MuseBlog Chautauqua*: Introducing (Graphics) Channels
Enceladus asked:
“Do they have channels in whatever graphics program you use? And if so, could you explain them? Because, essentially, there’ the one thing (aside from actual scripting) that I need to know to proclaim myself a GIMP wizard.”
Step inside our virtual tent to learn more about channels. Or just enjoy the pictures, if graphics aren’t your thing.
[*Chautauquas began in the 19th century as a sort of summer camp approach to continuing education. The events were often held in big circus-style tents. The name comes from the New York town where the first one took place. We’re borrowing the term as a convenient umbrella title for such occasional threads as might be more information oriented than the usual. Though really we simply like the name.]
At the most basic level, channels are greyscale maps that tell the hardware how to display or print your images. The number and content of channels in a given picture depend upon the image mode in which you’re working — more often than not, that will be RGB unless you’ve changed it. RGB stands for the primary colors of light: red, green, and blue. (Mixed together they make white.)
Each color has a channel. Each channel is represented by a greyscale image that measures out the amounts of that color to let through. Think of white as “on” and black as “off.” Shades of grey dole out all the mixed amounts between.
Here’s a typical layout, with a colorized version below.
The fifth row in each picture is an alpha channel, which I’ll return to later. First, here is a larger version of the three color channels, so you can see the differences better. Notice the man behind Midnight Fiddler on the right. His shirt contains a lot of red, so it is much lighter in the red channel than in the others. By comparison, Grant’s blue shirt requires more blue to show through so it is lightest in the blue channel. (Since the color is fairly light to begin with, none of the channels are very dark. Remember that the three colors combine to form white.)
Sometimes the contrasts can be quite dramatic, as in this example:
Even when at first glance the differences among channels are subtle, if you shuffle the color assignments around, you will see the differences are significant:
Alpha channels serve a different function. Unlike the color channels, which are essential components of every image file, alpha channels are added only when the user performs certain actions, such as saving a selection or creating a mask. In the example above, I made a mask to separate the image of Grant from the background. Here’s another way of showing the same thing (with the channels included for comparison). When the mask is active, it blocks everything it covers from being edited.
By the way, color channels can do more than simply reflect what you compose in the document window. They can be edited directly too. They have dozens of practical uses for sharpening, masking, and so forth. But they can be edited for special effects as well. Just for fun I applied different filters to each of the channels in the demonstration photo:
And, as is the case with so many things in life, channels provide new opportunities for bunnification:
Date: July 5, 2010
Categories: Sound and images, The Universe
Even though I pretty much just enjoyed the graphics, that was still very cool. Except for the bunnification, maybe…
