Add error diffusion to palette based rendering. Only used for bitmap and scaled bitmap plots. Doesn't do serpentine path, since that would need changes to the common bitmap rendering code.

1 files changed, 125 insertions, 5 deletions

diff --git a/include/palette.h b/include/palette.h
index 845c1bc..f975ef2 100644
--- a/include/palette.h
+++ b/include/palette.h
@@ -27,21 +27,35 @@ struct nsfb_palette_s {
 	enum nsfb_palette_type_e type; /**< Palette type */
 	uint8_t last; /**< Last used palette index */
 	nsfb_colour_t data[256]; /**< Palette for index modes */
+
+	bool dither; /**< Whether to use error diffusion */
+	struct {
+		int width; /**< Length of error value buffer ring*/
+		int current; /**< Current pos in ring buffer*/
+		int *data; /**< Ring buffer error values */
+		int data_len; /**< Max size of ring */
+	} dither_ctx;
 };
 
+
 /** Create an empty palette object. */
-bool nsfb_palette_new(struct nsfb_palette_s **palette);
+bool nsfb_palette_new(struct nsfb_palette_s **palette, int width);
 
 /** Free a palette object. */
 void nsfb_palette_free(struct nsfb_palette_s *palette);
 
+/** Init error diffusion for a plot. */
+void nsfb_palette_dither_init(struct nsfb_palette_s *palette, int width);
+
+/** Finalise error diffusion after a plot. */
+void nsfb_palette_dither_fini(struct nsfb_palette_s *palette);
+
 /** Generate libnsfb 8bpp default palette. */
 void nsfb_palette_generate_nsfb_8bpp(struct nsfb_palette_s *palette);
 
-
-static inline uint8_t nsfb_palette_best_match(
-		const struct nsfb_palette_s *palette,
-		nsfb_colour_t c)
+/** Find best palette match for given colour. */
+static inline uint8_t nsfb_palette_best_match(struct nsfb_palette_s *palette,
+		nsfb_colour_t c, int *r_error, int *g_error, int *b_error)
 {
 	uint8_t best_col = 0;
 
@@ -68,6 +82,9 @@ static inline uint8_t nsfb_palette_best_match(
 
 		best_col = col;
 		best_distance = cur_distance;
+		*r_error = dr;
+		*g_error = dg;
+		*b_error = db;
 
 		/* Index into grayscale part */
 		col = (( c        & 0xFF) +
@@ -82,6 +99,9 @@ static inline uint8_t nsfb_palette_best_match(
 		if (cur_distance < best_distance) {
 			best_distance = cur_distance;
 			best_col = col;
+			*r_error = dr;
+			*g_error = dg;
+			*b_error = db;
 		}
 		break;
 
@@ -97,6 +117,9 @@ static inline uint8_t nsfb_palette_best_match(
 			if (cur_distance < best_distance) {
 				best_distance = cur_distance;
 				best_col = col;
+				*r_error = dr;
+				*g_error = dg;
+				*b_error = db;
 			}
 		}
 		break;
@@ -108,4 +131,101 @@ static inline uint8_t nsfb_palette_best_match(
         return best_col;
 }
 
+/** Find best palette match for given colour, with error diffusion. */
+static inline uint8_t nsfb_palette_best_match_dither(
+		struct nsfb_palette_s *palette, nsfb_colour_t c)
+{
+	int r, g, b;
+	int current;
+	int error;
+	int width = palette->dither_ctx.width;
+	uint8_t best_col_index;
+
+	if (palette == NULL)
+		return 0;
+
+	if (palette->dither == false)
+		return nsfb_palette_best_match(palette, c, &r, &g, &b);
+
+	current = palette->dither_ctx.current;
+
+	/* Get RGB components of colour, and apply error */
+	r = ( c        & 0xFF) + palette->dither_ctx.data[current    ];
+	g = ((c >>  8) & 0xFF) + palette->dither_ctx.data[current + 1];
+	b = ((c >> 16) & 0xFF) + palette->dither_ctx.data[current + 2];
+
+	/* Clamp new RGB components to range */
+	if (r <   0) r =   0;
+	if (r > 255) r = 255;
+	if (g <   0) g =   0;
+	if (g > 255) g = 255;
+	if (b <   0) b =   0;
+	if (b > 255) b = 255;
+
+	/* Reset error diffusion slots to 0 */
+	palette->dither_ctx.data[current    ] = 0;
+	palette->dither_ctx.data[current + 1] = 0;
+	palette->dither_ctx.data[current + 2] = 0;
+
+	/* Rebuild colour from modified components */
+	c = r + (g << 8) + (b << 16);
+
+	/* Get best match for pixel, and find errors for each component */
+	best_col_index = nsfb_palette_best_match(palette, c, &r, &g, &b);
+
+	/* Advance one set of error diffusion slots */
+	current += 3;
+	if (current >= width)
+		current = 0;
+	palette->dither_ctx.current = current;
+
+	/* Save errors
+	 *
+	 *       [*]-[N]
+	 *      / | \
+	 *   [l]-[m]-[r]
+	 */
+	error = current;
+
+	/* Error for [N] (next) */
+	if (error != 0) {
+		/* The pixel exists */
+		palette->dither_ctx.data[error    ] += r * 7 / 16;
+		palette->dither_ctx.data[error + 1] += g * 7 / 16;
+		palette->dither_ctx.data[error + 2] += b * 7 / 16;
+	}
+
+	error += width - 2 * 3;
+	if (error >= width)
+		error -= width;
+	/* Error for [l] (below, left) */
+	if (error >= 0 && error != 3) {
+		/* The pixel exists */
+		palette->dither_ctx.data[error    ] += r * 3 / 16;
+		palette->dither_ctx.data[error + 1] += g * 3 / 16;
+		palette->dither_ctx.data[error + 2] += b * 3 / 16;
+	}
+
+	error += 3;
+	if (error >= width)
+		error -= width;
+	/* Error for [m] (below, middle) */
+	palette->dither_ctx.data[error    ] += r * 5 / 16;
+	palette->dither_ctx.data[error + 1] += g * 5 / 16;
+	palette->dither_ctx.data[error + 2] += b * 5 / 16;
+
+	error += 3;
+	if (error >= width)
+		error -= width;
+	/* Error for [r] (below, right) */
+	if (error != 0) {
+		/* The pixel exists */
+		palette->dither_ctx.data[error    ] += r / 16;
+		palette->dither_ctx.data[error + 1] += g / 16;
+		palette->dither_ctx.data[error + 2] += b / 16;
+	}
+
+	return best_col_index;
+}
+
 #endif /* PALETTE_H */