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Author Topic: Question re my Display Profile  (Read 5385 times)

Dave Ellis

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Question re my Display Profile
« on: July 22, 2015, 01:06:31 am »

I have only a limited knowledge of Colour management but I am currently trying to improve my understanding of how profiles work. In particular, I have been looking at my Benq monitor's default profile using the ICC Profile Inspector software. It tells me that this profile contains

RGB to XYZ 3x3 matrix data (3 tags)
1 x TRC for each of R,G and B
A2B0 tag containing an LUT for transforming from monitor to PCS (perceptual intent)
B2A0 tag containing an LUT for transforming from PCS to monitor (perceptual intent)

My understanding is that for simple profiles such as working space profiles that only contain the 3x3 matrix and TRC's, rendering intent has to be relative colourmetric. But with profiles that contain LUT's , perceptual rendering intent can be used.

My question is this - why does my profile have both the matrix/TRC data as well as the LUT data ? Presumably the LUT data would be used but why is the matrix and TRC data included ?

Your advice would be most welcome.

Dave
« Last Edit: July 22, 2015, 01:15:27 am by Dave Ellis »
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Lundberg02

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Re: Question re my Display Profile
« Reply #1 on: July 22, 2015, 02:32:33 am »

Because it would counterproductive to do relative to your monitor display, so you have to have perceptual from your image file to the monitor. You will see whatever the image is in perceptual without  further clipping. But you have to remember that you can't see anything but relative when you convert your working space from say argb to srgb. You can also get perceptual by soft proofing or by using Photoshop manages colors and selecting your printer's perceptual. If you take raw and develop to ProPhoto, you will see on the monitor perceptual in the monitor's space, hopefully argb if your monitor is wide gamut, so that's not too bad. If you convert the ProPhoto to srgb, you get relative. There are a couple of ways to get perceptual when doing that but it involves finding either the ICC v4 profile and installing it, or getting the other conversion profile from a member of this forum whose username I don't recall at the moment.
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Dave Ellis

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Re: Question re my Display Profile
« Reply #2 on: July 22, 2015, 05:06:55 am »

Hi Richard

Thanks for your response.

Because it would counterproductive to do relative to your monitor display, so you have to have perceptual from your image file to the monitor. You will see whatever the image is in perceptual without  further clipping.

I may have confused the issue by even mentioning rendering intent but I do realize that the LUT is there to get perceptual intent. My question was why are the matrix and TRC's in there as well as the LUT. Maybe I mis-understand the overall concept and the matrix and TRC's are required as well as the LUT.?

Dave
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GWGill

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Re: Question re my Display Profile
« Reply #3 on: July 22, 2015, 08:11:31 am »

RGB to XYZ 3x3 matrix data (3 tags)
1 x TRC for each of R,G and B
A2B0 tag containing an LUT for transforming from monitor to PCS (perceptual intent)
B2A0 tag containing an LUT for transforming from PCS to monitor (perceptual intent)
That is a legal Matrix profile, but not a legal cLUT profile. It would be quite reasonable for a CMM to completely ignore the cLUTs, since a cLUT Output profile requires all the A2B and B2A tables.
Quote
But with profiles that contain LUT's , perceptual rendering intent can be used.
Only if the perceptual tag points to a table containing perceptual information. It is legal for the perceptual tag to point to the Colorimetric table if there is not Perceptual information.
Quote
My question is this - why does my profile have both the matrix/TRC data as well as the LUT data ? Presumably the LUT data would be used but why is the matrix and TRC data included ?
Typically a profile might contain both a matrix and a valid set of LUTs to allow for CMMs that don't support cLUTs.  It's hard to guess what the intention is with your profile, since it only has a sub-set of the cLUTs that make up a valid cLUT type profile.
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Dave Ellis

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Re: Question re my Display Profile
« Reply #4 on: July 22, 2015, 03:44:28 pm »

Thanks for your response.

Typically a profile might contain both a matrix and a valid set of LUTs to allow for CMMs that don't support cLUTs. 

That makes sense and answers by basic question.

Thanks also for the other information. You've given me some more background to think about. Is there any way I can see (using the ICC Profile Inspector) if "the perceptual tag points to a table containing perceptual information" ?

Dave
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GWGill

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Re: Question re my Display Profile
« Reply #5 on: July 22, 2015, 08:56:50 pm »

Is there any way I can see (using the ICC Profile Inspector) if "the perceptual tag points to a table containing perceptual information" ?
I'm not familiar with ICC Profile Inspector.
Using ArgyllCMS iccdump you can tell by noting that the offset value for each tag is the same.
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Dave Ellis

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Re: Question re my Display Profile
« Reply #6 on: July 22, 2015, 11:32:47 pm »

I'm not familiar with ICC Profile Inspector.
Using ArgyllCMS iccdump you can tell by noting that the offset value for each tag is the same.

Here is a screenshot of the tag table from ICC Profile Inspector. Which tags are we looking to have the same offset value ?

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xpatUSA

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Re: Question re my Display Profile
« Reply #7 on: August 13, 2015, 12:38:08 pm »

That is a legal Matrix profile, but not a legal cLUT profile. It would be quite reasonable for a CMM to completely ignore the cLUTs, since a cLUT Output profile requires all the A2B and B2A tables.Only if the perceptual tag points to a table containing perceptual information. It is legal for the perceptual tag to point to the Colorimetric table if there is not Perceptual information.Typically a profile might contain both a matrix and a valid set of LUTs to allow for CMMs that don't support cLUTs.  It's hard to guess what the intention is with your profile, since it only has a sub-set of the cLUTs that make up a valid cLUT type profile.

Sorry to interrupt but I've sent you a PM on a related subject.

Thanks,
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best regards,

Ted
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