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Coloring text pixels using ReGIS

It is not possible to have multicolored text on the VT340 because the value of any pixel of text is 0111₂, which is an index into the colormap. No matter what color the colormap entry is changed to, all text will change simultaneously since it all points to the same place.

However, it is possible to modify the value of pixels on the screen by selectively erasing the bitplane using the ReGIS W(F()) operator.

Test program

Example code

DCS=$'\eP'
ST=$'\e\\'
# Erase screen. Writing Multiplier = 20, color Index = 0 (0000₂).
echo "${DCS}1p; S(E) W(M20,I0) ${ST}"
tput cup 10 25		# Cursor position
tput bold			# Bold text pixels are set to 15 (1111₂).
echo -n "ABCDEFGHIJKLMNOPQRSTUVWXYZ012345"

# Enable only certain fields (bitplanes) for writing.
echo "${DCS}0p;"
echo "P[250,200]"
for ((i=0; i<16; i++)); do
	# Filled Vector polygon with temporary Writing style of Field "i"
    echo "F(V(W(F${i})) 0642) v0 "
done
echo "${ST}"

...

    Side note

    The mysterious number 0642 is actually offset directions for the V (vector) operator. Here is the compass rose showing what each number means. Because the code has set W(M20) the Writing Multiplier is 20 pixels per step. So, 0642 goes east, south, west, and then north, making a 20x20 square. The final V0 moves the ReGIS cursor east to start the next box.


References

Potential problems

  • How fast is it?

    The above implementation is rather slow, taking nearly half a second on a VT340+ to colorize 32 characters. There is likely some faster way as no optimization has been attempted, yet.

  • What if the screen scrolls up?

    Colorized text scrolls up properly with the screen. Tested with '\n' (newline) and Esc D (index).

  • What if the screen scrolls down?

    Colorized text scrolls down properly with the screen. Tested with Esc M (reverse index) and with Esc [ M (delete line).

  • What if the text is shifted left?

    Colors are removed from the line when text is shifted left by Esc [ P (delete character).

  • What if the text is shifted right?

    Colors are removed from the line when the text is shifted right by Esc [ 4 h (insert mode).

Portability

This example code is lacking in that it ought to test for the size of the character cell instead of presuming it is 10x20. Due to this bug, it will not work on the VT340 in 132 column mode, nor does it work on terminal emulators that have different font sizes.