+static const double filter_scharr3[3][3] = {
+ { -3/32.0, 0, 3/32.0 },
+ { -10/32.0, 0, 10/32.0 },
+ { -3/32.0, 0, 3/32.0 }
+};
+
+static const double filter_prewitt3[3][3] = {
+ { -1/6.0, 0, 1/6.0 },
+ { -1/6.0, 0, 1/6.0 },
+ { -1/6.0, 0, 1/6.0 }
+};
+
+/* pathologic for inverting */
+static const double filter_sobel3[3][3] = {
+ { -1/8.0, 0, 1/8.0 },
+ { -2/8.0, 0, 2/8.0 },
+ { -1/8.0, 0, 1/8.0 }
+};
+
+/* pathologic for inverting */
+static const double filter_sobel5[5][5] = {
+ { -1/128.0, -2/128.0, 0, 2/128.0, 1/128.0 },
+ { -4/128.0, -8/128.0, 0, 8/128.0, 4/128.0 },
+ { -6/128.0, -12/128.0, 0, 12/128.0, 6/128.0 },
+ { -4/128.0, -8/128.0, 0, 8/128.0, 4/128.0 },
+ { -1/128.0, -2/128.0, 0, 2/128.0, 1/128.0 }
+};
+
+/* pathologic for inverting */
+static const double filter_prewitt5[5][5] = {
+ { -1/40.0, -2/40.0, 0, 2/40.0, 1/40.0 },
+ { -1/40.0, -2/40.0, 0, 2/40.0, 1/40.0 },
+ { -1/40.0, -2/40.0, 0, 2/40.0, 1/40.0 },
+ { -1/40.0, -2/40.0, 0, 2/40.0, 1/40.0 },
+ { -1/40.0, -2/40.0, 0, 2/40.0, 1/40.0 }
+};
+
+static const double filter_trivial[1][3] = {
+ { -0.5, 0, 0.5 }
+};
+