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Author Topic: Quick question about RawTherapee Lab values display  (Read 3023 times)
xpatUSA
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« on: May 20, 2013, 10:58:00 AM »
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I've started playing with RawTherapee v3.

Pardon my ignorance, but I can't find the range for the values shown for a* and b* in L*a*b* from the color picker.

Is a* and b* the standard -128 to +127?

Is L* the standard 0 - 100?

I ask because it took me a while to realize that the HSV values are all shown as 0-255 !

Thanks,

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best regards,

Ted
xpatUSA
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« Reply #1 on: May 21, 2013, 07:23:41 AM »
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I made an image with primary and secondary colors in it, plus a PhotoShop "50% gray".

The Lab numbers shown by RawTherapee (RT) are the same as given by Bruce Lindbloom's calculator to the nearest integer value.

Therefore it can be deduced that RT conforms to the L*a*b* standard.
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best regards,

Ted
BartvanderWolf
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« Reply #2 on: May 21, 2013, 08:38:54 AM »
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Therefore it can be deduced that RT conforms to the L*a*b* standard.

Hi Ted,

Thanks for the feedback. When I quickly compared L*a*b* values between Photoshop and RawTherapee yesterday, I found differences I could not immediately explain, so I wasn't sure what to think of that (probably caused by a missing profile assignment). Otherwise I could have confirmed your finding already, so I had to find a bit more time to delve into the matter. You beat me to it.

Photoshop of course doesn't display a* and b* readouts beyond the [-128, 128] range, but other than that things seem to look similar.

Cheers,
Bart
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MichaelEzra
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« Reply #3 on: May 21, 2013, 11:50:27 AM »
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1. I would recommend to use RawTherapee 4.x only as 3.0 is really outdated in comparison.
2. 4.0.11 should be released for all supported OS platforms in a few days.
3. In 4.x version the Lab L, RGB and HSV values in navigator were changed to %, which works great for internal float data representation. I don't recall the a and b units, I recall these are absolute values, not %.
4. All values displayed in Navigator are in the editing color space
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xpatUSA
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« Reply #4 on: May 21, 2013, 05:28:46 PM »
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Thanks, Gentlemen, for your comments and advice.

Yes, if I understand correctly, the CIELAB a and b channel values are not bounded, unlike for example XYZ. I was only concerned that some color, picked in RawTherapee, would be the same color if the same numbers were picked in another program. By which I mean that Lab is device-independent, of course.

I do have a tendency to lag behind the crowd on computers and software and cameras, as a grumpy old man properly should, so I'll continue with V3 for now but thanks for the tip.
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best regards,

Ted
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