Desaturating properly

This guide will explain how to achieve a good grayscale version of a colored image. Since the normal "Desaturation" option that can be found in Gimp via "Colors" -> "Desaturate" isn't that sophisticated and customizeable we will be using the Channel-Mixer instead. This one can be found at "Colors" -> "Components" -> "Channel Mixer". But first things first.
We need a image that we want to desaturate. I will first go into detail with the first image to state an example and show some further results with other pictures. So open up an image like this one for example:



Now go to "Colors" -> "Components" -> "Channel Mixer" and check the box that says "Monochrome".
(It might come handy if you also activate "Preserve luminosity" to avoid over exposed spots in the pic)



Now you can convert the three color channels into greyscale an the values will be added which will be representing the brightness of a pixel. The cool thing is that the Channel-Mixer also accepts negative values for the channels which will result in a substraction of brightness. What is important while looking for the right settings is that different colors in the pic will be represented by different grey-values to maintain the difference in color in the desaturated image. Let me show you an easy example what I mean:




The three different colors are made to the exact same grey since in the HSV color profile they have got the same V-Value which stands for the brightness and this value is simply taken for they greyscale which let's one asume that it also has been the same colour as before.
I fiddled a bit around with the channelmixer and this is what I can come up with concerning my example picture:


(Original, Desaturate, Channel-Mixer - from left)

As you can see the difference in this pic isn't _that_ huge but let's check out what we can do with the following pics:


(Original, Desaturate, Channel-Mixer - from left)

(Original, Desaturate, Channel-Mixer - from top)

Especially the last example shows the advantages in some cases. The conventionally desaturated image does barely show the clouds. But the image achieved with the Channel-Mixer preserves a certain contrast in the important parts of the image (i.e. sky). But still it massively depends on your source image which method results in better greyscales. You need to find it out via trial and error and much experience.