Processing a Halftone Bitmap
If you've added gray tones to line art for a cartoon that will print in black and white, you have two processing options. Obviously, you can simply use the 300 dpi TIFF, sacrificing some of the sharpness in the lines. Or you can opt to produce a halftone bitmap. The bitmap's advantages include a small file size and crisp lines. To process a halftone bitmap, you would follow the procedure for adding gray tones to line art discussed in the preceding section. However, you would not reduce the dpi. The dpi should remain at 600. This is a good time to save your work.
Converting to Halftone Bitmap
Once you're finished adding gray tones, go to Image, pull down to Mode and over to Bitmap. A dialogue box will appear with many choices of Methods. Instead of choosing 50% Threshold, choose Halftone Screen.
The halftone screen breaks down all the shades of gray into dots. Where there are many dots, close together, the gray will be dark. Where there are fewer dots, spread out, the gray will be light. The lines are still solid black.
Once you've converted your drawing from grayscale to halftone bitmap, you cannot convert back to grayscale. The conversion from gray tone to black dots is irreversible. For this reason, it is advisable to choose Save As after this step, saving your grayscale version for possible future modifications.
When you OK this dialogue box, a new one appears with fields for Frequency, Angle, and Shape. Frequency is measured in lines per inch (lpi). This is how many lines of dots fit into one square inch. One hundred lpi is a good frequency and will yield fields of gray that are not immediately noticeable as being composed of dots. At lower frequencies, the dots become more obvious. Choose 45° for the Angle to keep the art from looking mechanical. Choose Round for Shape.
A halftone bitmap at 53 lpi (on the left) and at 100 lpi (on the right)
Resizing and Moiré Patterns
When the halftone screen is being created, the dots are arranged in an equidistant pattern — that is, there is the same distance between all dots. If this didn't happen, your eye would perceive patterns between the dots — lines, circles, and so forth. That distracting pattern is known as a moiré (pronounced more-AY) pattern.
If your image is resized after the halftone screen is created, you will disrupt the equidistant spacing between the dots and risk creating a distracting moiré pattern. Always alert your client or your printer when you are sending a halftone bitmap. If the image needs to be resized, offer to do it for them from the grayscale version you saved before converting to bitmap. If this option is not viable, lower the halftone screen to about 53 lpi. This will result in a less-distracting moiré pattern if the image is resized.