magick.py: prevent floyd_steinberg() from modifying its input

magick.py

@@ -17,20 +17,21 @@ def find_closest_palette_color(color, palette):
 
 
 def floyd_steinberg(img, palette):
-    for y in range(img.shape[0]):
-        for x in range(img.shape[1]):
-            oldpixel = img[y, x]
+    result = numpy.array(img, copy=True)
+    for y in range(result.shape[0]):
+        for x in range(result.shape[1]):
+            oldpixel = result[y, x]
             newpixel = find_closest_palette_color(oldpixel, palette)
             quant_error = oldpixel - newpixel
-            img[y, x] = newpixel
-            if x + 1 < img.shape[1]:
-                img[y, x + 1] += quant_error * 7 / 16
-            if y + 1 < img.shape[0]:
-                img[y + 1, x - 1] += quant_error * 3 / 16
-                img[y + 1, x] += quant_error * 5 / 16
-            if x + 1 < img.shape[1] and y + 1 < img.shape[0]:
-                img[y + 1, x + 1] += quant_error * 1 / 16
-    return img
+            result[y, x] = newpixel
+            if x + 1 < result.shape[1]:
+                result[y, x + 1] += quant_error * 7 / 16
+            if y + 1 < result.shape[0]:
+                result[y + 1, x - 1] += quant_error * 3 / 16
+                result[y + 1, x] += quant_error * 5 / 16
+            if x + 1 < result.shape[1] and y + 1 < result.shape[0]:
+                result[y + 1, x + 1] += quant_error * 1 / 16
+    return result
 
 
 def convolve_rgba(img, kernel):

test.sh

@@ -114,16 +114,16 @@ fi
 cat << END | ( cd "$tempdir"; md5sum --check $status_arg - )
 cc611e80cde3b9b7adb7723801a4e5d4  alpha.png
 706175887af8ca1a33cfd93449f970df  gray16.png
-198e3333d7dc6158f94d1007e75c5bd3  gray1.png
-795ff9c5c59cdc95f5a0b9191a7247d9  gray2.png
+ff4d9f18de39be879926be2e65990167  gray1.png
+d51900476658a1c9dd26a7b27db8a21f  gray2.png
 722223ba74be9cba1af4a549076b70d3  gray4.png
 2320216faa5a10bf0f5f04ebce07f8e1  gray8.png
 35a47d6ae6de8c9d0b31aa0cda8648f3  inverse.png
 6ad810399058a87d8145d8d9a7734da5  normal16.png
 c8d2e1f116f31ecdeae050524efca7b6  normal.png
-c69edd665ee3546d9eb57fe10cd8095a  palette1.png
-504ecab83662aa2394042a62b1d2eafb  palette2.png
-90d1bbe7b26e210d8846cf8692a98b72  palette4.png
+18a3dfca369f976996ef93389ddfad61  palette1.png
+d38646afa6fa0714be9badef25ff9392  palette2.png
+e1c59e68a68fca3273b6dc164d526ed7  palette4.png
 50bf09eb3571901f0bf642b9a733038c  palette8.png
 END