Graphics File Formats
Perhaps the most difficult thing for people new to print production to get their heads around is the incorporation of graphics into their projects. Because the requirements of the computer screen are very different from the requirements of a printing press, the process often goes awry at this stage. A graphic that might look just fine on the monitor may lack sufficient resolution to print correctly, or might not print at all. Here’s a primer on preparing images for print:
Bitmap (Raster) Graphics
Most people are familiar with bitmap images. They’re made up of tiny dots, and are produced by scanners and digital cameras. All of the pictures that you see on the Internet are bitmap images. Most people have some experience creating and editing graphics in programs like Paint, Photoshop, or GIMP.
The best file formats for bitmap graphics are “lossless”, which means that they contain the maximum amount of information for the computer to use in displaying them. Tagged Image File Format (.tif or .tiff file extension) and Encapsulated Postscript (.eps) are generally regarded as the best bitmap files for print use. The least favored option is the common .jpeg file (named for the Joint Photographic Experts Group, inventors of the format) because it’s optimized for on-screen display, and sacrifices quality for a smaller file size. Use a .jpeg file only when there’s no other alternative.
Resolution is the density of dots that make up a bitmap image, measured in pixels per linear inch (dpi). It’s a ratio – if you double the size of a bitmap image, you will reduce its resolution by half. If the resolution in your image is too low, you will see the individual dots in the printed output. This is called “pixellation”, and is a big problem when using images taken from the Internet or other sources destined for display on the screen. The general rule is that you need half the resolution in your image as the final output will be. If your page will be printed at 300 dpi, your image needs to contain at least 150 dpi of information. If your job is to be output at 600 dpi, your images need to be at 300 dpi.
If you’re submitting bitmap files to a service bureau, ensure that you set the mode to “CMYK” in your editing software. This will save some headaches later.
Vector graphics are made up of lines. They’re completely unsuitable for photorealistic images, but fantastic for cartoons, illustrations, and corporate logos. They’re generally much smaller files than a bitmap, and are scalable, which means that no matter how big you make a vector image it won’t pixellate like a bitmap. Programs like Adobe Illustrator produce vector files with a variety of extensions, very commonly .eps and .ai. When you’re adding corporate logos to your project, always ask the person providing them if they’re available in a vector format.
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