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Author Topic: Perceptual colour profile conversion issue  (Read 20084 times)
Guillermo Luijk
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« on: April 27, 2008, 06:31:01 PM »
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I have done a colour profile conversion from ProPhoto to sRGB test comparing different methods in PS, basically Perceptual against the others. I thought the Perceptual colour profile conversion, at the cost of altering some of the colours found in the boundaries of sRGB close to the border with ProPhoto, would reallocate some ProPhoto tones close to those boundaries into sRGB, so colour relationships are maintained in an attempt to keep some colour gradation in the ProPhoto areas out of reach for sRGB.

But nothing of that seemed to happen. I converted a softly graduated patch in the R and G channels (B=128 in all of it, I took one of the central patches in Bruce Lindbloom test chart: Bruce Lindbloom -> Info -> An RGB Image Containing All Possible Colors), R varying from 0 to 255 in columns and G varying from 0 to 255 in rows, making use of all conversion models: Perceptual, Saturation, Rel Colorimetric and Abs Colorimetric, and all of them produced the same pixels going to 0 or 255 in the same channels (R or G).

The conversion was done in 16 bit and converted to 8 bit for output. Anyway I also did the conversion in 16 bit and analysed the 16-bit output image, and the same pixels went black/saturated so it is not a matter of integer rounding.

This is the patch assigned in ProPhoto and converted to sRGB:




These are the pixels where R and/or G channels went black (blue colour) or saturated (red colour) after the conversion:

[span style=\'font-size:14pt;line-height:100%\']R[/span]       .       [span style=\'font-size:14pt;line-height:100%\']G[/span]


And this is the intersection showing in gray colours those pixels that found a non-clippped correspondence in sRGB:




What am I missing? what is actually PS's Perceptual conversion doing?

Regards.
« Last Edit: April 27, 2008, 07:28:42 PM by GLuijk » Logged

Panopeeper
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« Reply #1 on: April 27, 2008, 06:42:25 PM »
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Perceptual in PS

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Gabor
Guillermo Luijk
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« Reply #2 on: April 27, 2008, 07:12:39 PM »
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A thread entirely worth reading, I missed that one thanks.
BTW what does exactly Shift-Ctrl-Y display? check gamut, in which sense? what's the meaning of the gray areas? (this gray changes when changing the work space)

BR
« Last Edit: April 27, 2008, 07:12:57 PM by GLuijk » Logged

walter.sk
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« Reply #3 on: April 27, 2008, 07:35:45 PM »
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Perceptual in PS

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[a href=\"index.php?act=findpost&pid=192197\"][{POST_SNAPBACK}][/a]
The Gamut warning (gray by default) is supposed to show you the colors in your image that are not in the gamut of the color space you are soft-proofing (based on the profile you tell the soft-proof dialog to use.

The reason for showing the O.O.G. colors is because there is little way of predicting what they will look like after you convert.  My education about that came from sending a Prophoto RGB file with some intense yellow-oranges to be printed on an HPZ3100 with Epson Enhanced Matte paper.  The few areas that were3 out of gamut came out reddish and gray in the print.   I learned quickly how to soft proof!
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Panopeeper
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« Reply #4 on: April 27, 2008, 07:38:20 PM »
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BTW what does exactly Shift-Ctrl-Y display? check gamut, in which sense? what's the meaning of the gray areas? (this gray changes when changing the work space)
See View, Proof Setup. The grey indicates that the color can not be expressed with that profile.
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Gabor
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« Reply #5 on: April 27, 2008, 08:01:04 PM »
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See View, Proof Setup. The grey indicates that the color can not be expressed with that profile.



Thanks, I configured it to provide gamut warning over sRGB and produced the same shape as I previously got. Again the correctly converted area doesn't change when changing method.
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Jonathan Wienke
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« Reply #6 on: April 27, 2008, 08:06:55 PM »
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Again the correctly converted area doesn't change when changing method.

Because the only valid options when converting from one matrix profile to another (such as sRGB, ProPhoto, etc.) are Relative Colorimetric and Absolute Colorimetric. Any other profile conversion setting is ignored.
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jbrembat
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« Reply #7 on: April 28, 2008, 02:45:40 AM »
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GLuijk,
 as Jonhathan said, matrix profiles do not have perceptual rendering. When you ask for perceptual, you get relative, and colors outside the destination gamut are clipped.
Pay attention to gamut warning there are many lut profiles that use hybrid intent. Relative well inside the destination gamut and perceptual near the boundaries. So gamut warning can't yield true values near the gamut boundaries.

