Dot Patterns; Color Graphics - Epson LQ-2550 User Manual

Because the dots overlap each other both horizontally and vertically
in the Letter Quality mode, it is difficult to see individual dots.
Instead, the letters and symbols seem to be made of unbroken lines.
In order for the dots to overlap vertically, the pins are in more than
one column, but the intelligence of the printer handles the timing of
pin firings so that the effect is of 24 pins arranged in a single vertical
column.

Dot patterns

Your printer's print head is able to print graphics as well as text
because graphic images are formed on the printer about the same
way that pictures in newspapers and magazines 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 360 dot positions per inch horizontally
and 360 dots vertically. The images printed by the printer can,
therefore, be as finely detailed as the ones at the beginning of this
section.

Color graphics

With the color ribbon and appropriate software, you can print in a
wide variety of colors. Even if your software doesn't support color,
printer command ESC r lets you select from seven basic colors,
including black. In addition, you can combine some of the basic
colors for a greater variety.
4-14 Using Software and Graphics