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Author Topic: Perceptual colour profile conversion issue  (Read 20516 times)
madmanchan
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« Reply #20 on: April 29, 2008, 08:42:35 PM »
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Personally, I would rather leave the perceptual mapping defined by the profile rather than the CMM, because:

- it allows multiple perceptual mappings (each one stored in a separate profile) to be available, giving the user a choice as to which one to use for a particular image, and

- it allows all such perceptual mappings to be shared across CMMs

As an analogy, Bill Atkinson has several profiles available for the 9800, built from the same measurement data but using different software & software options. They can be used across CMMs. For a given image, the user can decide which profile produces the most pleasing perceptual rendering.

Andrew, sorry I don't any answer to you for your specific question. I am actually not that familiar directly with PS plans, since I don't interact with that group directly.
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digitaldog
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« Reply #21 on: April 29, 2008, 09:04:22 PM »
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Andrew, sorry I don't any answer to you for your specific question. I am actually not that familiar directly with PS plans, since I don't interact with that group directly.
[a href=\"index.php?act=findpost&pid=192581\"][{POST_SNAPBACK}][/a]

No worries. I think it might be some time before V4 Working space profiles are ready for prime time (or for Adobe to install them).
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Andrew Rodney
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Peter_DL
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« Reply #22 on: April 30, 2008, 04:04:37 PM »
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Personally, I would rather leave the perceptual mapping defined by the profile rather than the CMM, because:
- it allows multiple perceptual mappings (each one stored in a separate profile) to be available, giving the user a choice as to which one to use for a particular image, and
- it allows all such perceptual mappings to be shared across CMMs

As an analogy, Bill Atkinson has several profiles available for the 9800, built from the same measurement data but using different software & software options. They can be used across CMMs. For a given image, the user can decide which profile produces the most pleasing perceptual rendering.

Andrew, sorry I don't any answer to you for your specific question. I am actually not that familiar directly with PS plans, since I don't interact with that group directly.
[a href=\"index.php?act=findpost&pid=192581\"][{POST_SNAPBACK}][/a]
>> Peter, … I'm not really a believer in Perceptual rendering, at least in the ICC implementation. A perceptual rendering that handles all images identically seems guaranteed to fail on a majority of them, and that has pretty much been my experience. Maybe if we had a much smarter CMM that actually looked at the image rather than the space in which it resides, we could do adaptive perceptual renderings that actually worked, but I'm the last person on the planet to try to convince Tom Knoll that he needs to introduce systematic errors in ACR's colorimetry. He seems to feel pretty much the same way I do about perceptual rendering anyway.<<

Cited from a RG forum discussion, August 2005, by Bruce Faser
(sorry, the link doesn’t work anymore).

Peter

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madmanchan
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« Reply #23 on: April 30, 2008, 07:46:20 PM »
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At this point I think we're getting into the semantics. Either we would have (1) a bunch of profiles generated offline and then pick the one at runtime that we want to use for a given image, or (2) have a CMM that will generate image-dependent profiles on-the-fly, provided this could be done fast enough.

Image-dependent gamut-mapping has been covered in the research literature but hasn't found its way to market in a standardized form yet.
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Peter_DL
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« Reply #24 on: May 01, 2008, 12:15:25 AM »
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At this point I think we're getting into the semantics. Either we would have (1) a bunch of profiles generated offline and then pick the one at runtime that we want to use for a given image, or (2) have a CMM that will generate image-dependent profiles on-the-fly, provided this could be done fast enough.

Image-dependent gamut-mapping has been covered in the research literature but hasn't found its way to market in a standardized form yet.
[a href=\"index.php?act=findpost&pid=192775\"][{POST_SNAPBACK}][/a]
(1) not sure if I want to see a bunch of different profiles for standard encodings such sRGB or Adobe RGB, respectively, as for the target space.

