more accurate palette conversion

docs/source/todo.rst

@@ -1,7 +1,6 @@
 TODO
 ====
 
-- integrate MCIOUT into diskio.f_write()
 - revisit the seek example to see if write-seeking now works?
 - remove unused interned strings in the resulting code (for example from removed if/else blocks)
 - [on branch: shortcircuit] investigate McCarthy evaluation again? this may also reduce code size perhaps for things like if a>4 or a<2 ....

examples/cx16/sprites/convertsprite.py

@@ -2,10 +2,15 @@
 
 from PIL import Image
 
+
 def make_cx16_palette(palette: list[int]) -> bytes:
+    def to4bit(color: int) -> int:
+        return (color * 15 + 135) >> 8      # see https://threadlocalmutex.com/?p=48
     cx16palette = bytearray()
     for pi in range(0, len(palette), 3):
-        r, g, b = palette[pi] >> 4, palette[pi + 1] >> 4, palette[pi + 2] >> 4
+        r = to4bit(palette[pi])
+        g = to4bit(palette[pi+1])
+        b = to4bit(palette[pi+2])
         cx16palette.append(g << 4 | b)
         cx16palette.append(r)
     return cx16palette