Real World Adobe® Photoshop® CS2 [Electronic resources] : Industrial-Strength Production Techniques نسخه متنی

Pixels and Images

Photoshop lets you do all kinds of nifty stuff, but at its heart, it pretty much all comes down to pixels. To use Photoshop effectively, you need to understand the basic attributes of pixel-based images.

Every pixel-based image has three basic characteristics: dimension, bit depth, and color model (which Photoshop refers to as image mode).

Dimension

Dimension is the attribute that is loosely related to size. Pixel-based images are always rectangular grids made up of little squares, like checkerboards or chessboards or parquet floors in your kitchen; those little squares are individual pixels (see Figure 3-1). The dimensions of the pixel grid refer to the number of pixels along its short and long dimensions. A chessboard is always eight squares by eight squares. The grid of pixels that makes up your computer screen might be 1200 by 1600. A 6-megapixel digital camera typically produces an image that's 3072 by 2048 pixels.

Image dimensions are limited by the capabilities of your capture device, the amount of available storage space, your patiencethe more pixels in the image, the more space it needs, and the longer it takes to do things to itand in Photoshop, by the 300,000-by-300,000-pixel image size limit.

Dimension is only indirectly related to physical size or resolution: Until you specify how large each pixel is (called "resolution") an image has no specific size. But resolution and size aren't innate to the digital image; they're fungible qualities. That is, the same 100-by-100 pixels can be made as small as a postage stamp or as large as a billboard. We'll discuss resolution and why it's important in more detail later in this chapter.

Bit Depth

Bit depth is the attribute that dictates how many shades or colors the image can contain. Because each pixel's tone or color is defined by one or more numbers, the range in which those numbers can fall dictates the range of possible values for each pixel, and hence the total number of colors (or shades of gray) that the image can contain.

For example, in a 1-bit image (one in which each pixel is represented by one bit of informationeither a one or a zero) each pixel is either on or off, which usually means black and white. (Of course, if you printed with red ink on blue paper, the pixels would be either red or blue.)

With two 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 3-2). Eight bits of information give you 256 possible values; in 8-bit/channel RGB images, each pixel actually has three 8-bit valuesone each for red, green, and blue (see Figure 3-3), for a total of 24 bits per pixel. (In 8-bit/channel CMYK there are four channels rather than three, so a CMYK pixel takes 32 bits to describe.)