Hello Nic!
I tried your datas and got quite good results.
Here are my strings:
scanin -v -F 2622,1399,6880,1425,6880,4257,2582,4227 -p -dipn -G 1.0
chart-raw.tiff CC24Classic.cht CC24Classic-D50.cie
colprof -v -qh chart-raw
I've used your big file directly (usually I'm downsampling such big
files to 2-3 megapixels).
I've got quite good profile:
Profile errors: max. = 2.817343, avg. = 0.686372, RMS = 0.942445
After that I've made color conversion from RGB to LAB:
cctiff -v -ia chart-raw.icm chart-raw.tiff chart-raw_LAB.tif
Also I've embedded profile into RGB TIFF.
cctiff -v -ia -e chart-raw.icm chart-raw.tiff chart-raw_RGB_Embedded.tif
Both LAB-TIFF and RGB-TIFF files look very close in Photoshop.
They looks completely close after RGB-TIFF file conversion from RGB to
LAB in Photoshop.
So everything seems working correctly.
Best regards,
Alexey Gribunin.
On 28/04/2022 21:36, Nic Nilov wrote:
Hello,
I've been trying to create a simple camera calibration profile and more or less
succeeded (profcheck reports errors max. = 5.159180, avg. = 2.285364, RMS =
2.613159).
I'm now trying to apply this profile to an image of the CC24 target (the same
the profile is derived from) and am observing some inconsistencies I'd like to
find an explanation for.
When I do it via this cctiff command:
cctiff -v -ia chart-xyz.icc chart-xyz.tiff chart-xyz_corrected.tiff
the result is looking very nice. Notably, the middle gray patch measures L* 50%
with near zero color deviation. The resulting tiff has CIELab encoding, and
opens as such in Photoshop, so it needs a conversion to an RGB colorspace
before further processing. This concerns me as LAB has some encoding issues
which may be relevant even with 16bit images. I would prefer the cctiff output
to be in an RGB colorspace.
So, when I specify the -e option to embed the ICC profile like this:
cctiff -v -ia -e chart-xyz.icc chart-xyz.tiff chart-xyz_corrected.tiff
exactly that happens. The output tiff now has RGB encoding with the profile
embedded.
I open this file in Photoshop using the embedded profile, at which point the
image looks significantly darker, the middle gray patch measures L* 35.
I then convert the image to ProPhotoRGB colorspace using the Absolute
Colorimetric intent (which is the same as cctiff -ia does), but the image is
still darker than the CIELab version. The middle gray patch now measures L* 45.
If, instead of using cctiff, I open the input chart-xyz.tiff file in Photoshop
directly, assign the same ICC profile and then convert to ProPhotoRGB with
Abs.Col., I get exactly the same looking image as with the previous step
(cctiff -e).
The main question is why when the image is in an RGB colorspace the look is
darker?
The second one is does the use of CIELab as the output encoding present an
image quality concern (primarily due to quantization with the resulting banding
and gamut reduction) when the first step of the post-processing in the
Photoshop has to be the Lab to RGB conversion?
Thank you,
Nic Nilov.
