Epson GX-80 Operating Manual page 33

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MYSTERIES
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In many cases you may wish to use commercial programs to design
graphics on a screen. Most of these programs perform what is called
a screen dump, in which whatever is on the computer screen is sent
to, and printed by, the printer. This function is sometimes accom-
plished by means of a "print screen" command or a "copy" com-
mand. If you have a program for screen dumps or any other soft-
ware that produces graphics, all you need to know about Epson dot
graphics is how to use the software. It is usually far easier and faster
to use such software than to program the graphics yourself. As an
example, the chart on this page was created and printed in about 10
minutes. You should check before you buy the software that it does
allow a screen dump to an Epson printer. Because the graphics
commands on the GX-80 are compatible with previous printers, it
does not specifically have to mention the GX-80 either in the docu-
mentation or on the screen. If in doubt use an MX-80, RX-80 or
FX-80 option in that order.
If, on the other hand, you wish to do your own programming or
merely wish to understand how the GX-80 prints graphics, this chap-
ter will tell you how it is done.
Dot Patterns
The GX-80 forms graphic images approximately the same way that
pictures in newspapers and magazines are printed. If you look close-
ly at a newspaper photograph, you can see that it is made up of
many small dots. The GX-80 also forms its images with patterns of
dots. In designing the graphics on the printer, you have to use a
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program to decide which dots are filled in and which are left blank.
It is often better to use squared graph paper to make up the design,
and then transfer the dot pattern to the program. In order to do this
it is necessary to understand how the print head transfers the dots to
the paper.
Print Head
The graphics mode on the GX-80 is quite different from the text
modes. Instead of sending codes for letters and printing functions,
the codes you send are ones that print a pattern of dots in a column.
None of the predefined characters or symbols in the printer's mem-
ory is used. Your program controls exactly where each dot is
printed, column by column and line by line.
The columns are defined by the row of pins on the print head. In the
standard graphics mode it uses only the top eight pins on the print
head because the computer uses eight data wires to communicate
with. the printer. Each of the top eight pins of the print head corres-
ponds to one of the data wires. There is also a graphics mode which
allows all nine pins of the head to be used.
When you want to print figures more than eight dots high, it is
necessary to print on a new line, advancing the paper and printing
another line, just as you would with text. To keep the print head
from leaving gaps between the graphics lines as it does between the
text lines, the line spacing must be adjusted to eliminate this gap.
When the line spacing is properly adjusted, the GX-80 prints finely
detailed graphics images that give no indication that they are made
up of separate lines, each no more than 1/8 of an inch high.
To ensure the proper alignment of dots in figures that use more than
one line, the GX-80 abandons the bidirectional printing it uses for
draft text. Instead it prints from left to right only in graphics mode.
It is not necessary to fill the whole page or even an entire line with
your graphics figures. However, before you send the data corres-
ponding to the columns of the pattern, you must tell the printer how
much data to expect. This information is part of the graphics com-
mand.
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