Grayscale, Line Art, and Vectors 926 Selecting Halftone Screen in the Bitmap dialog box opens the Halftone Screen dialog box, as seen in Figure 27.7. Figure 27.7. The Halftone Screen dialog box is where you set your halftone screen frequency, screen angle, and dot shape. You'll look at this setting in more detail in Chapter 28, but basically the halftone screen frequency refers to how many halftone dots are in a linear inch. The greater the screen frequency, the finer the dots, and the less detectable they are. The screen angle refers to the angle at which the dots are oriented with respect to the horizontal. Halftone dots are most visible to the eye when aligned with either the horizontal (0 degrees) or the vertical (90 degrees). Therefore, the angle farthest from either axis is 45 degrees, where the dots are the least perceptible. You can also specify a dot shape other than round; you can use diamonds, ellipses, lines, squares, or crosses. Different shapes add different effects and help relieve some halftone problems, such as dot gain and/or a tendency for dots to "plug up"--or clump together--in the shadow areas. Figure 27.8 shows a grayscale image and its Bitmap mode/halftone screen equivalent. Figure 27.8. The original grayscale image (left) has been converted to a Bitmap mode image with a halftone screen (right), giving it a sort of "shot through a screen door" look. Vector Graphics and Printing If your Photoshop image includes vector data--such as shapes, type, and so on--you can choose to print the file using that vector data, rather than rasterizing the entire image. Why would you want to do this? Vector data prints at the resolution of the actual printer or other output device, regardless of the resolution of the Photoshop file. Raster data, on the other hand, is at the behest of the file's resolution as set in Photoshop's Image Size dialog box. For a discussion of raster versus vector data, see "The Two Types of Computer Graphics," p. 156, in Chapter 6, "Pixels, Vectors, and Resolution."