Initial revision

1 files changed, 193 insertions, 0 deletions

diff --git a/dither.c b/dither.c
new file mode 100644
index 0000000..2eaeb31
--- /dev/null
+++ b/dither.c
@@ -0,0 +1,193 @@
+/*
+ * ordered dither rotines
+ *
+ * stolen from The GIMP and trimmed for speed
+ *
+ */
+
+#include <stdlib.h>
+#include "dither.h"
+
+#define DITHER_LEVEL 8
+
+void (*dither_line)(unsigned char *, unsigned char *, int, int);
+
+static long     red_mult, green_mult;
+static long     red_dither[256];
+static long     green_dither[256];
+static long     blue_dither[256];
+static long     gray_dither[256];
+
+typedef unsigned long vector[DITHER_LEVEL];
+typedef vector  matrix[DITHER_LEVEL];
+
+#if DITHER_LEVEL == 8
+#define DITHER_MASK 7
+static matrix   DM =
+{
+    {0, 32, 8, 40, 2, 34, 10, 42},
+    {48, 16, 56, 24, 50, 18, 58, 26},
+    {12, 44, 4, 36, 14, 46, 6, 38},
+    {60, 28, 52, 20, 62, 30, 54, 22},
+    {3, 35, 11, 43, 1, 33, 9, 41},
+    {51, 19, 59, 27, 49, 17, 57, 25},
+    {15, 47, 7, 39, 13, 45, 5, 37},
+    {63, 31, 55, 23, 61, 29, 53, 21}
+};
+
+#endif
+
+#if DITHER_LEVEL == 4
+#define DITHER_MASK 3
+static matrix   DM =
+{
+    {0, 8, 2, 10},
+    {12, 4, 14, 6},
+    {3, 11, 1, 9},
+    {15, 7, 13, 5}
+};
+
+#endif
+
+void
+init_dither(int shades_r, int shades_g, int shades_b, int shades_gray)
+{
+    int             i, j;
+    unsigned char   low_shade, high_shade;
+    unsigned short  index;
+    float           red_colors_per_shade;
+    float           green_colors_per_shade;
+    float           blue_colors_per_shade;
+    float           gray_colors_per_shade;
+
+    red_mult = shades_g * shades_b;
+    green_mult = shades_b;
+
+    red_colors_per_shade = 256.0 / (shades_r - 1);
+    green_colors_per_shade = 256.0 / (shades_g - 1);
+    blue_colors_per_shade = 256.0 / (shades_b - 1);
+    gray_colors_per_shade = 256.0 / (shades_gray - 1);
+
+    /* this avoids a shift when checking these values */
+    for (i = 0; i < DITHER_LEVEL; i++)
+	for (j = 0; j < DITHER_LEVEL; j++)
+	    DM[i][j] *= 0x10000;
+
+    /*  setup arrays containing three bytes of information for red, green, & blue  */
+    /*  the arrays contain :
+     *    1st byte:    low end shade value
+     *    2nd byte:    high end shade value
+     *    3rd & 4th bytes:    ordered dither matrix index
+     */
+
+    for (i = 0; i < 256; i++) {
+
+	/*  setup the red information  */
+	{
+	    low_shade = (unsigned char) (i / red_colors_per_shade);
+	    high_shade = low_shade + 1;
+
+	    index = (unsigned short)
+		(((i - low_shade * red_colors_per_shade) / red_colors_per_shade) *
+		 (DITHER_LEVEL * DITHER_LEVEL + 1));
+
+	    low_shade *= red_mult;
+	    high_shade *= red_mult;
+
+	    red_dither[i] = (index << 16) + (high_shade << 8) + (low_shade);
+	}
+
+	/*  setup the green information  */
+	{
+	    low_shade = (unsigned char) (i / green_colors_per_shade);
+	    high_shade = low_shade + 1;
+
+	    index = (unsigned short)
+		(((i - low_shade * green_colors_per_shade) / green_colors_per_shade) *
+		 (DITHER_LEVEL * DITHER_LEVEL + 1));
+
+	    low_shade *= green_mult;
+	    high_shade *= green_mult;
+
+	    green_dither[i] = (index << 16) + (high_shade << 8) + (low_shade);
+	}
+
+	/*  setup the blue information  */
+	{
+	    low_shade = (unsigned char) (i / blue_colors_per_shade);
+	    high_shade = low_shade + 1;
+
+	    index = (unsigned short)
+		(((i - low_shade * blue_colors_per_shade) / blue_colors_per_shade) *
+		 (DITHER_LEVEL * DITHER_LEVEL + 1));
+
+	    blue_dither[i] = (index << 16) + (high_shade << 8) + (low_shade);
+	}
+
+	/*  setup the gray information  */
+	{
+	    low_shade = (unsigned char) (i / gray_colors_per_shade);
+	    high_shade = low_shade + 1;
+
+	    index = (unsigned short)
+		(((i - low_shade * gray_colors_per_shade) / gray_colors_per_shade) *
+		 (DITHER_LEVEL * DITHER_LEVEL + 1));
+
+	    gray_dither[i] = (index << 16) + (high_shade << 8) + (low_shade);
+	}
+    }
+}
+
+void
+dither_line_color(unsigned char *src, unsigned char *dest, int y, int width)
+{
+    register long   a, b;
+    long           *ymod, xmod;
+
+    ymod = DM[y & DITHER_MASK];
+
+    while (width--) {
+	xmod = width & DITHER_MASK;
+
+	b = red_dither[*(src++)];
+	if (ymod[xmod] < b)
+	    b >>= 8;
+
+	a = green_dither[*(src++)];
+	if (ymod[xmod] < a)
+	    a >>= 8;
+	b += a;
+
+	a = blue_dither[*(src++)];
+	if (ymod[xmod] < a)
+	    a >>= 8;
+	b += a;
+
+	*(dest++) = b & 0xff;
+    }
+}
+
+void
+dither_line_gray(unsigned char *src, unsigned char *dest, int y, int width)
+{
+    long           *ymod, xmod;
+    register long   a,g;
+
+    ymod = DM[y & DITHER_MASK];
+
+    while (width--) {
+	xmod = width & DITHER_MASK;
+
+#if 1
+	g = (src[0]*3 + src[1]*6 + src[2]) / 10;
+	a = gray_dither[g];
+	src += 3;
+#else
+	a = gray_dither[*(src++)];
+#endif
+	if (ymod[xmod] < a)
+	    a >>= 8;
+
+	*(dest++) = a & 0xff;
+    }
+}