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Tutorials-Articles: Converting an image to B&W

23/06/2009

Converting an image to B&W

When converting an image to B&W, The Gimp gives you an array of options to get the results you want.

Each one has its pros and cons.

Convert when importing with UFRaw

If you work with RAW files (NEFs in Nikon’s world), UFRaw will give the first opportunity to do this, but I find the options quite simple: basically you can play with the saturation, so I usually import the NEF in full color and I make the conversion inside The Gimp:

We’ll suppose that we don’t do the conversion in UFRaw (it has more advanced options than just playing with the saturation. More on this on another tutorial).

Once you have the image in The Gimp, you have several choices.

The most straightforward is to use the Image > Mode > Grayscale menu. This menu will convert your image to BW by computing the weighted-average gray for every pixel of the picture.
As you know, the image in the computer is created by mixing Red, Green and Blue colours. For example, full Red, full Green and full Blue is white.
When you use the Grayscale menu, the Gimp will do the following:

RedValue*21% + GreenValue*72% + BlueValue*7%

In Gimp 2.4, it use to be RedValue*30% + GreenValue*59% + BlueValue*11%

This will give it the grayscale value. The weights have their explanation. The colour that the human eye “sees” better is Green, followed by Red and finally Blue. Thus, we give more importance to the colours we “see” better, yielding better visual results this way.

While this is a really simple method to convert your image to B&W, there are more interesting ways to do it. Everything, but, spans from such a calculation, that is: playing with the weights of each colour.

Color > Components > Decompose

There is a plugin in The Gimp that breaks down the image in 3 (or more) different images, one for each component of the image.

When using this menu, with the default options (RGB), you get an image with 3 layers, each one containing a channel of the image, converted to gray. You can keep the one you want. Usually, the Green component has the best image for the reasons explained above.

The plugin has some other options:

For the record:

V = max(Red,Green,Blue)
S = max(Red,Green,Blue) - min(Red,Green,Blue)/max(Red,Green,Blue)
H = Depends on which one is the maximum (Red,Green or Blue)

The “Foreground as registration color” option leaves the pixels that have the foreground colour as black in the split. I cannot figure out a use for this…

Desaturate

You can also use the Colors > Desaturate menu. This will display the following window:

You just have 3 different options.:

The results are different in each image, so it’s difficult to recommend a method. Just test, it’s easy :)

Channel mixer

The Channel Mixer (Colors > Components > Channel Mixer) is the most powerful tool to convert to B&W. It allows you to set the weights of each channel.

The Channel Mixer has more uses. We’ll concentrate on B&W conversion, so we must click on the “Monochrome” checkbox.
Now we can play with the sliders, to set the weight of each component in the conversion.

Note that we can set the weight to more than 100% (up to 200%), so we can end up with a very bright image.
If we click on “Preserve Luminosity”, the program takes care that the sum of all the values is lower than 100%, keeping the relative weight of each channel.

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Page last modified on 25/06/2009