How to Convert a Photo to Black and White in GIMP

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How to Convert a Photo to Black and White in GIMP

There is more than one way to convert a photo to black and white in GIMP and which you choose will be a matter of convenience and personal preference. It may seem surprising to hear that different techniques produce different results, however, that is the case. With this in mind, I'll show you how you can take advantage of the Channel Mixer feature to produce more striking black and white photos in GIMP.

Before considering the Channel Mixer, let's look at the easy way to convert a digital photo to black and white in GIMP. Typically when a GIMP user wants to convert a digital photo to black and white, they'll go to the Colors menu and select Desaturate. While the Desaturate dialog does offer three options for how the conversion will be made, namely Lightness, Luminosity and the average of the two, in practice the difference is often very slight.

Light is made up of different colors and the proportions of the different colors will often vary from area to area within a digital photo. When you use the Desaturate tool, the different colors that make up the light are treated equally. 

The Channel Mixer, however, allows you to treat the red, green and blue light differently within an image meaning that the final black and white conversion may look very different depending on which color channel was emphasized.

For many users, the results of the Desaturate tool are perfectly acceptable, but if you want to take more creative control over your digital photos, then do read on.

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The Channel Mixer Dialog

The Channel Mixer dialog does seem to be hidden within the Colors menu, but once you start using it I'm sure that you will always turn to it whenever you convert a digital photo to black and white in GIMP.

First, you'll need to open a photo that you'd like to convert to mono, so go to File > Open and navigate to your chosen image and open it.

Now you can go to Colors > Components > Channel Mixer to open the Channel Mixer dialog. Before using the Channel Mixer tool, let's just stop and take a quick look at the controls. Because we are using this tool to convert a digital photo to black and white, we can ignore the Output channel drop down menu as this doesn't have any effect on mono conversions.

The Monochrome tick box will convert the image to black and white and once this has been selected, the three color channel sliders allow you to tweak the lightness and darkness of the individual colors within your photo. The Luminosity slider will often appear to have little or no effect, but in some cases, it can help to make the resulting black and white photo appear more true to the original subject.

Next, I'll show you how different settings within the Channel Mixer can produce quite different black and white results from the same original digital photo. On the next page I'll show you how I produced a mono conversion with a darkened sky and then the following page will show the same photo with the sky lightened.

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Convert a Photo to Black and White with a Dark Sky

Our first example of how to convert a digital photo to black and white will show you how to produce a result with a darkened sky that will make the white of the building really stand out.

Firstly click on the Monochrome box to tick it and you'll see that the preview thumbnail becomes black and white. We'll use this preview thumbnail to see how our adjustments are changing the appearance of our mono conversion. Do remember that you can click the two magnifying glass icons to zoom in and out if you need to get a better view of an area of your photo.

Note that when you first click the Monochrome box, the Red slider is set to 100 and the other two color sliders are set to zero. To ensure that the end results look as natural as possible, the total values of all three sliders should total 100. If the values end at less than 100, the resulting image will appear darker and a value higher than 100 will make it appear lighter.

Because I want a darker sky, I've dragged the Blue slider to the left to a setting of -50%. That results in a total value of 50 meaning that the preview looks darker than it should. To compensate for that, I need to move one or both of the other two sliders to the right. I settled upon moving the Green slider to 20, which lightens the foliage of the trees a little without having to much effect on the sky, and pushed the Red slider to 130 which gives us a total value of 100 across the three sliders.

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Convert a Photo to Black and White with a Light Sky

This next image shows how to convert the same digital photo to black and white with a lighter sky. The point regarding keeping the total values of all three color sliders to 100 applies just the same as before.

Because the sky is predominantly made up of blue light, to lighten the sky, we need to lighten the blue channel. The settings I used saw the Blue slider pushed to 150, the Green increased to 30 and the Red channel reduced to -80.

If you compare this image to the other two conversions shown in this tutorial, you'll see how this technique of using the Channel Mixer offers up the ability to produce very different results when you convert your digital photos to black and white in GIMP.