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Prepare a publication for commercial printing

3-2 Bitmap Graphics

Bitmap graphics are made up of pixels:

Callout 1 When viewed normally, the bitmap picture looks like a photographic image.
Callout 2 When you look closely at an area of the bitmap, you can clearly see the pixels that make up the picture.

Bitmap graphics are typically created using paint type programs, by scanning a piece of art, or by taking a digital photograph. Bitmap graphics are made up of pixels, which are individual, uniformly sized squares in a grid.

In most cases, when you create a bitmap graphic in a paint program or scan it, you can set the following:

  • Resolution    This is the number of pixels per linear inch in the bitmap. This is usually expressed as ppi, but is often referred to as dpi (dots per inch) as well.
  • Bit depth     This is the number of bits — or binary pairs — in each pixel. For example, a black and white bitmap consists of pixels with a single bit (21), which is either on (white) or off (black). A grayscale bitmap is usually 8-bit (or 28) providing 256 levels of gray. Color bitmaps are usually three 8-bit channels (for red, green, and blue) that combine as 24-bit (224) providing 16,777,216 colors.

The two important considerations for bitmap graphics are resolution and bit-depth. Resolution refers to the number of pixels per inch (PPI). The more pixels per inch, the more detail is possible because each pixel defines a smaller part of the whole picture.

Bit-depth refers to the range of color possible for each pixel. Like resolution, bit-depth is also important for detail in pictures. The greater the bit-depth, the wider the range of colors or shades of gray in the picture.

The higher the resolution and bit depth, the more detail you will get in the picture. However, high resolution will increase file size, which could make for longer print times.

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