Thread: [Lcms-user] Lab->Lab spac conversion?
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From: Kai-Uwe B. <ku...@gm...> - 2005-05-25 07:21:30
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Hi, I looked at the spec and it says nothing about the above conversion in a colour space profile. Is Lab not valid as source colour space? Lcms seems to use the A2B0/B2A0 table pair only in one direction, ignoring in the other. The result is string hue casts while the gray axis remains stable. Anyway with renaming the colour space from Lab to RGB the profile works. It is about Lindbloom's Uniform Perceptual Lab profile. regards Kai-Uwe Behrmann + development for color management + imaging / panoramas + email: ku...@gm... + http://www.behrmann.name |
From: Marti <ma...@li...> - 2005-05-25 08:04:24
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The problem is, lcms cannot guess which direction to use. Let's imagine you have a input profile from RGB to Lab, and then you connect the colorspace profile after that, finally you have an output profile implementing Lab ->RGB Which direction to use? Well, lcms line of reasoning is something like: "Ok, here I have a Lab space and then I have to connect it to da profile... let's try the input direction, oh, great, it matches the Lab space, ok, let's verify what happens to the output .. wow, also matches the Lab requirement of output, excellent!" And then fails miserably to detect the correct direction. Is for that reason Lab->Lab profiles are usually marked as abstract profiles. Those have only one direction. Just to prevent such kind of things. Also, If I recall correctly, Lindbloom's profile deals with a tweaked Lab, so I wonder if the profile should be tagged as using Lab at all. That's not the Lab space all we are used. Regards Marti. ----- Original Message ----- From: "Kai-Uwe Behrmann" <ku...@gm...> To: "Lcms Liste" <lcm...@li...> Sent: Wednesday, May 25, 2005 9:25 AM Subject: [Lcms-user] Lab->Lab spac conversion? > Hi, > > I looked at the spec and it says nothing about the above conversion in a > colour space profile. Is Lab not valid as source colour space? > Lcms seems to use the A2B0/B2A0 table pair only in one direction, > ignoring in the other. The result is string hue casts while the gray axis > remains stable. > > Anyway with renaming the colour space from Lab to RGB the profile works. > It is about Lindbloom's Uniform Perceptual Lab profile. > > regards > Kai-Uwe Behrmann > + development for color management > + imaging / panoramas > + email: ku...@gm... > + http://www.behrmann.name > > > > ------------------------------------------------------- > This SF.Net email is sponsored by Yahoo. > Introducing Yahoo! Search Developer Network - Create apps using Yahoo! > Search APIs Find out how you can build Yahoo! directly into your own > Applications - visit http://developer.yahoo.net/?fr=offad-ysdn-ostg-q22005 > _______________________________________________ > Lcms-user mailing list > Lcm...@li... > https://lists.sourceforge.net/lists/listinfo/lcms-user > > > > -- > No virus found in this incoming message. > Checked by AVG Anti-Virus. > Version: 7.0.322 / Virus Database: 266.11.16 - Release Date: 24/05/2005 > > -- No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.322 / Virus Database: 266.11.16 - Release Date: 24/05/2005 |
From: Kai-Uwe B. <ku...@gm...> - 2005-05-25 10:38:42
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Am 25.05.05, 10:04 +0200 schrieb Marti: > > The problem is, lcms cannot guess which direction to use. > > Let's imagine you have a input profile from RGB to Lab, and then you connect > the colorspace profile after that, finally you have an output profile > implementing Lab ->RGB > Which direction to use? Thanks for this explanation. > Is for that reason Lab->Lab profiles are usually marked as > abstract profiles. Those have only one direction. Just to > prevent such kind of things. The profile should be usable as working space. Abstracts are no option. > Also, If I recall correctly, Lindbloom's profile deals with > a tweaked Lab, so I wonder if the profile should be tagged as using Lab at > all. That's not the Lab space all we are used. Well, that [L]*ab is somewhat rotated. Calling it internally RGB should be ok. thanks Kai-Uwe Behrmann + development for color management + imaging / panoramas + email: ku...@gm... + http://www.behrmann.name |
From: Steve U. <up...@ch...> - 2005-05-25 20:16:27
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At 9:25 AM +0200 5/25/05, Kai-Uwe Behrmann wrote: >Hi, > >I looked at the spec and it says nothing about the above conversion in a >colour space profile. Is Lab not valid as source colour space? >Lcms seems to use the A2B0/B2A0 table pair only in one direction, >ignoring in the other. The result is string hue casts while the gray axis >remains stable. what if it is combined with a normal Lab profile? then you could do spac->Lab or Lab->spac depending on the transform you are looking for... Regards, Steve Upton ________________________________________________________________________ o Steve Upton CHROMiX www.chromix.com o (hueman) 866.CHROMiX ________________________________________________________________________ -- |