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CM Theory and Practice Profiles, color spaces, perception, science.

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  #1  
Old September 15th, 2010, 06:15 PM
Doug Kerr Doug Kerr is offline
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Default Rendering intents in PS CS5

This issue came up during some discussions with Kevin Stecyk.

As I understand it, when we convert from one color space to another, if the gamut of the "source" color space is not completely accommodated within the gamut of the "destination" color space, the way this is deal with varies with the "rendering intent" that is chosen. The choice of rendering intent also affects how, if at all, differences between the white points of the two color spaces are handled.

Again, as I understand it, for the three most important rendering intents, the major characteristics are:

Absolute colorimetric: No adjustment is made for differences in the white points; colors are mapped into the destination color space; colors that would map out-of-gamut in the destination color space are "clipped" to the gamut boundary.

Relative colorimetric: Adjustment is made for differences in the white points; after that, the colors are mapped to the destination color space; colors that would map out-of-gamut in the destination color space are "clipped" to the gamut boundary.

Perceptual: Adjustment is made for differences in the white points; after that, the colors are mapped into the new color space using a mapping that is "smoothly compressed" from the theoretical mapping so that the all colors from the source gamut will map "nicely" within the gamut of the destination color space.

Now, that having been said, I did some experiments in Photoshop CS5 to see this at work. I started with a file, in the Adobe RGB color space, having a red histogram with a substantial "hill" at the top end (but it went to ground before the end of the scale; there were only a few R pixels with value 255).

I then had PS convert this image into the sRGB color space (which has a smaller gamut), using different rendering intents for different trials.

I would expect that when using the perceptual intent, there would be no significant "pile-up" of R pixels at the top of the scale. But in fact there were a gigantic number of R pixels with value 255 (over 25% of all the R pixels).

What am I missing here?

Best regards,

Doug
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  #2  
Old September 15th, 2010, 07:19 PM
Andrew Rodney Andrew Rodney is offline
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Doug, simple Matrix based profiles, of which nearly all RGB working space are, only have the Colorimetric table. You can ask Photoshop for a Perceptual rendering intent but youíll get RelCol (or if you force the issue, Absolute Colorimetric if you ask for that).

Output profiles have all three tables, thatís one reason they are so much larger in size as stored on your drive.

Version 4 working space profiles, if every built and installed by Photoshop would allow you to have access to the other tables. But not today.
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  #3  
Old September 15th, 2010, 07:37 PM
Doug Kerr Doug Kerr is offline
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Hi, Andrew,

Quote:
Originally Posted by Andrew Rodney View Post
Doug, simple Matrix based profiles, of which nearly all RGB working space are, only have the Colorimetric table. You can ask Photoshop for a Perceptual rendering intent but youíll get RelCol (or if you force the issue, Absolute Colorimetric if you ask for that).

Output profiles have all three tables, thatís one reason they are so much larger in size as stored on your drive.

Version 4 working space profiles, if every built and installed by Photoshop would allow you to have access to the other tables. But not today.
Thank you so much.

Best regards,

Doug
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  #4  
Old September 15th, 2010, 07:41 PM
Kevin Stecyk Kevin Stecyk is offline
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Hi Andrew,

Have you watched "From Camera To Print - Fine Art Printing Tutorial" (sells for $34.95 - I am not affiliated in any manner)?

When I purchased and watched the video, my understanding was similar to what Doug provided in his write-up. Jeff Schewe advised to try both Relative and Perceptual. Choose whichever seemed best. He indicated the Relative was a chop and Perceptual was a compression. He further indicated that Photoshop made the assumption that most users are concerned with tonality, so Relative was chosen as the default. The tone was more important than the color. However, that's not always the case. Sometime the color is more important, and then you are more likely to chose Perceptual.

If I recall correctly, Michael Reichmann indicated that he used both about 50% of the time. He would try both and pick whichever best reflected his artistic intent.

From your response to Doug, I get the impression for sRGB, it doesn't matter. You're getting Relative whether you want it or not?

That aside, do you have any thoughts on my recollection of Michael Reichmann's and Jeff Schewe's video?
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Old September 15th, 2010, 07:46 PM
Doug Kerr Doug Kerr is offline
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Hi, Kevin,

Quote:
Originally Posted by Kevin Stecyk View Post
Jeff Schewe advised to try both Relative and Perceptual. Choose whichever seemed best. He indicated the Relative was a chop and Perceptual was a compression. He further indicated that Photoshop made the assumption that most users are concerned with tonality, so Relative was chosen as the default. The tone was more important than the color. However, that's not always the case. Sometime the color is more important, and then you are more likely to chose Perceptual.

If I recall correctly, Michael Reichmann indicated that he used both about 50% of the time. He would try both and pick whichever best reflected his artistic intent.
Likely, given the context, they were speaking of printer (output) color spaces, which Andrew tells us likely do have distinct tables for all the rendering intents.

Best regards,

Doug
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  #6  
Old September 15th, 2010, 07:48 PM
Andrew Rodney Andrew Rodney is offline
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I do indeed have that fine video. They are referring to an output profile (and soft proofing). If I understand what Doug is doing, heís converting from working space to working space (ProPhoto to sRGB). Both those profiles only have the Colorimetric table.

When we convert from a Matrix profile (sRGB) to a printer profile (MyPrinterRGB), its that later profile that has the tables we need here. Doug said ď I started with a file, in the Adobe RGB color spaceď and then ďI then had PS convert this image into the sRGB color spaceď and here lies the issue.
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  #7  
Old September 15th, 2010, 08:01 PM
Kevin Stecyk Kevin Stecyk is offline
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Hi Andrew,

I am glad you enjoyed that video too.

As far as Colorimetric tables and all three tables, you've gone beyond my limited knowledge in this area. So my key take away is that rgb has a limited amount of information to work with, so for all intents and purposes, you are getting Relative Colormetric. However, devices have more extensive information and then Perceptual and Relative have more bearing on the situation.

I need to learn more about this topic.

Thank you for your help Andrew.

Kevin
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  #8  
Old September 16th, 2010, 06:39 AM
Andrew Rodney Andrew Rodney is offline
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There are three possible tables in a LUT based profile: Colorimetric (Absolute and Relative share this table), Perceptual and Saturation. Simple Matrix profiles (sRGB, Adobe RGB (1998), all those RGB working space profiles, most display profiles, those that are small in size in terms of KB) have a single, Colorimetric table. By default, that means Relative Colorimetric.
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  #9  
Old September 16th, 2010, 07:05 AM
Kevin Stecyk Kevin Stecyk is offline
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Thank you Andrew for your helpful reply.

Earlier I came across this link, which just reinforces your message: EarthBoundLight.com
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