Batch-adjusting white balance relative to auto white balance

[Beep6581]

Originally reported on Google Code with ID 2445

I was asked on IRC whether it's possible to batch-adjust the white balance on a series
of shots taken where the temperature of the light changed between shots. I set out
to take four shots in such a situation, only my temperature differences were quite
extreme. You can get the four shots from here:
http://rawtherapee.com/shared/test_images/changing_light_1.pef (1.pef to 4.pef)

This involves two things:
1- Being able to set a unique white balance for each shot and then to batch-adjust
it relative to that unique value.
2- Being able to make it warmer or cooler by equal amounts.

To solve point 1, do the following:
1a- Select all shots, apply Neutral (or whatever), change White Balance method to Auto.
1b- Individually click on each photo and nudge the temperature up one click of the
arrow. This is to change from method "Auto" to "Custom" where "Custom" should use the
value from "Auto" (eh?). 
1c- Preferences > Batch Processing, change White Balance Temperature to "Add".
1d- Select all four shots, and now you can use the Temperature slider to change the
white balance relative to what it was in Auto.

This lets you warm up or cool down a series of shots taken in mildly differing light.
However, it is limited to small light temperature differences, because a 500 Kelvin
difference from 2600K is far stronger than a 500 Kelvin difference from 8000K. This
is where point 2 would be nice to be able to overcome. Maybe this Mired unit could
help?
http://rawtherapee.com/forum/viewtopic.php?f=2&t=5506&p=37767#p37767

You can see a screencast of me doing the above here:
http://rawtherapee.com/shared/videos/rt4.1_batch_adjusting_whitebalance_with_changing_light.webm
(4MB, should play straight in any modern browser or mplayer/mpv)

It is also silly that I had to click through each individual image in point 1b. One
should be able to select all Auto images and just change the temperature if the mode
is "Add". Would be nice if this was improved.

Reported by entertheyoni on 2014-07-08 09:07:08

[Beep6581]

Indeed in such a case using Mired instead of Kelvin would ease your work because ---
at least according to Wikipedia [1], myself not being a color expert --- one Mired
unit shifts the color temperature by the same "perceptual" difference regardless where
on the scale it is (unlike Kelvin).

[1] https://en.wikipedia.org/wiki/Mired

Reported by ciprian.craciun on 2014-07-08 12:54:19