Three SIXEL improvements from nincsnevem662 at gmail dot com in GitHub issue

4149:

- Pass P2 on the received image through when forwarding the image to the
  outside terminal to preserve transparency;

- Preserve raster attribute dimensions which may be smaller than the actual
  image (used to crop out parts of the image to work around the limitation that
  rows must come in groups of six);

- To avoid collapsing empty sixel lines, no longer ignore duplicate "-" new
  line specifiers.

image-sixel.c

@@ -37,8 +37,13 @@ struct sixel_image {
 	u_int			 xpixel;
 	u_int			 ypixel;
 
+	u_int			 set_ra;
+	u_int			 ra_x;
+	u_int			 ra_y;
+
 	u_int			*colours;
 	u_int			 ncolours;
+	u_int			 p2;
 
 	u_int			 dx;
 	u_int			 dy;
@@ -166,6 +171,10 @@ sixel_parse_attributes(struct sixel_image *si, const char *cp, const char *end)
 	si->x = x;
 	sixel_parse_expand_lines(si, y);
 
+	si->set_ra = 1;
+	si->ra_x = x;
+	si->ra_y = y;
+
 	return (last);
 }
 
@@ -268,7 +277,7 @@ sixel_parse_repeat(struct sixel_image *si, const char *cp, const char *end)
 }
 
 struct sixel_image *
-sixel_parse(const char *buf, size_t len, u_int xpixel, u_int ypixel)
+sixel_parse(const char *buf, size_t len, u_int p2, u_int xpixel, u_int ypixel)
 {
 	struct sixel_image	*si;
 	const char		*cp = buf, *end = buf + len;
@@ -282,6 +291,7 @@ sixel_parse(const char *buf, size_t len, u_int xpixel, u_int ypixel)
 	si = xcalloc (1, sizeof *si);
 	si->xpixel = xpixel;
 	si->ypixel = ypixel;
+	si->p2 = p2;
 
 	while (cp != end) {
 		ch = *cp++;
@@ -423,6 +433,18 @@ sixel_scale(struct sixel_image *si, u_int xpixel, u_int ypixel, u_int ox,
 	new = xcalloc (1, sizeof *si);
 	new->xpixel = xpixel;
 	new->ypixel = ypixel;
+	new->p2 = si->p2;
+
+	new->set_ra = si->set_ra;
+	/* clamp to slice end */
+	new->ra_x = si->ra_x < psx ? si->ra_x : psx;
+	new->ra_y = si->ra_y < psy ? si->ra_y : psy;
+	/* subtract slice origin */
+	new->ra_x = new->ra_x > pox ? new->ra_x - pox : 0;
+	new->ra_y = new->ra_y > poy ? new->ra_y - poy : 0;
+	/* resize */
+	new->ra_x = new->ra_x * xpixel / si->xpixel;
+	new->ra_y = new->ra_y * ypixel / si->ypixel;
 
 	for (y = 0; y < tsy; y++) {
 		py = poy + ((double)y * psy / tsy);
@@ -497,11 +519,15 @@ sixel_print(struct sixel_image *si, struct sixel_image *map, size_t *size)
 	len = 8192;
 	buf = xmalloc(len);
 
-	sixel_print_add(&buf, &len, &used, "\033Pq", 3);
+	tmplen = xsnprintf(tmp, sizeof tmp, "\033P0;%uq", si->p2);
-
-	tmplen = xsnprintf(tmp, sizeof tmp, "\"1;1;%u;%u", si->x, si->y);
 	sixel_print_add(&buf, &len, &used, tmp, tmplen);
 
+	if (si->set_ra) {
+		tmplen = xsnprintf(tmp, sizeof tmp, "\"1;1;%u;%u", si->ra_x,
+		    si->ra_y);
+		sixel_print_add(&buf, &len, &used, tmp, tmplen);
+	}
+
 	for (i = 0; i < ncolours; i++) {
 		c = colours[i];
 		tmplen = xsnprintf(tmp, sizeof tmp, "#%u;%u;%u;%u;%u",
@@ -552,7 +578,6 @@ sixel_print(struct sixel_image *si, struct sixel_image *map, size_t *size)
 
 		if (buf[used - 1] == '$')
 			used--;
-		if (buf[used - 1] != '-')
 		sixel_print_add(&buf, &len, &used, "-", 1);
 	}
 	if (buf[used - 1] == '$' || buf[used - 1] == '-')

input.c

@@ -2320,6 +2320,7 @@ input_dcs_dispatch(struct input_ctx *ictx)
 #ifdef ENABLE_SIXEL
 	struct window		*w;
 	struct sixel_image	*si;
+	int			 p2;
 #endif
 
 	if (wp == NULL)
@@ -2333,7 +2334,12 @@ input_dcs_dispatch(struct input_ctx *ictx)
 #ifdef ENABLE_SIXEL
 	w = wp->window;
 	if (buf[0] == 'q') {
-		si = sixel_parse(buf, len, w->xpixel, w->ypixel);
+		if (input_split(ictx) != 0)
+			return (0);
+		p2 = input_get(ictx, 1, 0, 0);
+		if (p2 == -1)
+			p2 = 0;
+		si = sixel_parse(buf, len, p2, w->xpixel, w->ypixel);
 		if (si != NULL)
 			screen_write_sixelimage(sctx, si, ictx->cell.cell.bg);
 	}

tmux.h

@@ -3493,7 +3493,7 @@ int		 image_scroll_up(struct screen *, u_int);
 
 /* image-sixel.c */
 #define SIXEL_COLOUR_REGISTERS 1024
-struct sixel_image *sixel_parse(const char *, size_t, u_int, u_int);
+struct sixel_image *sixel_parse(const char *, size_t, u_int, u_int, u_int);
 void		 sixel_free(struct sixel_image *);
 void		 sixel_log(struct sixel_image *);
 void		 sixel_size_in_cells(struct sixel_image *, u_int *, u_int *);