# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1444428865 -7200
# Node ID f789f95ee65637b7660a60a98832a258ffe77af6
# Parent  92c6d7aa93436ea007001133246f440b2e1dc3e4
Simplified further, raising an exception in get_value instead of returning None.

diff -r 92c6d7aa9343 -r f789f95ee656 optimiser.py
--- a/optimiser.py	Sat Oct 10 00:05:54 2015 +0200
+++ b/optimiser.py	Sat Oct 10 00:14:25 2015 +0200
@@ -141,15 +141,18 @@
 
     """
     Get an output colour for 'rgb', optionally limited to any specified 'chosen'
-    colours. If 'fail' is set to a true value, return None if the colour cannot
-    be expressed using any of the chosen colours.
+    colours. If 'fail' is set to a true value, raise ValueError if the colour
+    cannot be expressed using any of the chosen colours.
     """
 
     l = pattern(rgb, chosen)
-    limit = sum([f for f, c in l])
+    limit = 0
+    for f, c in l:
+        limit += f
+
     if not limit:
         if fail:
-            return None
+            raise ValueError
         else:
             return l[randrange(0, len(l))][1]
 
@@ -264,10 +267,14 @@
         for y in range(0, height):
             for x in range(0, width):
                 rgb = im.getpixel((x, y))
-                if saturate or desaturate:
-                    rgb = saturate_rgb(rgb, saturate and 0.5 / saturate or 2 * desaturate)
-                if darken or brighten:
-                    rgb = amplify_rgb(rgb, brighten and 0.5 / brighten or 2 * darken)
+                if saturate:
+                    rgb = saturate_rgb(rgb, 0.5 / saturate)
+                elif desaturate:
+                    rgb = saturate_rgb(rgb, 2 * desaturate)
+                if darken:
+                    rgb = amplify_rgb(rgb, 2 * darken)
+                elif brighten:
+                    rgb = amplify_rgb(rgb, 0.5 / brighten)
                 im.putpixel((x, y), rgb)
 
 def preview_image(im, half_resolution_preview=False):
@@ -299,16 +306,21 @@
         c = get_colours(im, y)
 
         for l in get_combinations(c, 4):
-            most = [value for f, value in l]
+            most = []
+            for f, value in l:
+                most.append(value)
             for x in range(0, width):
                 rgb = im.getpixel((x, y))
-                value = get_value(rgb, most, True)
-                if value is None:
+                try:
+                    value = get_value(rgb, most, True)
+                except ValueError:
                     break # try next combination
             else:
                 break # use this combination
         else:
-            most = [value for f, value in c[:4]] # use the first four
+            most = []
+            for f, value in c[:4]: # use the first four
+                most.append(value)
 
         for x in range(0, width):
             rgb = im.getpixel((x, y))