Combining vector graphics and bitmap imagesMany of the tools and commands in Illustrator focus on vector drawing; however, Illustrator also provides a variety of ways to incorporate bitmap images in your artwork. You can import bitmap images into an Illustrator document using the Open, Place, and Paste commands. (See Importing artwork.) You can also create bitmap effects in your artwork using filters, effects, and graphic styles. When combining vector graphics and bitmap images, it's important to remember that how your artwork looks in Illustrator isn't always how it will look in its final medium (whether commercially printed, printed on a desktop printer, or viewed on the Web). The following factors influence the quality of your final artwork: Transparency and flattening Many bitmap effects add partially transparent pixels to your artwork. You can also add transparency to vector objects manually using the Transparency palette. When your artwork contains transparency, Illustrator performs a process called flattening before printing or exporting. In most cases, the default flattening process produces excellent results. However, if your artwork contains complex, overlapping areas and you require high-resolution output, you will probably want to preview the effects of flattening. For more information, see About flattening transparent artwork. Image resolution Bitmap images contain a fixed number of pixels, usually measured in pixels per inch (ppi). An image with a high resolution contains more, and therefore smaller, pixels than an image of the same printed dimensions with a low resolution. For example, a 1-inch-by-1-inch image with a resolution of 72 ppi contains a total of 5184 pixels (72 pixels wide x 72 pixels high = 5184). The same 1-inch-by-1-inch image with a resolution of 300 ppi would contain a total of 90,000 pixels. For imported bitmap images, image resolution is determined by the source file. For bitmap effects, you can specify a custom resolution. (See Specifying raster effects settings.) To determine the image resolution to use, consider the medium of final distribution for the image. If you're producing an image for online display, the image resolution only needs to match the typical monitor resolution (72 or 96 ppi). However, using too low a resolution for a printed image results in pixelation--output with large, coarse-looking pixels. Using too high a resolution (pixels smaller than what the output device can produce) increases the file size and slows the printing of the artwork. Printer resolution and screen frequency Printer resolution is measured in the number of ink dots produced per inch (dpi). Most desktop laser printers have a resolution of 600 dpi, and imagesetters have a resolution of 1200 dpi or higher. Ink jet printers produce a microscopic spray of ink, not actual dots; however, most ink jet printers have an approximate resolution of 300 to 720 dpi. To determine the appropriate resolution for your image when printing to any laser printer, but especially to imagesetters, you must also consider screen frequency. Screen frequency is the number of halftone cells per inch used to print grayscale images or color separations. Also known as screen ruling or line screen, screen frequency is measured in lines per inch (lpi)--or lines of cells per inch in a halftone screen. The relationship between image resolution and screen frequency determines the quality of detail in the printed image. To produce a halftone image of the highest quality, you generally use an image resolution that is from 1.5 to at most 2 times the screen frequency. But with some images and output devices, a lower resolution can produce good results. To determine your printer's screen frequency, check your printer documentation or consult your service provider. Note: Some imagesetters and desktop laser printers use screening technologies other than halftoning. If you are printing an image on a nonhalftone printer, consult your service provider or your printer documentation for the recommended image resolutions. |