diff --git a/dither.py b/dither.py
index 23d6f79..ebd2637 100644
--- a/dither.py
+++ b/dither.py
@@ -55,11 +55,16 @@ def rgb_to_linear(rgb):
 
 PICO_RGB_LINEAR = [rgb_to_linear(x) for x in PICO_RGB]
 
-bayer = np.array([[0]])
-np.block([[4*bayer, 4*bayer+3],[4*bayer+2, 4*bayer+1]])
-bayer = np.block([[4*bayer, 4*bayer+3],[4*bayer+2, 4*bayer+1]])
-bayer = np.block([[4*bayer, 4*bayer+3],[4*bayer+2, 4*bayer+1]])
-bayer = np.block([[4*bayer, 4*bayer+3],[4*bayer+2, 4*bayer+1]])
+def make_bayer_matrix(size):
+    assert size > 0
+    assert size & (size - 1) == 0 # power of two
+    m = np.array([[0.0]])
+    while m.shape[0] < size:
+        m = np.block([[4*m, 4*m+3],[4*m+2, 4*m+1]])
+    return m
+
+bayer = make_bayer_matrix(4)
+
 def dither_pixel(img, xy, pal):
     global bayer
     pal2 = [np.array(rgb_to_linear(c)) for c in pal]
@@ -72,12 +77,12 @@ def dither_pixel(img, xy, pal):
         return index
     color = np.array(rgb_to_linear(img.getpixel(xy)))
     err = np.array([0.0,0.0,0.0])
-    for _ in range(64):
+    for _ in range(bayer.size):
         err += color
         i = nearest(err)
         counts[i] += 1
         err -= pal2[i]
-    thresh = bayer[y % 8][x % 8]
+    thresh = bayer[y % bayer.shape[0]][x % bayer.shape[1]]
     for i, count in enumerate(counts):
         thresh -= count
         if thresh < 0: