Hi,
Thanks, numbers helps a lot in locating what is happening.
The answer is, your sRGB values are chromatically adapted
to D50 using Bradford, so they are not correct at all.
This effect is clearly visible using Bruce Lindbloom's
color calculator:
http://www.brucelindbloom.com/
Select "Calc" and "CIE Color calculator" in order to
go to calculator page. Then select "sRGB".
True sRGB should be computed by selecting
"Adaptation" to "None". If you do so, the obtained
numbers closely matches those obtained by lcms.
If you select "Bradford", then you will obtain
the numbers you give. LittleCMS goes into lots of
trouble in order to undo the chromatic adaptation
of the profile, so the obtained error should be
pretty small.
Method 1 is the correct one.
icctrans -t 3 -i <my custom scanner profile> -o "sRGB Color Space
Profile.icm" -v
Hope this helps
Regards
Marti Maria
The littleCMS project
http://www.littlecms.com
----- Original Message -----
From: "Dietmar Dreyer" <[EMAIL PROTECTED]>
To: <lcms-user@lists.sourceforge.net>
Sent: Thursday, April 20, 2006 2:05 PM
Subject: [Lcms-user] Re: Inaccurate sRGB-Conversion?
Please make sure the patches are inside sRGB gamut.
If you got good Lab but bad sRGB, is quite probably you are
trying to get color that are not representable in sRGB.
Any RGB value close to 255 or 0 would be an indication of
out of gamut. If this is not the case, double check the
measurements. Otherwise, can you give me some numbers, i.e,
average error, maximum, etc.
Hi Marti,
the measurements are all okay.
I list some sample colors with their corresponding transformed values,
that I produced with help of icctrans.exe (using lcms 1.14). The chosen
colors are from a scan of an IT8/7 Target by W. Faust (reflective
R011201).
I used three different methods to convert from scanner RGB to sRGB:
1. Using a combined transform with scanner profile as input and sRGB
color space profile as output and rendering intent absolute
colorimetric:
# icctrans -t 3 -i <my custom scanner profile> -o "sRGB Color Space
Profile.icm" -v
2. Using separate transform from scanner RGB to Lab (rendering intent
absolute colorimetric) and from Lab to sRGB (rendering intent
perceptual); the output of the first serves as input to the last:
# icctrans -t 3 -i <my custom scanner profile> -o *Lab -v
# icctrans -t 0 -i *Lab -o "sRGB Color Space Profile.icm" -v
3. The same as 2., but using rendering intent absolute colorimetric for
both transforms:
# icctrans -t 3 -i <my custom scanner profile> -o *Lab -v
# icctrans -t 3 -i *Lab -o "sRGB Color Space Profile.icm" -v
Here come my results:
Patch C12:
Measured Scanner RGB: 216.492 151.585 119.869
Lab from target reference file: 74.33 17.58
38.57
Correct sRGB: 235 169
112
Transformation by 1. method
Lab (as shown by icctrans in verbose mode): 74.91 16.36
39.63
sRGB:
240.8 169.8 91.66
Transformation by 2. method
Lab: 74.83 16.82 39.54
sRGB: 229.61 171.8 111.45
Transformation by 3. method
Lab: same as 2.
sRGB: 241.2 169.35 91.61
Patch J13:
Measured Scanner RGB: 51.7216 110.287 165.963
Lab from target reference file: 56.17 -41.36
-34.20
Correct sRGB: 0 156
193
Transformation by 1. method
Lab (as shown by icctrans in verbose mode): 56.29
-43.37 -33.54
sRGB:
0.01 154.78 167.75
Transformation by 2. method
Lab: 56.29 -43.37 -33.55
sRGB: 0.01 155.86 192.3
Transformation by 3. method
Lab: same as 2.
sRGB: 0.01 154.78 167.74
Patch D18:
Measured Scanner RGB: 154.303 186.076 164.891
Lab from target reference file: 79.40 -23.69
16.64
Correct sRGB: 162 209
165
Transformation by 1. method
Lab (as shown by icctrans in verbose mode): 79.66
-23.89 16.54
sRGB:
176.21 207.53 141.14
Transformation by 2. method
Lab: 79.66 -23.89 16.54
sRGB: 158.55 209.85 165.28
Transformation by 3. method
Lab: same as 2.
sRGB: 176.13 207.6 141.11
I think it's quite striking, that the sRGB values of method 1 and 3 are
nearly identical.
Method 2 nearly always gives accurate results, but I cannot figure out a
way to accomplish this with a single transform.
Best regards
Dietmar
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-------------------------------------------------------
Using Tomcat but need to do more? Need to support web services, security?
Get stuff done quickly with pre-integrated technology to make your job easier
Download IBM WebSphere Application Server v.1.0.1 based on Apache Geronimo
http://sel.as-us.falkag.net/sel?cmd=lnk&kid=120709&bid=263057&dat=121642
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