Graphics

The print head

To understand dot graphics you should first learn how your printer’s print head works.

The print head has nine pins arranged in a vertical column. As the print head moves across the page, electrical impulses cause the pins to fire. Each time a pin fires, it strikes the inked ribbon and presses it against the paper to produce a small dot. The pins fire time after time in different patterns that produce letters, numbers, and graphic symbols.

Dot patterns

The print head is able to print both graphics and text because graphic images are formed on the printer in the same way that newspaper and magazine pictures are printed. If you look closely at a newspaper photograph, you can see that it is made up of many small dots. Your printer also forms its images with patterns of dots, as many as 240 dot positions per inch horizontally and 72 dots per inch vertically. The images printed by this printer can be as finely detailed as the one at the beginning of this section.

In its main graphics mode, your printer uses only the top eight of the nine pins to print one column of dots for each code it receives. Your graphics program, therefore, must send one code for each column in a line. Each code specifies the dot pattern for that column.

To print graphic images taller than eight dots, the print head must make more than one pass. After printing one line, the printer advances the paper and prints another, just as it does when printing text.

To keep the print head from leaving gaps between graphics lines as it does between text lines, the line spacing must be changed using the ESC A command described in Chapter 9. Since an 8-dot column prints lines that are 8/72-inch high, you generally want to set the line spacing to 8/72nds, as you’ll see in a sample program later in this chapter.

4-10 Software and Graphics