Gimp 101: Converting to Black and White

June 29, 2009

I take all my photos in color. But I’m in love with a good black and white photo now and then.

You might be thinking, “oh, c’mon iffles, I’m not that stupid, I saw the Desaturate in the Colors menu. I know that makes it black and white! I don’t need your help!” And, ok, if you want to settle for what GIMP thinks are the best settings for black and white – go for it.

I, on the other hand, like a bit more control. I like using the Channel Mixer. Here’s the photo I’m going to use as an example. It’s a picture of our friend Josh (”our” meaning John and myself. Chances are, Josh isn’t your friend). It was taken the morning he got married! There was really no reasoning behind picking this photo, I just kinda randomed into it in my collection and decided to play with it.

Josh in Color

First let me show you what the photo looks like just using Colors –> Desaturate. If you bring up that dialog box you’ll see you have three options – Lightness, Luminosity and Average. Let’s check out Lightness:

Using Desaturate - Lightness

Hmm… It looks kinda flat, Josh is the subject of the photo and he has hardly any contrast. We could fix this by playing with contrast and levels, but we shouldn’t have to. Let’s check out Luminosity:

Using Desaturate - Luminosity

Wow, that’s worse! I didn’t even know it was possible! Let’s do this ourselves with the Channel Mixer, which you get to by going to Colors –> Components –> Channel Mixer (at least, you do in my version of Gimp. I understand it’s been moved a few times, so if you can’t find it, leave me a comment with what version of Gimp you have [which you can find by going to Help --> About], and I’ll help you find it)

You should now see this dialog box:

Channel Mixer

Be sure to check the Monochrome checkbox near the bottom there, and now you can determine how much of the Red Filter, Green Filter and Blue Filter will go into your monochrome image. Typically you would adjust these so that the percentages add up to 100%. It’s not necessary, but the experts say to do it, and they’re usually right – if I slide my sliders so they don’t add up to 100% the brightness of my image seems off. That being said, sometimes you want that effect, so don’t be tied down to the 100% thing.

…so… uh, what exactly are these filters and how do I know what percentage to put them each at? Well, the easiest explanation is that the red filter is turning everything Red into White, and everything that’s exactly the opposite of Red into Black. Honestly? I can never envision what any of them are going to do, so I always try each one at 100% (leaving the others at 0). Like so:

Channel Mixer - All Red
All Red

Chanel Mixer - All Green
All Green

Channel Mixer - All Blue
All Blue

After looking at these, I can see that I like All Red the best, and All Blue the worst. To me that means I want the highest percentage with my Red slider and the lowest with my Blue (on a side not, my blue slider is almost always the lowest, if I even set it all. It’s always the red and the green competing for the top slot) I’m sure there’s a technical way of figuring out what sliders to put where by looking at your photo and thinking “well, his shirt is green and I want that to be dark”, but honestly, I just figure out which one I like best and then play around with my numbers to make ‘em add up to 100. First, I typically set my Blue around 10. I hate leaving it out completely, I feel bad for Blue, I don’t want Blue to feel useless. Then I start futzing around with different combinations of the Red and Green channel that add up to 90 – always giving Red the higher number since I liked that channel at 100% more. I finally settle with 70/20, so my final Channel Mixer looks like this:

Channel Mixer - Final

Making my final image look like this:

Final Image

That’s a big improvement over the default that Desaturate gave me, in my opinion – Josh’s face looks a lot softer, and there’s more contrast between his face and his shirt. I know this is more work than just doing the Desaturate, and I admit to “cheating” – if doing one of the Desaturate options gets me close to what I’m looking for, I’ll just do it and then adjust the levels to get even closer to what I want, but if it’s something I want to show to other people and/or print, I’ll always use the Channel Mixer, just to be more confident about what I’m getting.

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