(2) a smart CMM would selectively treat those oog colors which are really given in an image, rather than trying to compress the whole source space such as ProPhoto RGB, this or that way, into a smaller target space. Can’t see how to realize this with (1). Even if we go for a smaller source space assumption with the Lut for perceptial compression, there’s no differentiation by color range & hue angle.

Nonetheless, the key challenge I see is to improve RelCol at all. This might require a very smart CMM which recognizes what we perceive as posterization – very color range selective, very dependent on the image content and very smooth regarding the degree of moving colors “inwards”.

Peter

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« Last Edit: May 01, 2008, 12:27:36 AM by DPL » Logged
digitaldog
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« Reply #25 on: May 01, 2008, 08:15:11 AM »
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Nonetheless, the key challenge I see is to improve RelCol at all.

Actually, RelCol should need no tweaks, its a colorimetric intent. All profiles, in theory should produce the same values using this intent, unlike Perceptual, which is supposed to have some secrete sauce built into it by the profile building software (hence the reason we have no less than three options in ProfileMaker Pro for doing this). Every manufacturers Perceptual rendering can be, and probably IS different. I'm not suggesting this is an elegant solution, but a colorimetric intent should be a strictly defined process.
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Andrew Rodney
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Chris_T
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« Reply #26 on: May 01, 2008, 11:17:40 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
[a href=\"index.php?act=findpost&pid=192253\"][{POST_SNAPBACK}][/a]

What is a "matrix profile"? Is Epson printer's media profile one?

I recall reading somewhere that while there are definitions for intents, the profile vendors can build profiles according to their whims. And don't tell you exactly how they do it.
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jbrembat
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« Reply #27 on: May 01, 2008, 01:32:17 PM »
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A matrix profile is a profile that include a matrix to go from RGB to PCS (XYZ or Lab) and back.
In other words RGB are linear combination of PCS coordinates.

Every synthetic color space (sRGB,Adobe 1998, ProPhoto,....) is generate from XYZ fixing the Red,Green,Blue,White and gamma.

Printing profile are Lut profile.
A lut profile has up to 3 tables that correlate RGB values to PCS coordinates.
This is a semplified schema: you can have a mix of lut and matrices.

ICC specs fix a framework and rules for profiles, but the true profile creation is vendor specific.

Jacopo
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bjanes
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« Reply #28 on: May 01, 2008, 01:44:27 PM »
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What is a "matrix profile"? Is Epson printer's media profile one?

I recall reading somewhere that while there are definitions for intents, the profile vendors can build profiles according to their whims. And don't tell you exactly how they do it.
[{POST_SNAPBACK}][/a]

It is usually wise to to a Google search:

[a href=\"http://www.ppmag.com/reviews/200501_rodneycm.pdf]http://www.ppmag.com/reviews/200501_rodneycm.pdf[/url]
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Guillermo Luijk
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« Reply #29 on: May 01, 2008, 06:29:34 PM »
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So any conversion involving an origin or destination matrix profile cannot be applied using other than Relative Colorimetric? or that's only for the origin? or that's only for the destination?

e.g.: from sRGB to a printer table based ICC profile?
from a camera table based ICC profile to sRGB?
any of those can be Perceptual or none?

Thanks for the link Bill
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jbrembat
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« Reply #30 on: May 02, 2008, 04:30:28 AM »
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So any conversion involving an origin or destination matrix profile cannot be applied using other than Relative Colorimetric? or that's only for the origin? or that's only for the destination?
No.

When you transform colors from an original color space into a different color space, 2 profiles are involved: the original (colorspace1) and the detination profile (colorspace2).

2 steps are performed:
  - step1: from colorspace1 to Lab (or XYZ) D50
  - step2: from Lab (or XYZ) D50 to colorspace2

The Lab (or XYZ) D50 is the PCS (Profile Connection Space) fixed by ICC

So, if you go from sRGB to printer profile, the following steps are perfomed:
 - step1: from sRGB to XYZ D50  (relative)
 - step2: from XYZ D50 to printer color space (perceptual or relative,depending on user selection)

Jacopo
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Guillermo Luijk
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« Reply #31 on: May 02, 2008, 05:16:28 AM »
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When you transform colors from an original color space into a different color space, 2 profiles are involved: the original (colorspace1) and the detination profile (colorspace2).

