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Bug #12279

Wrong LAB values for blue colors

Added by Hong-Vu Dang 12 days ago. Updated 2 days ago.

Status:
New
Priority:
Low
Assignee:
-
Category:
Darkroom
Target version:
-
Start date:
08/04/2018
Due date:
% Done:

0%

Affected Version:
2.4.4
System:
Windows
bitness:
64-bit
hardware architecture:
amd64/x86

Description

I have an issue with reading Lab values, for some strange reason all values read out correctly compared to photoshop or lightroom, except for blue colors. I am on Windows 10 usiing the latest DT version, Photoshop CS6 and Lightroom 6.14
I have the exact same picture opened in lightroom, photoshop and darktable, with several different colors, a gray card that I use to calibrate L, a and b values. The gray card, other gray tints, yellow and red give the same Lab values, but not for blue. This is really weird. So this particular blue has an LAB value in photoshop/lightroom of L=29, a=-5, b=-58. You can make this particular blue by using edit/fill/color and then entering these LAB numbers in photoshop. I have a photo with this color (which I attached to this issue), in lightroom I apply a dcp profile (don't have the icc profile so can't do this in darktable), then I set the exposure and white balance to match the gray card in the photo. Then I export to jpg, tried both prophoto and adobergb. The reason why is so that the dcp profile is now embedded, and I am not sure what darktable does to my raw files when I open them (still new to DT)
Then I use the color picker tool in DT, I totally love the live samples function. All the LAB values of all the colors I measured up until now match the values that I get when using Lightroom/Photoshop, EXCEPT for this blue. I havent tried other blue colors yet, but I don't have this issue with red and yellow and various white, gray and black tints
The LAB values of that particular blue I get in DT is 33.438, 7.825, -53.621, quite different from the values I get in PS/LR: 29,-5,-58. I attached the photo with the blue color that is giving a mismatch

_MG_0140-3.jpg (1.11 MB) Hong-Vu Dang, 08/04/2018 12:26 PM

Untitled-1.jpg (34.2 KB) Hong-Vu Dang, 08/14/2018 12:30 AM

History

#1 Updated by Ulrich Pegelow 2 days ago

Please note that the color picker tool (left side panel) shows you the color values of the image converted into the screen color space. As a consequence values will change depending on your installed monitor profile. E.g. here on my computer with a wide gamut Eizo monitor I get values of 31, -2, -57.

As a way to see what Lab values darktable sees when reading the image you can open the tone curve module with "scale chroma" set to manual mode. The color picker in that modules gives me values of 30, -6, -58.

#2 Updated by Hong-Vu Dang 2 days ago

Hi Ulrich, thanks for your reply, however, that didn't do the trick for me: I still get the same value, see attached image.

Maybe I did something wrong here, I am very new to Darktable. And if this indeed is the solution, is it possible to set scale chroma to manual mode by default for all photos?

Kind regards,

Hong

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