> 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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