Pixels and Images
To use Photoshop effectively, you need to understand the basic attributes of pixel-based images: dimension, bit depth, and color model (which Photoshop refers to as image mode).
Pixel-based images are rectangular grids made up of little squares, like floor tiles; those little squares are individual pixels (see Figure 2-2). The dimensions of the pixel grid (pixel dimensions) refer to the number of pixels along its width and height. The grid of pixels that makes up your computer screen might be 1680 by 1050 pixels, while an image reduced for display on a Web page may be 600 by 400 pixels.
Figure 2-2 The grid of squares that makes up an image. The nonprinting pixel grid lines are visible in Photoshop CS4 when you zoom in over 500 percent.
The original pixel dimensions of an image are determined by the capabilities of the sensor in the digital camera or scanner that you use to create the image. For example, a 10-megapixel digital camera may produce an image that’s 3888 by 2592 pixels. The more pixels there are in an image, the more disk space it uses, and the more processing time it needs.
Pixel dimensions aren’t the same as resolution. You can’t actually know the resolution (pixels per inch) of your 3888-by-2592-pixel image until you declare how large you’re printing it. We’ll talk about this more in “Resolution,” later in this chapter.
Bit depth tells Photoshop how many shades or colors the image can contain. For example, in a 1-bit image (one in which each pixel is represented by 1 bit of information—either a 1 or a 0) each pixel is either on or off, which usually means it’s black or white. (Of course, if you printed with red ink on blue paper, the pixels would be either red or blue.)
With 2 bits per pixel, there are four possible combinations (00, 01, 10, and 11), hence four possible values and four possible colors or gray levels (see Figure 2-3). Eight bits of information give you 256 possible values; in 8-bit/channel RGB images, each pixel actually has three 8-bit values—one each for red, green, and blue—for a total of 24 bits per pixel. (In 8-bit/channel CMYK [cyan, magenta, yellow, and black] there are four channels rather than three, so a CMYK pixel takes 32 bits to describe.)
Figure 2-3 Bit depth
In Photoshop, you can work with bit depths up to 32 floating-point bits per channel in HDR (High Dynamic Range) mode. Most image editing is done at 8 bits per channel (24-bit RGB and 32-bit CMYK), while some high-end photographers have 16 bit/channel (48-bit RGB) workflows.
How many bits are enough? A bit depth of 8 bits per channel provides 16.7 million possible RGB color definitions, which is much more than the number of unique colors the human eye can distinguish, and certainly much more than the number of unique colors we can print.
Why capture many more colors than we can print, or even see? The simple answer is that the larger number of bits gives us more editing flexibility. As you’ll see in Chapter 7, “Image Adjustment Fundamentals,” every edit opens up gaps between some adjacent pixel values and smooshes others together, reducing the total number of shades.
Pixel dimensions and bit depth each tell part of the story, but the third essential attribute of images, the image mode, is the one that dictates whether all those numbers represent either shades of gray or colors, and how many.
In general, the numbers that describe pixels relate to tonal values, with lower numbers representing darker tones and higher ones representing brighter tones. In an 8 bit/channel grayscale image (256 levels per channel), 0 represents solid black, 255 represents pure white, and the intermediate numbers represent intermediate shades of gray.
In the color image modes, the numbers represent shades of a primary color rather than shades of gray. So an RGB image is actually made up of three grayscale channels: one representing red values, one representing green values, and one representing blue values (see Figure 2-4). A CMYK image has four grayscale channels: one each for cyan, magenta, yellow, and black.
Figure 2-4 Color images (like the one in the top-left corner) can be described with RGB data (top) or CMYK data (bottom). Note that for press reproduction, the red, green, and blue channels in this figure had to be simulated using cyan, magenta, and yellow press inks.
Indexed Color mode is an exception—its colors are not built using channels. In Indexed Color mode, each value represents a color from a lookup table, a list of up to 256 colors actually used in the image. Many Photoshop features don’t work in Indexed Color mode because of the arbitrary nature and limited number of colors available in the lookup table. For example, you can’t easily make a color area more blue, because the blue shades you want may not exist in the lookup table. Indexed color images have become much less common because indexed-color GIF files are increasingly being replaced by 24-bit JPEG and PNG images.