From c0ab9b7b3fe778ccfbeb69001afe25797b9d3f96 Mon Sep 17 00:00:00 2001
From: Michael Grant <mgrant@grant.org>
Date: Wed, 5 Aug 2026 21:38:28 +0100
Subject: [PATCH] =?UTF-8?q?Revert=20"Removed=20unconditional=20Floyd?=
 =?UTF-8?q?=E2=80=93Steinberg=20dithering=20from=20image-sixel.c:1077.=20K?=
 =?UTF-8?q?itty=20images=20now=20use=20the=20adaptive=20256-colour=20media?=
 =?UTF-8?q?n-cut=20palette=20with=20direct=20nearest-colour=20mapping."?=
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This reverts commit ab591ae4302f52f6a9775dce4965419f16dd7ea6.
---
 image-sixel.c            | 49 ++++++++++++++++++++++++++++++++++++++--
 regress/image-support.sh |  4 ++--
 2 files changed, 49 insertions(+), 4 deletions(-)

diff --git a/image-sixel.c b/image-sixel.c
index e81ff5554..01dc57fb0 100644
--- a/image-sixel.c
+++ b/image-sixel.c
@@ -978,6 +978,16 @@ sixel_nearest_colour(struct sixel_rgb *palette, u_int ncolours,
 	return (best);
 }
 
+static u_int
+sixel_clamp_colour(int colour)
+{
+	if (colour < 0)
+		return (0);
+	if (colour > 255)
+		return (255);
+	return (colour);
+}
+
 static const u_char *
 sixel_from_image_pixel(struct image *im, u_int sourcex0, u_int sourcey0,
     u_int sourcewidth, u_int sourceheight, u_int sx, u_int sy, u_int x,
@@ -1003,7 +1013,9 @@ sixel_from_image(struct image *im, u_int ox, u_int oy, u_int cells_x,
 	struct sixel_rgb		 palette[SIXEL_PALETTE_SIZE];
 	const u_char			*pixel;
 	uint16_t			*cache;
-	u_int				 x, y, sx, sy, index;
+	int				*current, *next, *tmp;
+	int				 red_error, green_error, blue_error;
+	u_int				 x, y, sx, sy, index, error_index;
 	u_int			 sourcex0, sourcey0, sourcewidth, sourceheight;
 	u_int			 red, green, blue, colour, i, ncolours;
 	uint64_t		 destination_width, destination_height;
@@ -1076,22 +1088,55 @@ sixel_from_image(struct image *im, u_int ox, u_int oy, u_int cells_x,
 
 	cache = xmalloc(SIXEL_HISTOGRAM_SIZE * sizeof *cache);
 	memset(cache, 0xff, SIXEL_HISTOGRAM_SIZE * sizeof *cache);
+	current = xcalloc(((size_t)sx + 2) * 3, sizeof *current);
+	next = xcalloc(((size_t)sx + 2) * 3, sizeof *next);
 	for (y = 0; y < sy; y++) {
 		for (x = 0; x < sx; x++) {
 			pixel = sixel_from_image_pixel(im, sourcex0, sourcey0,
 			    sourcewidth, sourceheight, sx, sy, x, y);
 			if (pixel[3] < 128)
 				continue;
+			error_index = (x + 1) * 3;
+			red = sixel_clamp_colour((int)pixel[0] +
+			    current[error_index] / 16);
+			green = sixel_clamp_colour((int)pixel[1] +
+			    current[error_index + 1] / 16);
+			blue = sixel_clamp_colour((int)pixel[2] +
+			    current[error_index + 2] / 16);
 			colour = sixel_nearest_colour(palette, ncolours, cache,
-			    pixel[0], pixel[1], pixel[2]);
+			    red, green, blue);
 			if (sixel_set_pixel(si, x, y, colour + 1) != 0)
 				goto fail;
+
+			red_error = (int)red - palette[colour].red;
+			green_error = (int)green - palette[colour].green;
+			blue_error = (int)blue - palette[colour].blue;
+			current[error_index + 3] += red_error * 7;
+			current[error_index + 4] += green_error * 7;
+			current[error_index + 5] += blue_error * 7;
+			next[error_index - 3] += red_error * 3;
+			next[error_index - 2] += green_error * 3;
+			next[error_index - 1] += blue_error * 3;
+			next[error_index] += red_error * 5;
+			next[error_index + 1] += green_error * 5;
+			next[error_index + 2] += blue_error * 5;
+			next[error_index + 3] += red_error;
+			next[error_index + 4] += green_error;
+			next[error_index + 5] += blue_error;
 		}
+		tmp = current;
+		current = next;
+		next = tmp;
+		memset(next, 0, ((size_t)sx + 2) * 3 * sizeof *next);
 	}
+	free(current);
+	free(next);
 	free(cache);
 	return (si);
 
 fail:
+	free(current);
+	free(next);
 	free(cache);
 	sixel_free(si);
 	return (NULL);
diff --git a/regress/image-support.sh b/regress/image-support.sh
index 39e1ad3d7..8265d7933 100755
--- a/regress/image-support.sh
+++ b/regress/image-support.sh
@@ -125,7 +125,7 @@ $TMUX2 select-window -t:1 || exit 1
 sleep 1
 [ "$($TMUX2 display-message -p '#{cursor_x},#{cursor_y}')" = "2,1" ] || exit 1
 $TMUX capture-pane -pS0 -E3 >$TMP || exit 1
-[ "$(sed -n 3p $TMP)" = "  -" ] || exit 1
+[ "$(sed -n 3p $TMP)" = "  :" ] || exit 1
 [ "$(sed -n 4p $TMP)" = "  *" ] || exit 1
 
 # With normal cursor movement, scrolling is calculated from the full image
@@ -139,7 +139,7 @@ $TMUX2 select-window -t:2 || exit 1
 sleep 1
 [ "$($TMUX2 display-message -p '#{cursor_y}')" = 3 ] || exit 1
 $TMUX capture-pane -pS0 -E3 >$TMP || exit 1
-[ "$(sed -n 1p $TMP)" = "-" ] || exit 1
+[ "$(sed -n 1p $TMP)" = ":" ] || exit 1
 [ "$(sed -n 2p $TMP)" = "*" ] || exit 1
 [ "$(sed -n 3p $TMP)" = "@" ] || exit 1