Dynamically Redefined Character Set support

[davidrg]

Aside from the VT100 confidence tests (which don't make any sense for a terminal emulator), this is the only VT220 feature not currently supported by K95. With this implemented, K95s VT220 emulation would be as comprehensive reasonably possible.

This feature may also not be especially difficult either. I guess the implementaiton would probably look something like:

• Implement DECDLD Device control string which will involve parsing Sixels, turning them into an HBITMAP or something and sticking them into one of the DRCS tables. This will really be the hard part.
• Then when the soft character set is designated, instead of doing character set conversion we'd set a soft-character attribute of some kind. This would work if only one soft character set is supported, but what about multiple soft character sets?
• At rendering time, if the soft character set attribute is set rather than rendering the character with a font, the characters bitmap would be looked up in the DRCS table, resized to fit the font size, and then painted in that character position.

Unanswered questions:

• If we take the attribute path, how do we handle multiple soft character sets? While the VT320 (and, I assume, the VT220) only has one DRCS table, the VT510 has three. So we would probably need at least two attribute bits, and there is currently only one free. Might need to increase the size of the char attribute (which may not actually increase memory use depending on structure padding)
• How do we handle soft characters in scrollback? Perhaps when a soft character set is redefined we go clear the attributes for anything in scrollback?

Reasons to do this:

• Completes VT220 emulation, and might also do the same for VT320 emulation
• Provides sixel handling code for future graphics terminal features
• Ability to play Termtris

Priority:
Low - nothing much uses it. Target some release post-3.0.

[j4james]

If you'd like some more games to test with, I have a few:

• https://github.com/j4james/vtnibbler
• https://github.com/j4james/vtinvaders
• https://github.com/j4james/vtrex

The last one requires some VT420 features I don't think are supported in the beta 7 release, but might already be working in your development build (rectangular area ops, horizontal scrolling, and paging).

Some other test cases:

• https://github.com/jmason/gif320/tree/main/examples (images created from soft fonts)
• https://github.com/jhamby/cmatrix (a fork of CMatrix using a soft font for the Japanese characters)

[davidrg]

Your games are a large part of my motivation for getting this feature done. Before I stumbled on them, this and the recent VT420 features (paging, macros) were really just boxes that needed checking before I could justify adding a VT420 emulation option with no obvious software that actually makes use of any of it.