The "Four" Colours of SG6 Mode

On Cocotalk last week L. Curtis Boyle mentioned my "Tank Capture" port (from a TRS-80 Model 1/3 game) and noted the "stripy" green characters left as trails behind the tanks.  I made a short video about these green characters.  Not all the “undefined” characters (0-127) from the MC6847 SG6 character set appear as “stripy” characters.  Two characters (ASCII characters 0 and 85) appear as solid dark green and solid light green characters on real hardware, probably due to some NTSC artifact effect. You can see them in the video (the dark green block on the last few seconds is one of the target blocks).  In many of my SG6 games I use these two characters to provide some visual variety to the otherwise limited RED/BLUE semi-graphic block characters.

Flywheel with light green sky and dark green wheels

XRally with dark green fuel bar and light walls.

From what I have been told, the 0-128 characters of the character set in SG6 mode are a result of the fact that the lines for the MC6847 VDG chip are not wired up fully for that mode.  Other computers like the NEC PC6001 do wire up the VDG to exploit all its abilities.  You can see that demonstrated here:

The NEC PC6001 really pushes the VDG to its max.

You can see that there are actually 4 semigraphic block colors available and text mixed in, so how the VDG is wired up can be much more complex. It's interesting to think about what the possibilities would have been for games if some extra wiring had been added to the MC-10 to exploit those modes to their maximum.