- made the grey background of the cave tiles true black. the imagemagick command
  used to do this (rather imperfect, but maybe a workable baseline) was:

   convert src.png -write mpr:o \
              -fill white -opaque \#444444 -fill black -opaque \#777777 -write mpr:a \
              -compose multiply \
              \( mpr:a -median 2 \) -composite \
              \( mpr:a -median 3 \) -composite \
              \( mpr:a -median 4 \) -composite \
              \( mpr:a -median 5 \) -composite \
              -negate \
              mpr:o -composite \
              -transparent white \
              dst.png