From a3fe29531694a36557b816bc690b81e7b231f8ad Mon Sep 17 00:00:00 2001
From: Walter Bender <walter@sugarlabs.org>
Date: Sun, 26 Dec 2010 13:00:20 +0000
Subject: catch zero-divide situation in set_gray

---
diff --git a/TurtleArt/tacanvas.py b/TurtleArt/tacanvas.py
index 3e4dcf6..ed71c4b 100644..100755
--- a/TurtleArt/tacanvas.py
+++ b/TurtleArt/tacanvas.py
@@ -61,7 +61,10 @@ def calc_gray(c, g, invert=False):
     if g == 100:
         return c
     if invert:
-        return int(((c * 100) - (32768 * (100 - g))) / g)
+        if g == 0:
+            return c
+        else:
+            return int(((c * 100) - (32768 * (100 - g))) / g)
     else:
         return int(((c * g) + (32768 * (100 - g))) / 100)
 
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