Re: [Lcms-user] conversion sRGB to L*a*b* fatal error

[Mart� Maria]

Hi,   >good news for you. >conversion with icctrans works quite well - in both directions. >  now i know how to deal with it :-) values are the same as in >  photoshop - except for rounding differences. > >i must admit - hmmh - i made a beginners mistake... >when choosing color values with photoshop 'color picker' i was in >  L*a*b* color space - and so was very often out of RGB gamut... >  got a hot flush when i realised this. > sorry Mart� for this false alarm, my only excuse is that i _am_ a >  newbie to color models and color handling.   Don't apologize :-) You have done a great job checking the engine and then reporting what seemed a bug to the list. This is the way we uncovered bugs, so this is really a contribution, and it is welcome.   >in that situation i threw my eyes upon the L*a*b* color model which >  hopefully gives me what i want - at least it has one value to adjust >  'lightness'.   Yep. Lab is very adequate for doing image manipulation. In first place, is near perceptually uniform. This is good for most operations. Then, it has Luma separated from chroma, again a nice feature.   Here are some "tricks" on Lab:    - Addition works far better than in RGB. Adding two Lab values does give the resulting Lab color.  - To modify "brigtness" just add o substract a constant to L  - To modify "contrast" apply a exp() on L.  - You can also use polar form of Lab LCh. There are a couple of functions to convert Lab <-> LCh  - You can "desaturate" or "oversaturate" color by incrementing/decrementing C in LCh  - Saturation of color is given by s = C/L  - Hue angle is given by h   Best Regards, Mart� Maria The little cms project http://www.littlecms.com [EMAIL PROTECTED]     ----- Original Message ----- From: Andreas Sch�mann To: [EMAIL PROTECTED] Sent: Saturday, March 16, 2002 9:34 AM Subject: Re: [Lcms-user] conversion sRGB to L*a*b* fatal error hi Mart�,   good news for you. conversion with icctrans works quite well - in both directions.   now i know how to deal with it :-) values are the same as in   photoshop - except for rounding differences.   i must admit - hmmh - i made a beginners mistake... when choosing color values with photoshop 'color picker' i was in   L*a*b* color space - and so was very often out of RGB gamut...   got a hot flush when i realised this.   sorry Mart� for this false alarm, my only excuse is that i _am_ a   newbie to color models and color handling.   maybe you like to know how i came to lcms... i intend to make a tool to adjust brightness of each individual pixel   of a picture - for now JPEGs. to do so i modified the IJG source code - needed three days of   hard work... when i came to the point where i had access to the sample data i   wondered how to modify the RGB values in order to modify   brightness - but without changing the 'color' or at least my   perception of a color becoming 'lighter'. whatever this _really_ means - need to find out more about it -   i figured this must be difficult modifying three independent color   values at once. in that situation i threw my eyes upon the L*a*b* color model which   hopefully gives me what i want - at least it has one value to adjust   'lightness'. but how to convert RGB to L*a*b* from within C code? now i know   the answer and am very happy with it.   thank you Mart� and all others involved in lcms project!   andreas   ----- Original Message ----- From: Mart� Maria To: Andreas Sch�mann ; [EMAIL PROTECTED] Sent: Saturday, March 16, 2002 3:14 PM Subject: Re: [Lcms-user] conversion sRGB to L*a*b* fatal error Hi,   > i am trying to validate the 'icctrans'-values for sRGB IEC61966-2.1 to L*a*b* conversion.   Humm... I have checked this utility extensively, and seemed to work. Are you sure Photoshop is using sRGB as workspace (Edit-Color settings-Working spaces RGB)?   I'm also attaching latest source, with some small additions. But anyway the old one did work properly to me...   This would give sRGB to Lab   icctrans -v   And this Lab to sRGB   icctrans -i*Lab   If this don't work to you, could please send any values that makes it fail? Thanks.   Regards, Mart� Maria The little cms project http://www.littlecms.com [EMAIL PROTECTED]           ----- Original Message ----- From: Andreas Sch�mann To: [EMAIL PROTECTED] Sent: Friday, March 15, 2002 5:10 PM Subject: [Lcms-user] conversion sRGB to L*a*b* fatal error hello marti,   i am trying to validate the 'icctrans'-values for sRGB IEC61966-2.1 to L*a*b* conversion. i want to validate against adobe photoshop - i simply use the 'color picker' to get values. i am unlucky :( neither the built in profile 'cmsCreateLabProfile(NULL)' nor 'lcmstiff8.icm' nor 'tifflab8spac.icm' give me the photoshop values.   i built lcms 1.08 with cygwin on win2000, build looked ok.   what do you think? maybe i missed something? i'm not sure whether i use the right profiles.   regards, andreas