Values lockable color picker Gimp/Art differ somewhat

[paulmatth (Paul Matthijsse)]

Hello, I discovered a strange thing this morning (at least to me it seems strange).
I made a box in Gimp and filled it with red, green and blue, plus white and black boxes. I set the RGB value for red in Gimp to 255,0,0 (so pure red), blue is 0,0,255, etc.

That looks like this in Gimp.

Now I open this image in Art and place some lockable color pickers on the colors, like so.

Now, why is red noted as 99,6, 0, 0 instead of 100,0,0? Same for the blue and green boxes. Any ideas?

[Jade_NL (Jacques)]

Are you sure that GIMP and ART are both using the same colour spaces and working profiles? If that isn’t the case then there will be differences.

Had a bit of a hard time getting my head around that one, but it was explained to me by @XavAL rather well in this part and onwards in a topic I started:

Translating a specific colour(-code) to a 0-->1 number: Colour Toning -> L*a*b blending

To me, @age is right. But to get the right number you have to know both: the color space those patches refer to the working profile you are using Bear in mind that when using the Color Toning tool and adjusting by hand those numbers, both color spaces must be the same. Your best bet would be following these steps: check the working profile you will be using in your image (you can’t change it from the moment you use the Color Toning tool onwards), and make sure it’s the same as the one the…

[paulmatth (Paul Matthijsse)]

@Jade_NL

Are you sure that GIMP and ART are both using the same colour spaces and working profiles? If that isn’t the case then there will be differences.

Hello, Gimp uses its standard built-in sRGB when creating a new image. Changing in Art the working profile (default ProPhoto) to another profile does not change those values. They do change though when I choose another output profile, but that’s not relevant here, in my opinion.

My guess is that Gimps default profile sRGB differs a little bit from Arts sRGB or RTv4_sRGB.

[agriggio (Alberto)]

Hi,

@paulmatth

My guess is that Gimps default profile sRGB differs a little bit from Arts sRGB or RTv4_sRGB.

could be. You can test by using the same profile in GIMP and check…

[Thanatomanic (Roel)]

I remember an issue in the RT tracker that noted something very similar, but I could not find it again just now…
I did find these two Color picker Lab readouts depend on the output profile (and are almost always wrong)) · Issue #5408 · Beep6581/RawTherapee · GitHub Lockable color picker circle does not reflect actual sampling region when zoomed · Issue #5767 · Beep6581/RawTherapee · GitHub which may be relevant in some way…

[agriggio (Alberto)]

@agriggio

You can test by using the same profile in GIMP and check…

seems everything is fine to me:

[AdmFubar]

This sounds a bit like an issue with math, specifically calculating binary to decimal
see:
https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html

Round-off error

[paulmatth (Paul Matthijsse)]

Hello all, thanks for your replies, I was away a couple of days.

Today I discovered something funny. If I make a red square in Gimp (again at 100% red) and save it as a jpg, Art says again 99,6% red. But if I save that thing as a tif, Art reports 100% now! The jpg was saved with quality 100 (max), so here comes the difference from, apparently.

Alberto used indeed a tif above.

Lesson learned: never save in jpg again!