaptation (E.G. going from D65 toD50 or viceversa) is like a "rotation" in 3D of the color space. It is not really a hue shift, because the reference white is different.

When comparing different color spaces it is better to chose a reference white and perform the adaptation to the other color spaces to that reference. If you don't do it, then you may derive wrong conclusions, such as "There are colors in Adobe RGB outside the gamut of ProPhotoRGB" (try it if you don't believe).

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xpatUSA
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« Reply #7 on: May 28, 2013, 12:56:57 AM »
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The effect of chromatic adaptation (E.G. going from D65 toD50 or viceversa) is like a "rotation" in 3D of the color space. It is not really a hue shift, because the reference white is different.

Interesting - I did not know that.

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When comparing different color spaces it is better to chose a reference white and perform the adaptation to the other color spaces to that reference.

Sorry, a choice of reference white was not possible because the RAW converter (DCraw) embeds a profile not under my control. And if if I edited the extracted profile to be D50 and its gamma to be 1.8 then that profile would not match the image data, if I am not mistaken.

Thanks,

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Ted
FranciscoDisilvestro
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« Reply #8 on: May 28, 2013, 02:23:10 AM »
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I'm not sure what is involved when you say "Edit" the profile. You cannot just change the white point leaving the same cromaticity values. If you do that you end up with a non-neutral color space (where R=G=B =/= neutral). You should perform chromatic adaptation to the cromaticities (such as Bradford) either to the color profile or the image to a common white point before comparing color spaces.

My understanding from DCRaw, is that it uses profiles adapted to D65, (and it is true that starts with a conversion to sRGB) so further conversions do not require "cromatic adaptation" (this seems to be a rather common approach, according to the quote from Bruce Lindbloom at the end of this message).

Also in DCRaw, the gamma is not really 1.92 but a TRC function corresponding to the standard B.T. 709, with an initial slope of 4.5 and then a gamma correction of 2.222, but you could specify your desired parameters with option -g power toe_slope. In any case, it seems that all calculations are performed in linear space and TRC is applied just before writing the output file.

A couple of years ago there was a rather long thread about color space conversions here in Lu-La, where I posted a response from Bruce Lindbloom (with his permission) from an equiry, which I think it is relevant for this discussion:

From Bruce Lindbloom:
Quote
The resolution of the issues boils down to agreement of the premises. If one takes the position that AdobeRGB blue lies outside the gamut of ProPhotoRGB, then one must also agree that the AdobeRGB grayscale (R=G=B) is not neutral. This is the case where chromatic adaptation is not used (absolute colorimetric). AdobeRGB and ProPhotoRGB whites are outside of each other’s gamuts.

I believe that all of the reference RGB color systems share the basic premise that R=G=B is neutral. Of course, neutral is relative to the color system’s reference white. When considering two RGB systems having different reference whites, the only way to reconcile the neutrality of the grayscales is to use chromatic adaptation (relative colorimetric). In most real-world applications and workflows, this is the only thing that makes practical sense. It is what ICC, Photoshop, Lightroom, etc. are based on. Although AdobeRGB is specified relative to D65, its ICC profile contains the primaries adapted from D65 to D50 using the Bradford chromatic adaptation method (use a profile inspector to examine the rXYZ, gXYZ and bXYZ tag contents). So any ICC-aware app that uses this profile is transforming colors in the D50 world, not D65. R=G=B is a D50 neutral, no matter what the native reference white of the color space.

If one agrees that R=G=B is neutral for both profiles, regardless of the reference whites, then chromatic adaptation must be used to bring them both to the same reference. In that context, AdobeRGB blue lies inside the gamut of ProPhotoRGB.

Regards,

Bruce
--
Bruce J. Lindbloom

Regards
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Schewe
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« Reply #9 on: May 28, 2013, 08:45:45 AM »
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A couple of years ago there was a rather long thread about color space conversions here in Lu-La, where I posted a response from Bruce Lindbloom (with his permission) from an equiry, which I think it is relevant for this discussion:

Oy vey...I remember that discussion...EVERYTHING you ever wanted to know about comparing ProPhoto RGB and Adobe RGB (but were smart enough not to ask :~).
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xpatUSA
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« Reply #10 on: May 28, 2013, 10:24:26 AM »
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I'm not sure what is involved when you say "Edit" the profile. You cannot just change the white point leaving the same cromaticity values. If you do that you end up with a non-neutral color space (where R=G=B =/= neutral). You should perform chromatic adaptation to the cromaticities (such as Bradford) either to the color profile or the image to a common white point before comparing color spaces.

Yes, I fully agree and may have misunderstood the earlier comment. The main point of the OP was that, even though the DCraw profile is not standard, images from an application would look the same provided it is color-managed. Thus a D65 "PhotoPro" should work, with that proviso. In creating the comparison image in my earlier post, I did use a profile converter, which I can only assume applies chromatic adaptation, set to relative colorimetric intent.

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My understanding from DCRaw, is that it uses profiles adapted to D65, (and it is true that starts with a conversion to sRGB) so further conversions do not require "cromatic adaptation" (this seems to be a rather common approach, according to the quote from Bruce Lindbloom at the end of this message).

That is also my understanding.

Quote
Also in DCRaw, the gamma is not really 1.92 but a TRC function corresponding to the standard B.T. 709, with an initial slope of 4.5 and then a gamma correction of 2.222, but you could specify your desired parameters with option -g power toe_slope. In any case, it seems that all calculations are performed in linear space and TRC is applied just before writing the output file.

That is indeed the default correction. Thank you for the -g clarification - I had thought it was peculiar to sRGB, we live and learn (slowly, at my age). I got the odd gamma number from ColorThink's Profile Inspector which reported rTRC as "Gamma-defined curve Value = 1.92" which, in my naivety, I took as correct.

Quote
A couple of years ago there was a rather long thread about color space conversions here in Lu-La, where I posted a response from Bruce Lindbloom (with his permission) from an equiry, which I think it is relevant for this discussion

Thanks for the link to the relevant topic, which I will read avidly, and I had Bruce's site already bookmarked and use his calculator a lot but many of his writings hurt my brain ;-)

In ColorThink V2, A comparison of Adobe 1998 vs, ProPhoto does indeed show Adobe sticking out of the 3D upper surface on the blue side but only because of the different white point. Not surprisingly, Dave Coffin's profile encompasses Adobe 1998 quite nicely!
« Last Edit: May 28, 2013, 10:48:30 AM by xpatUSA » Logged

best regards,

Ted
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