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Author Topic: Shouldn't absolute colorimetric leave colors be that are in gamut in src & dst?  (Read 2217 times)
darlingm
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« on: October 18, 2012, 03:12:35 AM »
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If I have a document in "ProfileA", which is completely within ProPhoto RGB, and I convert this document using absolute colorimetric to ProPhoto RGB, and I convert this using absolute colorimetric back to "ProfileA", shouldn't the LAB values remain the same throughout the same process?  (Except for possible rounding issues, which I'm willing to consider negligible, especially in 16bit color mode.)  Isn't the whole point of absolute colorimetric that in gamut values don't change?  Everything starts out in gamut in "ProfileA", and since ProPhoto RGB fully contains "ProfileA", then everything should be in gamut there, and likewise back again to "ProfileA"?

I have a color in "ProfileA" of RGB 35/43/35 LAB 4/-3/3.  Converting that to ProPhoto RGB, it becomes RGB 24/25/24 LAB 13/-2/2.  (RGB of course changes - it's the LAB change that confuses me.)  Converting it back to "ProfileA", it becomes RGB 53/57/55 LAB 7/-4/3.  I'm sort of willing to overlook a change of 1, although in 16bit color, I wouldn't expect it often.  But where did L* changing from 4 to 13 to 7 come into play?
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BartvanderWolf
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« Reply #1 on: October 18, 2012, 04:06:59 AM »
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If I have a document in "ProfileA", which is completely within ProPhoto RGB, and I convert this document using absolute colorimetric to ProPhoto RGB, and I convert this using absolute colorimetric back to "ProfileA", shouldn't the LAB values remain the same throughout the same process?  (Except for possible rounding issues, which I'm willing to consider negligible, especially in 16bit color mode.)  Isn't the whole point of absolute colorimetric that in gamut values don't change?  Everything starts out in gamut in "ProfileA", and since ProPhoto RGB fully contains "ProfileA", then everything should be in gamut there, and likewise back again to "ProfileA"?

I have a color in "ProfileA" of RGB 35/43/35 LAB 4/-3/3.  Converting that to ProPhoto RGB, it becomes RGB 24/25/24 LAB 13/-2/2.  (RGB of course changes - it's the LAB change that confuses me.)  Converting it back to "ProfileA", it becomes RGB 53/57/55 LAB 7/-4/3.  I'm sort of willing to overlook a change of 1, although in 16bit color, I wouldn't expect it often.  But where did L* changing from 4 to 13 to 7 come into play?

Hi,

Hard to say, but do both profiles use the same illuminant (D50)? Any Chromatic adaptation? Which color engine is used for the conversion? I saw in another thread that you used Adobe ACE, have you tried another (e.g. Microsoft or Argyll)?

Cheers,
Bart
« Last Edit: October 18, 2012, 04:13:39 AM by BartvanderWolf » Logged
FranciscoDisilvestro
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« Reply #2 on: October 18, 2012, 07:54:42 AM »
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Hi.

Adding to the questions Bart asked, is "Profile A" an icc v4 profile or a  icc v4 display profile? Also, how are you sure that Profile A is within ProPhoto RGB in an absolute colorimetric scenario? Even Adobe RGB has color outside ProPhoto RGB if you don't perform chromatic adaptation.

Could you provide the "Profile A" .icc or .icm file?
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Scott Martin
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« Reply #3 on: October 18, 2012, 09:52:37 AM »
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Keep in mind that PS is calculating the Lab values using the conversion options specified in PS's Color Settings dialog. The rendering intent, dithering, BPC, etc options all have an effect on how the Lab values are calculated.

Yes, your colors should be perfectly preserved if converted with the correct and desired options.

Lot of learning curve to this stuff isn't there?
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Scott Martin
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« Reply #4 on: October 21, 2012, 09:24:09 AM »
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darlingm - what do you think? Was the rendering intent the cause of confusion?
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digitaldog
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« Reply #5 on: October 21, 2012, 12:15:37 PM »
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The rendering intent, dithering, BPC, etc options all have an effect on how the Lab values are calculated. [

Dither can totally ruin these kinds of tests if you are intending to end up with dE values. Just subtract the two in PS* and you can often see significant visual differences.

*http://digitaldog.net/files/Apply_Image.pdf
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Andrew Rodney
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MarkM
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« Reply #6 on: October 21, 2012, 02:26:30 PM »
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One thing to keep in mind: output profiles are different from simple matrix-based profiles like ProPhotoRGB. Output profiles are based on multi-dimensional lookup tables which use interpolation from a handful points. This is inexact. On top of that, output profiles have different lookup table depending on whether you are going from the device to PCS (profile connection space) or from PCS to device space (these are the A2B and B2A tags in the profile). These two LUTs are quite different, which is one of the reasons you almost never arrive back in exactly the same place if you make a round trip from output profile to working space and back to output profile, even if everything is within gamut. Having said that, it does look like the discrepancies you showed are quite large. I don't get that kind of error when I try it with my own profiles. What exactly is ProfileA that you're using?
« Last Edit: October 21, 2012, 03:55:14 PM by MarkM » Logged

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