The best introductory book to ReGIS is probably the GIGI/ReGIS Handbook despite it being rather out of date (1981) compared to the release of the VT340.
VT340 graphics can be in the form of Sixel bitmaps or ReGIS commands.
While I tend to use Sixels more often these days, ReGIS graphics are
surprisingly fun due to the interactive interpreter mode. Just do
printf "\eP3p" and start experimenting. Use Ctrl-[ \ to exit ReGIS.
ReGIS offers a way to draw text at any color, size, or angle. Here's a color text example which uses the colormap index numbers:
DCS=$'\eP'
ST=$'\e\\'
echo "${DCS}3p;P[27,285];T(W(I3))' Key: Good ';T(W(I2))'Fair ';T(W(I1))'Poor';${ST}"
The ReGIS Text command is T and it has multiple suboptions, such as
Sn for size and W(In) to pick the color index for writing:
echo "${DCS}p;P[27,200];T(S16,W(I4))' Great';${ST}"
Another useful ReGIS command is S (Screen) which (among many other
things) changes the colors in the colormap with the suboption M.
Note that what the manual calls the "RGB system" is not like modern RGB colors. Instead, it uses a single letter to specify one of eight colors: Red, Green, Blue, Cyan, Magenta, Yellow, Dark, White.
To access all 4096 different colors the VT340 offers, use the Hue-Lightness-Saturation color space. HLS has much to recommend it as a more natural way for humans to express colors.
echo "${DCS}p;S(M 1(H60 L80 S60) 2(H150 L50 S60))';${ST}"
While slower than using the built-in fonts, you could use the Hershey vector fonts. Here is an example you can send to your screen which is the output from running plotutils hersheydemo -T regis --rotation 90.
Speaking of ReGIS demos, there are some nice ones written in ODE (ordinary differential equation solver) which, due to the slow speed of drawing, have a nice animated effect. You can find the files here: https://github.com/hackerb9/vt340test/tree/main/regis/ode