The lost art of making custom terminal fonts
============================================
I've been playing with my old terminals quite a bit, a wyse-325 and a
honeywell terminal - the wyse terminal has a way to emulate a vt-220
among a few other terminals, and I noticed it supports loading custom
fonts. Now, That's kind of neat, because you could combine multiple
custom font characters into a graphical image. Wooo, GFX on an old
text terminal! sign me up! It took a bit of research to figure out
how to do this. I combined my notes below along with a little script
that should help if you want to try this on your terminal.
Say you wanted to have an O, or a circle, or whatever. First draw the
character in a 8x12 grid and split the grid in a top and bottom half.
Then, for each half, add up the columns into a value, treating each
column as a series of bits top to bottom:
---------------------
1| | | |X| | | | | Calculate values by reading
2| | |X| |X| | | | bits top to bottom.
4| |X| | | |X| | | For every pixel denoted with X,
8| |X| | | |X| | | you will be adding up
16|X| | | | | |X| | the value in the first
32|X| | | | | |X| | column to get the total)
--+-+-+-+-+-+-+-+-+--
48,12,2,1,2,12,48,0 <- top half total
---------------------
1| |X| | | |X| | |
2| |X| | | |X| | |
4| | |X| |x| | | |
8| | | |x| | | | |
16| | | | | | | | |
32| | | | | | | | |
---------------------
0,3,4,8,4,3,0,0 <- bottom half total
Note that we didn't put any pixels in the rightmost column.
This is because that column is kind of special. Any pixel
you put there, gets doubled-up to the right, meaning, the
terminal will automatically provide a sort of 'bridge'
between this character and the next character at any place
there is a pixel in the rightmost column. This is so
characters can look like they are 'connected', for instance
if you wanted to draw a solid, uninterrupted line.
Next, we pick a font bank, a place in the bank, and a name
for that character etc,... and we can form our magic
string to load and activate the new character.
For the vt300 and vt220 and probably some other compatible
terminals, this seems to be DCS followed by the font bank,
and a whole bunch of other params, ending with an ST char.
I wrote a little bash script which I'll share below that
automates and explains the whole process. The script will
load a smiley-face character into the "!" character, so,
if you execute this on a vt300/vt220/(/others) terminal,
you will now see a smiley face where there would normally
be an exclamation point.
# Font definition (calculated from grid, TSL=top, BSL=bottom)
TSL="{AX@@XA\x3f"
BSL="BCHIIHC\x3f"
# Destination font bank ( 0 or 1 )
FB="1"
# Position in which the first character of the font set is to be
# loaded (0-95)
BP="1"
# What part of the font bank should be erased:
# 0,2 = entire font set ; 1 = only loaded characters
ER="1"