2 steps are performed:
  - step1: from colorspace1 to Lab (or XYZ) D50
  - step2: from Lab (or XYZ) D50 to colorspace2

The Lab (or XYZ) D50 is the PCS (Profile Connection Space) fixed by ICC

So, if you go from sRGB to printer profile, the following steps are perfomed:
 - step1: from sRGB to XYZ D50  (relative)
 - step2: from XYZ D50 to printer color space (perceptual or relative,depending on user selection)
And a conversion from a table colour space (like a camera calibration profile for instance) into a matrix colour space (like sRGB) would also allow a real Perceptual conversion in the step 1 from camera to XYZ, and relative in step 2 from XYZ to sRGB?

BR
« Last Edit: May 02, 2008, 05:16:45 AM by GLuijk » Logged

jbrembat
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« Reply #32 on: May 02, 2008, 05:23:06 AM »
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Yes, if camera profile has a perceptual intent embedded.

But camera profile does not knows the destination color space, so I think it is useless.

Jacopo
« Last Edit: May 02, 2008, 05:29:38 AM by jbrembat » Logged
madmanchan
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« Reply #33 on: May 02, 2008, 09:15:43 AM »
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(2) a smart CMM would selectively treat those oog colors which are really given in an image, rather than trying to compress the whole source space such as ProPhoto RGB,

Yes, but how? There are an infinite number of ways to perform this transformation. No single method is perfect for all images. As Andrew said, every profile-building software has its own secret sauce for creating perceptual mappings. It all depends on the color optimization metric being used.

A smart CMM that picks one particular method of performing perceptual mappings would be great as long as you agreed that its results were good for your images. But it wouldn't be so great for someone else who doesn't like its results.

To get around this issue, a smart CMM could offer multiple options for doing perceptual mappings, letting the user choose on an image-by-image basis which one works best. But that is not that different from having multiple pre-baked perceptual profiles in the first place. That was the point I'm trying to make.
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bjanes
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« Reply #34 on: May 02, 2008, 09:50:25 AM »
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And a conversion from a table colour space (like a camera calibration profile for instance) into a matrix colour space (like sRGB) would also allow a real Perceptual conversion in the step 1 from camera to XYZ, and relative in step 2 from XYZ to sRGB?

BR
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One could apply perceptual rendering in step 1, camera space to XYZ, but it would not make sense. The purpose of perceptual rendering is to map out of gamut colors in a larger color space into a smaller space. Since XYZ is a large space and it contains all the colors that humans can see, there is no need to map the camera space into XYZ.

As [a href=\"http://www.adobeforums.com/webx?14@@.3bc112e8/27]Bruce Fraser[/url] explains in this link on the Adobe forum, the current dumb perceptual rendering has no knowledge of the source space or how much of it is populated by colors actually in the image. It merely compresses the source colors by a predetermined amount whether such compression is needed or not. If the source contains no out of gamut colors, this is not desirable.

" Given the many limitations of perceptual rendering (ignorance of source space, ignorance of how much of the source space is actually populated by the image, treating a black cat in a coal cellar and a polar bear on an ice floe identically, etc), I generally find that when a straight relcol conversion from PPRGB to sRGB fails in some regard, it's easier to simply edit the file than to come up with a magic rendering that works for all images."

With Camera Raw many use ProPhotoRGB as their default color space. If you need the final image to be in sRGB, it would probably be easier to use the ACR controls to map the colors into the sRGB space without clipping rather than initially rendering into ProPhoto and subsequently editing the image in Photoshop.

If you are working in Lightroom, the clipping display is for ProPhotoRGB (actually Mellisa), you don't have this option. The clipping will occur when you export the image to sRGB.

