+static const GLubyte kernel[16] = {
+ 0*16, 8*16, 2*16, 10*16,
+ 12*16, 4*16, 14*16, 6*16,
+ 3*16, 11*16, 1*16, 9*16,
+ 15*16, 7*16, 13*16, 5*16,
+};
+
+#if DITHER
+#define DITHER_COMP(X, Y) kernel[((X) & 0x3) | (((Y) & 0x3) << 2)]
+
+#define DITHER_CLAMP(X) (((X) < CHAN_MAX) ? (X) : CHAN_MAX)
+#else
+#define DITHER_COMP(X, Y) 0
+
+#define DITHER_CLAMP(X) (X)
+#endif