# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1444173667 -7200
# Node ID 96453447feab2a6fb521b95845d1a6e34c68a77f
# Parent  075e5f1f12e316c69e5f2f4c83bcbbf5c3d10fbd
Spread any error across two pixels on the following row.

diff -r 075e5f1f12e3 -r 96453447feab optimiser.py
--- a/optimiser.py	Wed Oct 07 00:49:40 2015 +0200
+++ b/optimiser.py	Wed Oct 07 01:21:07 2015 +0200
@@ -17,7 +17,7 @@
     return min(max(v, lower), upper)
 
 def clip(v):
-    return within(v, 0, 255)
+    return int(within(v, 0, 255))
 
 def distance(rgb1, rgb2):
     r1, g1, b1 = rgb1
@@ -249,9 +249,14 @@
                 im.putpixel((x, y), value)
 
                 if y < height - 1:
-                    rgbn = im.getpixel((x, y+1))
-                    rgbn = tuple(map(lambda i: clip(i[0] + i[1] - i[2]), zip(rgbn, rgb, value)))
-                    im.putpixel((x, y+1), rgbn)
+
+                    # Spread the error across adjacent pixels.
+
+                    error = tuple(map(lambda i: (i[0] - i[1]) / 2.0, zip(rgb, value)))
+                    for xn in range(x, min(x + 2, width)):
+                        rgbn = im.getpixel((xn, y+1))
+                        rgbn = tuple(map(lambda i: clip(i[0] + i[1]), zip(rgbn, error)))
+                        im.putpixel((xn, y+1), rgbn)
 
         im.save(output_filename)