UNDERSTANDING PLACED IMAGE RESOLUTION

We've talked a little about issues with low-resolution images and why it's important to maintain links to the original graphics files. Part of the assumption there is that you will want to output the documents you are preparing in InDesign. But output them where? And how?

These are questions you'll have to answer yourself, and some a commercial printer will have to answer for you. Before you send a document to print, it's important to know what your printer expects from you and needs to print your document as you want. One specification you will need to know is resolution.

Resolution is communicated in terms of pixels per inch, or ppi. The measurement is literally how many pixels are contained within an inch of display on your page. Lower numbers mean fewer pixels. Fewer pixels mean less information being displayed. Depending on your output devicea computer monitor for web pages, a laser printer for interoffice communications, a printing press for your corporate magazineyou need more or less resolution for your images to print correctly.

In general, commercial printers need anywhere from 150300 ppi to print an image at high quality. If you are printing documents on a desktop printer, you may print documents with images from 72 ppi or higher, depending on the output resolution of your printer. Most monitors do not display images higher than 72 ppi or 96 ppi, so any document being created for strictly onscreen display can contain lower-resolution images without losing display quality. Figure 28.3 shows an image that does not have enough resolution to display properly; you can see the jagged edges along the curves of the water droplets.

Figure 28.3. Images that are placed without sufficient resolution for the output device will be pixelated when they print in your final document.

When you create graphics or save images in Illustrator or Photoshop, you'll be able to set the resolution of the file. Resolution is closely tied to image size: a 1- by 1-inch image at 100 ppi displays at 50 ppi when enlarged to 2 by 2 inches. Wherever you set the resolution in your graphic-editing program, know that you must keep the image at 100% or smaller when you place it in InDesign. If you enlarge the graphic, you lose resolution.

Fortunately, InDesign gives you a good tool to track image resolution within the program. The Info palette displays the actual ppi and effective ppi of a selected graphic (see Figure 28.4). The actual ppi tells you the resolution at which the graphic was originally saved. The effective ppi tells you the resolution of the image as it prints or displays. Watch the effective ppi of graphics in your document to ensure you get the output you want.

Figure 28.4. The Info palette is a good source of information about items in your document, including resolution of placed images.