OK, I got a little bit fed up with software “pseudo reviews”, so let’s talk about photography for a change… Oh! wait…
The case for black and white
Black and white was the first type of photography available and even with color becoming widely used, B&W still has this “something” that makes it invaluable. Sebastiao Salgado said in a recent interview “my photography is black and white. I don’t even know where I stored the few color slides I shot years ago”.
So while color photography focuses and on, well, colors and tints, B&W strongly reveals composition and lines of force. So in the digital age, B&W still has a crucial role to play in photography – the purpose of this entry is to see how to obtain the best result in converting a color file to B&W.
Obviously, the files we get from our camera is in color and needs conversion. So here is a color (Red-Green-Blue) file:
And here is the desaturated version standard from The Gimp:
But that is not the end of the story. Here is the red channel…
here is the green channel…
… and here is the blue channel
Just for fun, here is the light channel (from hue / saturation / light decomposition)
So when we talk about converting an image to B&W, there are really several possibilities with very different results.
So what do we do?
Basically, there are two main ways to get a good result. For high quality, let’s forget the decompose option since we don’t know what it exactly does.
- Using the Light channel: you use this channel as your B&W starting point. You can then post-process it as normal with curves, local contrast USM and so on. This method needs a bit more curves adjustments than mixing channels but its advantage is less noise (so it supports more curves adjustment, precisely)
- Mixing RGB channels: you combine different proportions of the red green and blue channels to a B&W image.
The Gimp 2.4 offers a “channel mixer” which you can use to blend your channels and check the output “live”. Since a digital sensor captures 25% of its pixels in blue, 25% in red and 50% in green (through a bayer filter) the blue and red channels are more noise prone than the green. So even if it is tempting to put a lot of the red channel since it generally gives the most dramatic result, noise can become an issue with too much red.
Hence a general rule of thumb is to use green to 70-80% the rest of red; blue is generally too unnatural and can be ignored. So here is what our example would look like in The Gimp channel mixer:
Make sure you check the “monochrome” option, obviously 🙂
The Gimp allows values from -200 to +200 for each channel. I don’t really see what to do with negative values; in most cases, you want to make sure the addition of R+G+B = 100%, say red 20% green 80% blue 0%. If you tick “preserve luminosity”, you will get a 100% output no matter what – so red 0% green 0% blue 1% will in effect be 100% blue. I know that is a bit confusing but (dare I say?) don’t hesitate to experiment.
8bits / channel thing bites again, and badly
Now the problem is that The Gimp only processes images at 8bits / channel and B&W really is… one channel. So your B&W picture is really processed in The Gimp with 256 levels from its pitch dark to pure white nuances. Add to it a bit of curves adjustment and you are left with 220 levels. Not good at all.
As you now know, Cinepaint is the solution that allows 16bits / channel on Linux – about 65000 nuances per channel, what we need to keep all the B&W richness. However, Cinepaint being a bit more “primitive” than The Gimp, it doesn’t offer a channel mixer. You have the solution to decompose the image to its red, green and blue components as three separate (B&W) images, which you can then copy / paste to three different layers. By playing with the transparency of these layers (between 0% and 100%), you have a very nice channel mixer substitute. A bit of a pain to set up, but very usable.
So my 2 cents would be: play around with The Gimp to see how your image works in B&W and once you are happy with the mix values, just fire Cinepaint to get the final version. It is absolutely critical for good B&W output.
Since a B&W image can be stunning it deserves the best possible quality.