Digital Prints

There are three things you need to know about bitmaps to fully understand the nuances
of printing digital images: pixels and bit depth, resolution, and halftoning and dithering.
(Color is another issue, but because it’s such a huge subject, it gets its own chapter—
Chapter 4.) Let’s take them one at a time.
Pixels and Bit Depth
Pixels are the basic elements that make up a bitmap image. Pixels actually have no shape
or form until they are viewed, printed, or otherwise “rendered.” Instead, they are little
points that contain information in the form of binary digits or “bits” (ones and zeros—a
“0” represents something, a “1” represents nothing or empty space). Bits are the smallest
unit of digital information.
A 1-bit image is the lowliest of all bitmaps. There are only two digits to work with—a 1
and a 0, which means that each picture element is either on or off, black or white (I’m
keeping this to a simple one-color example to start with). But a 2-bit image is much more
detailed. Now you have four possibilities or values for each pixel: 00, 01, 10, 11 (black,
white, and two shades of gray). Keep going, and you see that three bits yields eight values,
four bits 16, eight bits 256, and so on (see Figure 2.3). In mathematical terms, this is called
the power of two: 2 2 equals four choices (2 × 2), 2 8 is 256 choices (2 × 2 × 2 × 2 × 2 × 2
× 2 × 2). Generally speaking, a one-color digital image needs to be at least 8-bit (256 tones)
to be “photorealistic” or “continuous-tone” in appearance. Study the eye image variations
in Figure 2.3, and you’ll see what I mean.
Chapter 2 ■ Understanding Digital Printing 39
Figure 2.3 The more bits, the more
realistic the image.