Bill
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Chris_T
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« Reply #35 on: May 03, 2008, 07:17:30 AM »
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To get around this issue, a smart CMM could offer multiple options for doing perceptual mappings, letting the user choose on an image-by-image basis which one works best. But that is not that different from having multiple pre-baked perceptual profiles in the first place. That was the point I'm trying to make.
[a href=\"index.php?act=findpost&pid=193114\"][{POST_SNAPBACK}][/a]

Sounds like a wonderful and reasonable idea. Are there any profile vendors who offer "multiple pre-baked perceptual profiles"?

Like many tools (sharpner, noise reducer, stitcher), to get the best results from them, different image types need different settings/tweakings. One size does not fit all.
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digitaldog
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« Reply #36 on: May 03, 2008, 09:32:24 AM »
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Sounds like a wonderful and reasonable idea. Are there any profile vendors who offer "multiple pre-baked perceptual profiles"?

ProfileMaker Pro offers three "tweaks". Only one I find useful (Colorful).
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Andrew Rodney
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Hermie
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« Reply #37 on: May 03, 2008, 07:09:17 PM »
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> Are there any profile vendors who offer "multiple pre-baked perceptual profiles"?

Practically all packages make a silent assumption on source color space.
The free Argyll CMS (command line based) allows you to specify the source color space (sRGB, aRGB etc.) when creating the perceptual intent.
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madmanchan
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« Reply #38 on: May 03, 2008, 10:22:12 PM »
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Sounds like a wonderful and reasonable idea. Are there any profile vendors who offer "multiple pre-baked perceptual profiles"?

Like many tools (sharpner, noise reducer, stitcher), to get the best results from them, different image types need different settings/tweakings. One size does not fit all.
[a href=\"index.php?act=findpost&pid=193283\"][{POST_SNAPBACK}][/a]

The analogy here is the one I posted earlier in the thread: Bill Atkinson providing several profiles for the printer/paper combos he's profiled. For example, consider the Epson 9800 with Premium Luster. He put a lot of effort into getting a set of solid, clean measurements, then used that data to generate several profiles, using different software & software options. He calls this a "bouquet" of profiles. So when it comes to making a print, you turn on soft proofing and flip through the profiles and pick the one that works best for that image.
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Guillermo Luijk
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« Reply #39 on: May 04, 2008, 02:29:22 PM »
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If you need the final image to be in sRGB, it would probably be easier to use the ACR controls to map the colors into the sRGB space without clipping rather than initially rendering into ProPhoto and subsequently editing the image in Photoshop.
Bill, I was looking into DCRAW's source code but I feel a bit lost to answer this: DCRAW uses Camera -> XYZ matrices (provided to Coffin by Adobe), and then another transformation XYZ -> output colour space used. In fact both matrices are multiplied in advance to do Camera -> output colour space in just one step.

The question is: is it possible that the exposure level of a given pixel has an influence to determina whether it will fall inside or outside the output colour profile space?

Think of a pixel in a scene captured with RGB levels=(10,22,45), if it was shot 1 f-stop more exposed, for sensor linearity it would be coded as (20,44,90).
Is it possible that (10,22,45) when converted to the output colour space falls into sRGB gamut, but (20,44,90) gets clipped in sRGB?

I want to believe then that if (20,44,90) clips in the output colour profile but (10,22,45) doesn't clip, is just for saturation (luminosity) reasons, not because the Hue determined by the pixel of the scene can alternatively belong to or not belong to the output profile gamut. Right?

If the answer to all that is yes, it means the degree of exposure of the RAW data will affect the possibility of converting levels on a pixel to a given output colour profile, so the Exposure slider in ACR is a good tool, not only to cancel the effect of the white balance, but also to avoid clipping due to excesive input exposure.

Am I right?

BR
« Last Edit: May 04, 2008, 02:31:57 PM by GLuijk » Logged

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