On 11/16/05, Timo Steuerwald wrote:

Hi all,

can anyone explain how the automatical white balance works? - I can not find anything about it in the documentation. It can be found at the menu in Layers->Colours->Auto->White Balance. I'm not sure if this are exactly the right words because I use the german version. How does this feature know which pixel should be white? - I expected something like the manual white balance in digital cameras: Pointing and selecting an area which should be white, the rest of the picture will be changed accordingly.

Open Levels dialog (Layer- Colour-Levels), click on the right color picker in its bottom that is in charge for white and then pick a color that seems to be the whitest one. Changes will apply.

You can also "show" this color levels tool the "blackest" and the "grayest" dots ;-)

Alexandre

On 11/17/05, Timo Steuerwald wrote:

Hey, that's a cool feature! Thanks a lot! I didn't know that up to now, but anyway: How does this automatical white balance work?

I don't know how exactly this engine ticks, but I can to to guess: it searches for the lightest pixel in the image and makes GIMP believe that it is white and then it shifts other colors appropriately. But that's my guess. Probably gimp-help package contains information you want to know.

Some info on Levels dialog ("Werte" im Deutschen, soweit ich das verstehe) is here: http://docs.gimp.org/de/ch06s05s06.html

Alexandre

On Thu, Nov 17, 2005 at 01:10:19AM +0300, Alexandre Prokoudine wrote:

On 11/17/05, Timo Steuerwald wrote:

Hey, that's a cool feature! Thanks a lot! I didn't know that up to now, but anyway: How does this automatical white balance work?

I don't know how exactly this engine ticks, but I can to to guess: it searches for the lightest pixel in the image and makes GIMP believe that it is white and then it shifts other colors appropriately. But that's my guess. Probably gimp-help package contains information you want to know.

I'd like to understand it better too, if someone has an actual explanation or links to documentation, please post (I searched a bit for gimp white balance ... not much in terms of explanation)).

I think the auto white balance is the same thing as Tools -> Color Tools -> Levels, then select "auto", but you have flexibility with the levels tool.

It looks like the auto leveling just sets each color such that they are evenly distributed, by setting "dark" and "light" tones (that is what the help menu calls them) such that they are at the low and high range of where each color shows up. Just play with it some and you can see what I mean ...

Read this for a good explanation on using the level tool:

http://world.std.com/~mmcirvin/gimp_tutorial/levels_curves.html

Try auto leveling, and then look and play with the setting for each of the three *color* channels (i.e. not the default "Value" channel, auto does not modify it). I'll often adjust the values towards their original values in order to get more accurate colors.

This must be similar to in-camera auto white balance. And even the in-camera auto white balance is sometimes wrong (and probably why you are trying to fix this in gimp ... or maybe you picked "sunny" white blance, and forgot to switch when moving indoors under incandescent light).

It would be nice if there were gimp level settings like "cloudy", "tungsten" etc., like we have for dcraw, but I guess those can only be applied to raw images (or to images with no modification to their white balance), else it would be somewhat a useless.

-- Patrick Mansfield