If you are taking fotos indoor, the light there will be of a different temperatur and colour than the light outside. This can lead to interesting colour effects, but aside of those effects, you normally want to capture the whole scene in a constistent light. The white balance can not handle two kind of lights so you will have to adjust the scene flashing (which won’t help always) or covering the windows completely or with coloured material. If you don’t want or can’t to do this and you only have some highlighted areas with the light you don’t want, there are possibilities to adjust this light colour in post processing with GIMP or any other decent image editor. Here are the steps I used to correct the bright areas in the lower part of this photo of a staircase in paris.

The photograph with and without light adjustment

GIMP Layers

1. Determine the colour you want to use to adjust the light. You can use an average colour of the while image or part of the image which fits best to the surroundings of the areas you want to correct.
2. Create a layer with layer mode “Color” overlaying the background layer.
3. Determine the areas you want to adjust. Create a black layer coloring it white where you want the colour correction to be applied. This can be done using the “Threshold” tool or something similar. The use of the channel of the colour you want to adjust only, in my case the blue channel, may give very good results. If you have too much time, you can do this manually, too. Smooth the edges using Gaussian Blur filter and use this layer as layer mask for the colour layer you created before.

The layer mask used, before and after blurring it

This photo on Flickr.

Any comments welcome.

Update: Interesting, John Arnold of Photowalkthrough.com is doing the same with a similar technique in lightroom in his latest podcast.

Especially if you want to convert photographs to black and white and create a nice dark sky, it may happen that you will have some noise in the sky, even if you shoot with low ISO. This happens if you use a channel mixer or a similar tool for black and white conversion. As the sky is (often) blue, it will appear darker in the black and white version if you reduce the blue channel. But in general you will have more noise in the red channel than in the blue and the green channel.

Of course this depends on the sensor type and the camera hardware and software you have. This per channel noise phenomena is very deeply documented on libraw.org, I recommend to read it, if you are interested in the technical stuff of (your) digital camera. There is another article there testing the results regarding channel noise with a magenta filter.

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With the new versions of GIMP there is a menu called “GEGL Operation…” with a bunch of more or less interesting image operations. GEGL will be the graphics library for the future GIMP releases and brings 16bit support among other features to GIMP, when the transition will be completed.

GEGL c2g dialog with default settings

Within those GEGL Operations there is one operation called simply c2g (color2gray). Applying c2g is quite slow, but the results are at least interesting. It seems to be similar to the HDR and tone mapping hype of over-processing images and the results are similar, as this is a kind of tone mapping, just with black and white conversion applied.

There are a lot of bad examples tagged c2g on Flickr. Have look at this images. Doesn’t it remind you of the HDR groups in Flickr (or wherever)? Horrible halos, overall dull pictures as the high contrast and lighting is the same in all areas of the image. Aggressive, because of the high contrasts and because of the business of the subjects.

This method was tested by Rolf in his Meet The Gimp Podcast and Joel Cornuz is blogging in his Linux Photography Blog about it with the title “Could this be the ultimate black and white converter??”.

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I am doing a lot of black and white postprocessing lately, inspired by some of my photos, by other photos I saw and by the two books about black and white photography and post processing I read. They are writing a lot about toning black and white photographs in those books, especially with selen and sepia colors. This lead me to experiment a little bit with GIMP to try to get similar effects.
There are some toning examples and tutorials about how to tone photographs with one additional color (apart from black), and there is a GIMP script written by Alexios named “Duotone” doing this.

I took a similar approach and tried to tone black and white photographs with both, selen and sepia color, at once. The idea is to tone the darker areas with one and the brighter areas with another color keeping the completely white areas white and the black areas black. Using this technique carefully you can increase visibly the contrast of black and white photos without making them look colored. In some cases the observer won’t even notice that there is any color in the image while not looking explicitly for color. Additionally it will give a bit of old (or “darkroom”) feeling to the images.

Here my steps explained shortly for GIMP, should be easy in other applications supporting layers, masks and layer modes, too.

• Create a layer and fill it with the color you want to use for toning the brighter areas and set it to “Color” mode. Add a layer mask with the grayscale of the original photo to it.
• Create another layer, fill it with the dark toning color and set it to “Color” mode, too. Then add a layer mask to this layer using the negative of the mask used before.
• Reduce the opacity of both layers to something between 5 and 30. If you use saturated fill colors you won’t need more normally. Use it carefully.

You can modify the curves afterwards. The duotone script works with a toning curve with its hightest point in the middle and going down to 0 at the very left and the very right side. Instead of masking the darker toning color with the negative of the image, you could use the normal image there too and then apply curves like the curve used in the duotone script, but moved a little bit to the left (or to the right for the brighter toning color).

By the way, the sharpening layer used in this file is set to “Value” mode. This way you sharpen the image without touching the original layer nor the toning layers. You can download the sample GIMP xcf image file I created. And the border is created with Alexio’s Full Frame Script.

Based on the duotone script mentioned above I created a script for the GIMP which does exactly those steps. As default color values I used the more saturated colors of a selenium/sepia tone and a bluish, like cyanotype. After running this script you’ll need to adjust the opacity of the toning layers.

• tri-tone.scm

You can install it the same way as Alexios is documenting for his script.

Rolf, who is hosting the Meet The Gimp Video Podcast will perhaps include this technique in one of his shows.

Update: I corrected what I said about selenium. Selenium will give a brown-reddish tone, not a cold one. See http://en.wikipedia. … graphic_print_toning and the documentation about Ilford Multigrade Paper. Here you can find a lot of possible toning colors. Thanks, Frank, for the hint.
Although I am a bit irritated as I find a lot of bluish photos tagged with “selenium”…

Update 2: Rolf wrote about this post. From the comments there I got another link to a site of toning examples: http://www.gimpguru. … orials/SampleToning/. Great.