Index: www/study/study.py
===================================================================
--- www/study/study.py	(revision 1927)
+++ www/study/study.py	(revision 1930)
@@ -47,4 +47,5 @@
     for y in range(256):
         gradient256bw.setGray(x, 255 - y, y / 255.)
+gradient256bw.writePng("gradient256bw.png")
 
 # Output 1: 50% threshold
@@ -143,4 +144,63 @@
         dest.setGray(x, y, c)
 dest.writePng("pat003.png")
+
+# Pattern 4: two different 25% patterns
+dest = Image((320, 80))
+for y in range(80):
+    for x in range(80):
+        c = (x / 2 & 1) and (y / 2 & 1)
+        dest.setGray(x, y, c)
+    for x in range(80, 160):
+        c = (x & 1) and (y & 1)
+        dest.setGray(x, y, c)
+    for x in range(160, 240):
+        c = (x & 1) and ((y + x / 2) & 1)
+        dest.setGray(x, y, c)
+    for x in range(240, 320):
+        c = (x / 2 & 1) and ((y / 2 + x / 4) & 1)
+        dest.setGray(x, y, c)
+dest.writePng("pat004.png")
+
+# Output 7: 4x4 Bayer dithering
+def test4(src, mat, name):
+    (w, h) = src.size()
+    dest = Image((w, h))
+    dx = len(mat[0])
+    dy = len(mat)
+    for y in range(h):
+        for x in range(w):
+            c = src.getGray(x, y)
+            threshold = (1. + mat[y % dy][x % dx]) / (dx * dy + 1)
+            c = c > threshold
+            dest.setGray(x, y, c)
+    dest.writePng(name)
+
+mat = [[  0,  8,  3, 11],
+       [ 15,  4, 12,  7],
+       [  2, 10,  1,  9],
+       [ 13,  6, 14,  5]]
+test4(lenna256bw, mat, "out007.png")
+test4(gradient256bw, mat, "grad007.png")
+
+mat = [[ 12,  5,  6, 13],
+       [  4,  0,  1,  7],
+       [ 11,  3,  2,  8],
+       [ 15, 10,  9, 14]]
+test4(lenna256bw, mat, "out008.png")
+test4(gradient256bw, mat, "grad008.png")
+
+mat = [[ 13,  7,  0,  4, 10],
+       [  9,  3,  1,  8, 14],
+       [ 11,  5,  2,  6, 12],]
+test4(lenna256bw, mat, "out009.png")
+test4(gradient256bw, mat, "grad009.png")
+
+##############################################################################
+# Only temporary cruft below this
+import sys
+sys.exit(0)
+
+
+
 
 # Pattern 4: gamma-corrected 50% gray, black-white halftone, 50% gray
@@ -213,7 +273,4 @@
 
 
-##############################################################################
-# Only temporary cruft below this
-
 src = lenna256bw
 src = gradient256bw
@@ -224,24 +281,26 @@
        [  2, 10,  1,  9],
        [ 13,  6, 14,  5]]
-mat = [[  6,  7,  8,  9],
-       [  5,  0,  1, 10],
-       [  4,  3,  2, 11],
-       [ 15, 14, 13, 12]]
-mat = [[ 12,  5,  9, 13],
-       [  8,  0,  1,  6],
-       [  4,  3,  2, 10],
-       [ 15, 11,  7, 14]]
-mat = [[ 35, 24, 13, 14, 25, 32],
-       [ 31, 12,  5,  6, 15, 26],
-       [ 23,  4,  0,  1,  7, 16],
-       [ 22, 11,  3,  2,  8, 17],
-       [ 30, 21, 10,  9, 18, 27],
-       [ 34, 29, 20, 19, 28, 33]]
-mat = [[  0, 20, 30,  3, 23, 29],
-       [ 12, 32, 18, 15, 35, 17],
-       [ 27,  8,  4, 24, 11,  7],
-       [  2, 22, 28,  1, 21, 31],
-       [ 14, 34, 16, 13, 33, 19],
-       [ 25, 10,  6, 26,  9,  5]]
+#mat = [[  6,  7,  8,  9],
+#       [  5,  0,  1, 10],
+#       [  4,  3,  2, 11],
+#       [ 15, 14, 13, 12]]
+#mat = [[ 12,  5,  9, 13],
+#       [  8,  0,  1,  6],
+#       [  4,  3,  2, 10],
+#       [ 15, 11,  7, 14]]
+size = 4
+#mat = [[ 35, 24, 13, 14, 25, 32],
+#       [ 31, 12,  5,  6, 15, 26],
+#       [ 23,  4,  0,  1,  7, 16],
+#       [ 22, 11,  3,  2,  8, 17],
+#       [ 30, 21, 10,  9, 18, 27],
+#       [ 34, 29, 20, 19, 28, 33]]
+#mat = [[  0, 20, 30,  3, 23, 29],
+#       [ 12, 32, 18, 15, 35, 17],
+#       [ 27,  8,  4, 24, 11,  7],
+#       [  2, 22, 28,  1, 21, 31],
+#       [ 14, 34, 16, 13, 33, 19],
+#       [ 25, 10,  6, 26,  9,  5]]
+#size = 6
 dest = Image((w, h))
 for y in range(h):
@@ -249,11 +308,11 @@
         c = src.getGray(x, y)
         i = Gamma.CtoI(c)
-        threshold = mat[x % 6][y % 6]
-        c = math.floor(i * 35.999) > threshold
+        threshold = mat[x % size][y % size]
+        d = math.floor(i * (size * size + .9999)) > threshold
+        if c > 0.95:
+            print c, i, i * (size * size + .9999)
+        c = d
         dest.setGray(x, y, c)
 dest.writePng("out008.png")
-
-import sys
-sys.exit(0)
 
 # Create a dot-matrix pattern