Tuesday, February 1, 2005

Using channels to get the best black and white from your color images


By David Gewirtz

Over the past month or so, we've been talking more and more about creating black and white photographs from color originals. While you can certainly desaturate an image, one of my favorite techniques is to choose among the various color channels and use the best-looking channel as the basis for my black and white image. I'll describe how that's done in this article.

Let's start with some very simple basics that'll help you understand what I'm doing. There's a very arcane science of color, which I'm not going to even attempt to discuss today. Most color photographs consist of a blend of colors, and the science of colors helps determine various ways those colors are represented. By far, the most common color mode is RGB, standing for Red, Green, and Blue.

In most Photoshop-like programs, these color mixes are represented in channels. Channels are grayscale images that store different types of information. For our purposes, it's enough to know that, in RGB color mode, there are four channels: red, green, blue, and a combined, RGB channel. Actually, you don't even need to know that much. All you really need to know is how to open up the Channels palette and click between the channels.

In Photoshop, you can open the channel palette, shown in Figure A, by selecting Channels from the Window menu.


The Channels palette shows the three channels and the mixed channel. (click for larger image)

Notice that the Channels palette looks very similar to the Layers palette. The eye icon shows what's visible on the screen, and the rows represent each channel. In the example above, all the rows are selected, meaning the image shown is the full color image.

Switching between channels is easy. Simply click one of the rows, and the image displayed will change. Let's start with a simple picture I took one day this summer, shown in Figure B.


Here's a full-color image to get us started. (click for larger image)

If I click the individual channels, we'll get a black and white image, but it'll be measurably different because different information is stored in each channel. Figure C shows all three variations, one above the next, so you can see how each channel compares.


Look at how the three channels compare. (click for larger image)

Notice that the main block seems much more clear in the red channel. Everything seems a bit more washed out in the green channel. And the blue channel seems much darker. In fact, in the blue channel, the truck and container in the upper right of the image stand out more, primarily because the detail of the trees is now in shadow.