Issue 8235. Certain -I -R in grdlandmask create horizontal bands

src/gmt_shore.c

@@ -1422,6 +1422,23 @@ int gmt_assemble_shore (struct GMT_CTRL *GMT, struct GMT_SHORE *c, int dir, bool
 	if (c->ns > 0) p = gmt_M_memory (GMT, p, P, struct GMT_GSHHS_POL);	/* Trim memory */
 
 	for (ku = 0; ku < P; ku++) gmtshore_path_shift2 (p[ku].lon, p[ku].n, west, east, c->leftmost_bin);	/* Deal with possible longitude -/+360 issues */
+	if (c->lon_sw >= east) {	/* This bin sits at or beyond the east border, so its polygons were shifted west above.
+		 * A point that lay exactly on that border is not shifted by the test above, which tears the polygon across
+		 * the entire map: its bounding box then spans every column and the containment test paints unrelated nodes
+		 * clear across the grid [issue #8235].  A polygon cannot be wider than its bin, so when one is we know the
+		 * points left behind belong with the shifted part. */
+		for (ku = 0; ku < P; ku++) {
+			double lon_min = DBL_MAX, lon_max = -DBL_MAX;
+			unsigned int kp;
+			for (kp = 0; kp < (unsigned int)p[ku].n; kp++) {
+				if (p[ku].lon[kp] < lon_min) lon_min = p[ku].lon[kp];
+				if (p[ku].lon[kp] > lon_max) lon_max = p[ku].lon[kp];
+			}
+			if ((lon_max - lon_min) <= c->bsize) continue;	/* This one is intact */
+			for (kp = 0; kp < (unsigned int)p[ku].n; kp++)
+				if (p[ku].lon[kp] >= east && (p[ku].lon[kp] - 360.0) >= west) p[ku].lon[kp] -= 360.0;
+		}
+	}
 
 	*pol = p;
 	return (P);	/* Return list of polygons found */

src/grdlandmask.c

@@ -439,12 +439,24 @@ EXTERN_MSC int GMT_grdlandmask (void *V_API, int mode, void *args) {
 					if (p[k].lat[i] < ymin) ymin = p[k].lat[i];
 					if (p[k].lat[i] > ymax) ymax = p[k].lat[i];
 				}
-				col_min = MAX (0, irint (ceil (xmin * i_dx_inch - Grid->header->xy_off - GMT_CONV8_LIMIT)));
-				if (col_min > nx1) col_min = 0;
-				/* So col_min is in range [0,nx1] */
-				col_max = MIN (nx1, irint (floor (xmax * i_dx_inch - Grid->header->xy_off + GMT_CONV8_LIMIT)));
-				if (col_max <= 0 || col_max < col_min) col_max = nx1;
-				/* So col_max is in range [1,nx1] */
+				col_min = irint (ceil (xmin * i_dx_inch - Grid->header->xy_off - GMT_CONV8_LIMIT));
+				col_max = irint (floor (xmax * i_dx_inch - Grid->header->xy_off + GMT_CONV8_LIMIT));
+				if (wrap) {	/* Keep the wrap-around treatment of a global grid */
+					if (col_min < 0 || col_min > nx1) col_min = 0;
+					/* So col_min is in range [0,nx1] */
+					if (col_max > nx1) col_max = nx1;
+					if (col_max <= 0 || col_max < col_min) col_max = nx1;
+					/* So col_max is in range [1,nx1] */
+				}
+				/* For a non-global grid the same fallbacks would turn a round-off miss just outside the grid into
+				 * the full grid width, so the winding test then ran over unrelated columns and made a mess of the
+				 * mask [issue #8235].  Clip a genuine overlap instead, and skip a polygon that has none. */
+				else if (col_min > nx1 || col_max < 0 || col_max < col_min)
+					continue;
+				else {
+					if (col_min < 0) col_min = 0;
+					if (col_max > nx1) col_max = nx1;
+				}
 				row_min = MAX (0, irint (ceil ((GMT->current.proj.rect[YHI] - ymax) * i_dy_inch - Grid->header->xy_off - GMT_CONV8_LIMIT)));
 				/* So row_min is in range [0,?] */
 				row_max = MIN (ny1, irint (floor ((GMT->current.proj.rect[YHI] - ymin) * i_dy_inch - Grid->header->xy_off + GMT_CONV8_LIMIT)));
@@ -604,14 +616,15 @@ EXTERN_MSC int GMT_grdlandmask (void *V_API, int mode, void *args) {
 				if (col_max < col_min) col_max += (int)Grid->header->n_columns;
 			}
 			else {	/* Make sure we are inside our grid */
+				/* lon_w and lon_e are the bin's edges measured east from the grid's west edge, so they must be
+				 * wrapped against 360, not compared to the absolute wesn limits: doing the latter shifted a bin
+				 * lying east of the west edge into negative offsets, so col_max came out below col_min and the
+				 * bin painted nothing at all [issue #8235] */
 				double lon_w, lon_e;
-				lon_w = c.lon_sw - Grid->header->wesn[XLO];	lon_e = c.lon_sw + c.bsize - Grid->header->wesn[XLO];
-				if (lon_w < Grid->header->wesn[XLO] && (lon_w+360.0) < Grid->header->wesn[XHI]) {
-					lon_w += 360.0;	lon_e += 360.0;
-				}
-				else if (lon_e > Grid->header->wesn[XHI] && (lon_e-360.0) > Grid->header->wesn[XLO]) {
-					lon_w -= 360.0;	lon_e -= 360.0;
-				}
+				lon_w = c.lon_sw - Grid->header->wesn[XLO];	lon_e = lon_w + c.bsize;
+				while (lon_w < 0.0) { lon_w += 360.0;	lon_e += 360.0; }
+				while (lon_w >= 360.0) { lon_w -= 360.0;	lon_e -= 360.0; }
+				if (lon_e > 360.0) { lon_w -= 360.0;	lon_e -= 360.0; }	/* Bin straddles the seam; use its western part */
 				col_min = irint (ceil (lon_w * HH->r_inc[GMT_X] - Grid->header->xy_off));
 				col_max = irint (floor (lon_e * HH->r_inc[GMT_X] - Grid->header->xy_off));
 				if (col_min < 0) col_min = 0;