ubb-vga: documented single and double mode

- README: describe how single and double mode work
- res.fig: illustrate the mapping in single and double mode

ubb-vga/README

@@ -40,6 +40,27 @@ clock that's about 10% slower than the original. Luckily, most monitors
 don't mind.
 
 
+Single-double mode
+------------------
+
+In single mode, only one set/clear can be performed per pixel. They are
+arranges such that the first pixel can only turn on channels, the
+following pixel can only turn off channels, and so on.
+
+In double mode, a set and a clear is performed per pixel. This changes
+the timing such that it differs substantially from VGA, but some
+monitors can still synchronize with this.
+
+The resulting signals for various bit patterns are shown in res.fig
+Each group shows the input pixels, and signal for single mode, and the
+signal for double mode. "S" indicates a point where the channel can be
+turned on, "C" indicates a point where it can be turned off.
+
+Note that there are probably other mapping algorithms that would yield
+better results. The mapping is not a very performance-critical part of
+ubb-vga. Improvements are welcome.
+
+
 Compatibility
 -------------