Black and White digital photo processing part 2: using Raw Therapee

Raw Therapee, an open-source program for digital photo processing, has both simple and sophisticated tools for black and white images.

This post is a follow on to Black and white digital photo processing-part 1. In that post I describe how black and white digital images are formed from the three color channels: red, green and blue.

The link for Raw Therapee’s excellent documentation is: RawPedia.

Raw Therapee allows you to work with color using different models: the RGB model, native to your camera, and the L*a*b model. The L*a*b model models how the human eye and brain tend to see color. A good reference for learning more about this is: Seeing in Colour: How our Eyes Sense and Cameras Record.

Ways to create black and white using Raw Therapee

In Raw Therapee you can transform an image into black and white in different ways.

#1: Remove saturation: In the Exposure tab simply take the saturation slider to -100%.

#2: Remove chromaticity: In the L*a*b Adjustments tool, at the bottom of the Exposure tab, slide the “Chromaticity” slider to -100%. As you can see, this is not exactly equivalent to #1. This is just the “L” channel.

#3: Use the Black-and-white tool in the Colors tab. This sophisticated tool has a lot of options.The Rawpedia entry for this tool is called “black-and-white addon“.

Black-and-white tool options

The tool gives you a choice of three methods. Before describing the methods: Here are three tools they have in common:

Gamma.

Gamma emulates the effects of rendering on paper. It is not a channel mixing tool for the content of the image. The channel effects add together, so you can get a very,very dark or very,very light image if you choose extreme values for all three channels.

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‘Before’ curve

This curve allows you to apply a tone curve to the image before the chosen black-and-white method is applied. The Tone curves article in RawPedia describes how the curve works. This tool can give the image a customized boost in contrast before desaturating it.

‘After’ curve

Here you specify a curve to be applied after the chosen black-and-white method. There are no choices for mode, e.g., standard or film-like, as the image is in black and white. Use this tool to fine tune the tones after all black and white adjustments are made.

Available methods in the Black-and-White tool

Desaturation

Desaturation uses a simple algorithm combining the RGB channels for a neutral gray image: 0.299*red + 0.587*green + 0.114*blue.

Luminance equalizer

This tool allows you to pick specific colors to be lighter or darker. You will notice the base image is different from the Desaturation method, because the formula used is 0.333*red + 0.333*green + 0.333*blue.

Select the color you want to change in one of two ways:

First: locate the point corresponding to the color you want to change. You click and hold that point and move it upwards to lighten the color and downwards to darken it. Add intermediate points by clicking on the color line, you can then pull the new point up and down to adjust.

Second: use the pipette tool to choose the color you want to change on the image. Hold the left mouse button down then press the ctrl key and drag the mouse away from you to lighten the color and toward you to darken it. (Click the right mouse button to turn off the pipette tool when you are done with it.)

Channel Mixer

Hard to know where to start with this tool. Depending on the option you select, it can be either simple or complex Here is what you see when you select this method:

Presets

There are 15 presets as I write this. Of them, 11 are preset percentage values for the channel mixer.

Name of preset	% red	% green	% blue
Normal contrast	40.6	31.1	28.3
High contrast	29.9	25.4	44.8
Luminance	30	59	11
Landscape	56	24	10
Portrait	49.1	40	10.9
Low sensitivity	27	27	46
High sensitivity	30	28	42
Panchromatic	33.3	33.3	33.3
Hyper panchromatic	41	25	34
Orthochromatic	0	42	58
Infra-red	-28	139.9	-11.9

These presets are simple to use: select a preset then experiment to see if you like one of the color filters. If, after you try one of these, you feel like you are close but want to fine tune, choose one of these four presets: Absolute RGB, Relative RGB, Absolute ROYGCBPM, Relative ROYGCBPM. That will move you into more sophisticated tools with the preset you chose already set.

Channel Mixing and Special Effects using the RGB and ROYGCBPM preset options

The other four presets, Absolute RGB, Relative RGB, Absolute ROYGCBPM, Relative ROYGCBPM, are more complex. Each allows you to input specific values for the red, green and blue channels. And the ones labeled “ROYGCBPM” let you adjust specific colors as well.

Both RGB and ROYGCBPM have a relative and absolute option. When you choose relative, the program recalculates the percentages for the channels so the total is always 100%. This keeps the overall brightness of the image constant. The “absolute” choice allows the total to vary. If the total is less than 100% then the picture will be darker than the original; if it is greater it will be brighter.

In the above example I made a small adjustment to the purple (P), you can see that the total percentage is greater than 100%. If I had chosen relative the program would have adjusted values and the image would have been darker.

I haven’t really got a handle on the way the algorithm for OYCPM (the colors between red, green and blue) works. But it is not difficult to play around.

In conclusion

Raw Therapee has tools that allow you to create great black and white digital photos. You can get great results simply using presets, but it also has sophisticated tools to really fine tune your images.