Jacopo
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Guillermo Luijk
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« Reply #8 on: April 28, 2008, 03:45:20 AM »
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Ok thanks all, I understand now what soft proofing is.

Going back to the profile conversion itself, I have not yet had time to read carefully the insteresting thread referenced by Panopeeper, but had a rough look at it. Can we conclude then that Perceptual conversion in Photoshop is a bit fake when using matrix defined profiles and is far from the idea to keep textures and gradation even if the destination colour profile is smaller than the original? in such a way as:




And why cannot PS set some interpolated correction when dealing with matrix defined profiles to 'imitate' the desired Perceptual behaviour?
« Last Edit: April 28, 2008, 09:00:52 AM by GLuijk » Logged

madmanchan
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« Reply #9 on: April 28, 2008, 03:16:43 PM »
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When matrix-based profiles are used in PS, PS always does Relative Colorimetric during conversion (even if Perceptual is selected). Thus when going from ProPhoto to sRGB, the white points are mapped (chromatic adaptation) and then any points that lie outside the sRGB gamut are clipped (these would be negative values in sRGB space or values that exceed 1.0 (in normalized floating-point coordinates)).

Perceptual intent makes no sense for matrix-based profiles.
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Guillermo Luijk
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« Reply #10 on: April 28, 2008, 05:07:25 PM »
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When matrix-based profiles are used in PS, PS always does Relative Colorimetric during conversion (even if Perceptual is selected). Thus when going from ProPhoto to sRGB, the white points are mapped (chromatic adaptation) and then any points that lie outside the sRGB gamut are clipped (these would be negative values in sRGB space or values that exceed 1.0 (in normalized floating-point coordinates)).

Perceptual intent makes no sense for matrix-based profiles.
[a href=\"index.php?act=findpost&pid=192350\"][{POST_SNAPBACK}][/a]

Right, but the story changes when using the Microsoft ICM engine instead of Adobe ACM for the conversion. I have repeated the tests with the MS algorithms and:
- Perceptual, Saturation and Relative colorimetric yield almost identical results, and they prevent more pixels from clipping after the conversion than the Adobe engine.
- And now Absolute colorimetric works different: it prevents more pixels to go 0 than the former three, but it saturates more pixels even than the Adobe engine:

This is the result of the same conversion as before, I took the R channel:

Per/Sat/Rel vs Abs
 .  


To see the differences compared to Adobe ACM engine, I overlaped in layers and difference (or substraction, no idea how is in the English PS) blending mode the result of the MS conversions and the Adobe ACM:

Per/Sat/Rel(MS)-Adobe vs Abs(MS)-Adobe
 .  
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digitaldog
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« Reply #11 on: April 28, 2008, 05:59:48 PM »
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Right, but the story changes when using the Microsoft ICM engine instead of Adobe ACM for the conversion.

Sure it does, but you're still NOT getting any perceptual mapping. Its impossible. There's no table to do so.
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Andrew Rodney
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Guillermo Luijk
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« Reply #12 on: April 28, 2008, 07:13:32 PM »
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Sure it does, but you're still NOT getting any perceptual mapping. Its impossible. There's no table to do so.

how are non-matrix profiles codified? they are made of 3 curves? they are made of some 3D mapping? and why is possible to implement perceptual mapping on them and not through a mathematically defined matrix?

Sorry for such basic questions but I am very new to these matters.

BR
« Last Edit: April 29, 2008, 07:55:33 AM by GLuijk » Logged

madmanchan
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« Reply #13 on: April 28, 2008, 09:11:05 PM »
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Hi,

A perceptual transform in an ICC profile is usually a multidimensional (often 3D) lookup table. In general, however, it can be a concatenation of several individual transforms (e.g., a matrix multiplication, followed by a set of 1D linear curves, followed by a multidimensional lookup table, followed by another set of 1D linear curves).

When you consider the perceptual intent as implemented, say, in a standard ICC printer profile, it's basically all done with the lookup table.
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bjanes
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« Reply #14 on: April 29, 2008, 06:09:39 AM »
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Sure it does, but you're still NOT getting any perceptual mapping. Its impossible. There's no table to do so.
[{POST_SNAPBACK}][/a]

Impossible is a bit of an overstatement. Currently in Photoshop with the Adobe CMM perceptual rendering when converting matrix profiles may not be possible since there are no lookup tables, but that does not mean that a smarter CMS could not do the job.

Profiling a device such as a printer requires quantifying the characteristics of the device. Since printers are not well behaved devices, it is difficult to characterize them mathematically and lookup tables are a convenient brute force method of characterizing them.

