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Fundamental Concepts: Channels
Before we start looking at some of the advanced ways that images can be controlled an manipulated in your image editing software, we need to cover a few fundamental concepts such as channels, histograms, and colour theory. Understanding these will enable you to get much more out of the sections that follow, and ultimately much better results.
Channels
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All images store colour information in channels - a JPG image as produced by your camera has three channels: red, green and blue. When you look at the full colour image on the screen you are actually seeing a composite of all three channels which appears in the Photoshop channels palette (right) as the RGB channel, although it is not actually an independent channel.
Another possible format for images (used by the printing industry) is CMYK. This creates the image from 4 separate channels - cyan, magenta, yellow and black (K). If you have an inkjet printer you will probably be familiar with these four colour as the ink cartridges you need to buy. |
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If you look at an individual channel, you will see it looks just like a black and white image. This is actually showing you the lightness of that individual colour (i.e. white in the green channel means fully saturated green (it just doesn’t get any greener than this). The illustration below will hopefully make this clearer.
A colour seperation is a grey scale image that represents just one of the channels in the full colour image.
If you pick out a red toy brick in the combined RGB image and find the same brick in the colour separations, you will see that it is very light in the red channel (indicating a large amount of that colour), but nearly black in the green and blue channels. A yellow brick appears light in both the red and the green channel as red and green combine to make yellow.
Many of the functions and filters that you can apply in photoshop can be done either on the whole image (i.e. the RGB ‘composite channel’) or on an individual colour channel. For example is an image has a reddish cast that you want to remove, you could reduce the brightness of just the red channel.
If you convert an image to greyscale, you are combining the 3 channels into a single channel image which contains only the luminance (the perceived brightness) of each pixel. See the ‘Nerd Box’ in the next section for a fuller explanation of brightness and luminance.
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Photographs
This is a site about photography so I'm sure you are expecting to see plenty of pictures.
For now, why not take a peek at the flickr galleries belonging to the two authors of this site.
Colin's Flickr Page
Phil's Flickr Page
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