From Custom White Balance to Color Temperature ?

Usually custom white balance is defined by clicking on a neutral pixel.
I'd like to know if there is a formula that allows to calculate the color temperature when I clic on a pixel of the image that represent a neutral area ?

Let's say that this pixel has the value R,G and B initially.
what is the function that allows to calculate CT=f(R,G,B) ?
CT being the color temperature

Since none of our many members of Measurebaters Unanimous have popped in here to answer, I'm going to ask you why CT=f(R,G,B) is important to you?

Personally, regarding WB generally, reality isn't everything. Would we try to make a sunset neutral? The "Correct" WB may not be the "Right" WB for a image. Remember, you're the judge of your own image, & if your monitor, browser, & post processing utility are properly calibrated & color managed, I think it's more useful to adjust by eye for a pleasing image.

Heh! Although at first glance it does seem like a strange request, but I imagine yb98 is trying to work out an algorithm for his DPP++ application so that an RGB reading of a netral target taken from the eyedropper can be calculated in the formula into a color temp which can be used to apply a WB correction in DPP++ without resorting to the "regular" DPP WB tool.

Am I in the ballpark, yb98?

tonylong wrote in post #11265607
Heh! Although at first glance it does seem like a strange request, but I imagine yb98 is trying to work out an algorithm for his DPP++ application so that an RGB reading of a netral target taken from the eyedropper can be calculated in the formula into a color temp which can be used to apply a WB correction in DPP++ without resorting to the "regular" DPP WB tool.

Am I in the ballpark, yb98?

Exactly !!!

I have the feeling that the white balance issued from the use of the eyedropper cant not be always converted just to a color temperature but color temperature + something else (tint ?), because sometimes I get a white balance from the eyedropper that I can not obtain with the color temperature slider. Am I right ?

Hue/tint is also part of the formula.

Very probable.

DPP doesn't use a tint slider like LR does, but it has the "Color tone" slider, which does about the same: left = magenta, right = green (with a more limited adjustment region then the "tint" slider in LR)

I presume the "tint" is set "under the hood" as the midpoint of the Color tone slider.

Heh! I've never bothered studying the DCRaw code to try to understand what goes on under the hood of a Raw processor -- the thought of going back to the days of being a C Programmer and then trying to work that into being an amateur/hobbyist photographer makes my brain ache!

I guess in Auto WB DPP has to do what all the other Raw processors to to determine a WB estimate. I imagine that involves reading an RGB value for each pixel, averaging it out, then using that average to determine temperature/tone/hue whatever. Obviously that becomes hit-or-miss with Auto WB, and with other processors who can't read a Fixed/Custom WB like DPP can.

But how the eyedropper works has to do a similar "average" of a few pixels then set them to "medium" but I think it would fail to determine the temp of the scene, just of the target. Let's see, take the RGB values of a target and you compare it to "medium" and you then can determine the comparative yellow-blue and red-green values to "medium", right? Something like that? You'd have to have a value range for temp and hue...of course I'm just doing speculative babbling here...

I have found this : http://en.wikipedia.or​g/wiki/Color_balance
It explains how to apply a custom white balance but it doesn't say how to convert it in a color temperature unit...

Well, Lightroom breaks things up into Temp, which goes from Blue to Yellow, which is a combination of Red and Green. Tint goes from Green (Blue+Yellow) to Red.

So, I guess you could approach it really simply by first comparing the Blue value with the (R+G)/2 value so that the difference would represent the Temp and then compare the Red to the (G+B)/2 to come up with the relative tint/hue. DPP has a different layout from LR but in the camera you can set a Temp and also a Hue/Color Tone rather than a Custom WB or whatever.

Like I said, that's probably a very simplistic approach...