Matrix spaces are mathematically defined and are fully characterized by their parameters. A smart CMS should have no trouble in converting from ProPhotoRGB to sRGB with perceptual rendering. The required gamut compression could be performed mathematically rather than derived from a lookup table.

[a href=\"http://www.colorwiki.com/index.php?title=Vistas_New_Color_Management_System_-_WCS&printable=yes]Steve Upton [/url] discusses smart CMSs in a white paper. He describes the current situation as "smart profiles, dumb CMSs". Dumb CMSs do not even look at the colors actually present in the image and perform a fixed amount of compression when doing perceptual rendering even if the image contains no out of gamut colors in the source space. I do not know if the Microsoft WCS can perform an intelligent perceptual rendering from ProPhotoRGB to sRGB without lookup tables. However, the task should be possible with a sufficiently smart CMS.

Another way to perform the perceptual rendering would be for some enterprising color expert to make a set of lookup tables for the above purpose.

Some readers might find this thread on Photo.net interesting. It discusses some of the above, but reaches no conclusion.

Bill
« Last Edit: April 29, 2008, 06:12:18 AM by bjanes » Logged
digitaldog
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« Reply #15 on: April 29, 2008, 07:51:14 AM »
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Impossible is a bit of an overstatement. Currently in Photoshop with the Adobe CMM perceptual rendering when converting matrix profiles may not be possible since there are no lookup tables, but that does not mean that a smarter CMS could not do the job.

Yes and perhaps with a smatter CMM, pigs could fly too.

If you feel better if I say "with the current CMM, its not possible", fine. I'd hate to mince words with you lurking around....
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Andrew Rodney
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Guillermo Luijk
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« Reply #16 on: April 29, 2008, 07:54:48 AM »
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calm down men.
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jbrembat
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« Reply #17 on: April 29, 2008, 10:42:24 AM »
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On ICC web site there is a V4 sRGB profile (beta version).

Jacopo
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digitaldog
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« Reply #18 on: April 29, 2008, 10:56:47 AM »
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On ICC web site there is a V4 sRGB profile (beta version).
[a href=\"index.php?act=findpost&pid=192497\"][{POST_SNAPBACK}][/a]

Yup, and I have access to other working space profiles that follow this too, providing now other options. No need for Uber flying pig CMM's. But today, with V2 profiles, you can have any rendering intent you wish from simple matrix profiles, as long as its Colorimetric.

Now what would be a useful question for Eric, if he can speak about this is, when we might see the one application that would move V4 profiles forward, Photoshop? If and when the more common working space profiles are installed as V4 profiles, only then will they take off in any meaningful way.
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Andrew Rodney
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Peter_DL
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« Reply #19 on: April 29, 2008, 06:27:54 PM »
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calm down men.
[{POST_SNAPBACK}][/a]
This subject was heavily discussed back in 2004/06 when ProPhoto RGB became popular. Just went over a couple of saved threads, and there are essentially two questions which seem to remain unsolved:

Would we like “Perceptual“ at all, if available say for ProPhoto RGB to sRGB conversion ?

/> There was Oscar Rysdyk’s proposal for a CMM based color clipping algorithm rather than channel clipping:
[a href=\"http://www.theimagingfactory.com/excalibur/clipping.htm]http://www.theimagingfactory.com/excalibur/clipping.htm[/url]

/> There was ICC3D from a graduate of the Norwegian Color Research Laboratory:
http://www.colorlab.no/icc3d

/> Not to forget PhotoGamut RGB which is a Lut-type RGB working space:
http://www.photogamut.org/E_ICC_profile.html

/> Fwiw: here’s another proposal on this (post #24):
http://luminous-landscape.com/forum/index....pic=10883&st=23

and in addition there are many different ways for manually operated editing and a selective reduction of color saturation towards & inwards a tiny target space such as sRGB.  All in all I’m staying unconvinced.


Another fundamental question is about the relevance with regard to real-world images. How to quantify the degree of image degradation due to RelCol conversion ?  Often enough, image details seem to bear sufficient differences on the lightness channel to survive a reduction of chroma contrast as introduced by the RelCol intent. For sure, the amount of channel clipping as indicated by the RGB histogram is quite meaningless.


That said, here’s interesting quote about the capabilities of color engines:

“Devices such as digital cameras and printers perform embedded (typically proprietary) perceptual renderings to and from standard color encodings like sRGB. … Finally, a color management system (CMS) may offer color rendering or re-rendering capabilities beyond that built into any source and destination profiles.”
http://www.color.org/ICC_white_paper_2_Per...g_use_cases.pdf


Peter

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