Fix issue "Oblique projection (Ob): Partly wrong annotations"

src/gmt_init.c

@@ -6858,7 +6858,11 @@ EXTERN_MSC void gmtinit_conf_classic (struct GMT_CTRL *GMT) {
 	GMT->current.setting.given_unit[GMTCASE_MAP_ANNOT_OFFSET_PRIMARY] = 'p';
 	GMT->current.setting.given_unit[GMTCASE_MAP_ANNOT_OFFSET_SECONDARY] = 'p';
 	/* MAP_ANNOT_OBLIQUE */
-	GMT->current.setting.map_annot_oblique = GMT_OBL_ANNOT_LON_HORIZONTAL | GMT_OBL_ANNOT_LAT_HORIZONTAL | GMT_OBL_ANNOT_EXTEND_TICKS;
+	/* The anywhere bit was lost when this setting went from a bit-sum to a keyword list; without it an oblique frame
+	 * only annotates longitudes on the S/N borders and latitudes on the W/E borders, which throws away most of the
+	 * annotations as soon as the graticule is rotated far from the frame axes.  The documentation has said "default
+	 * is anywhere" all along, and that is what GMT 6.1.0 did [issue #8418] */
+	GMT->current.setting.map_annot_oblique = GMT_OBL_ANNOT_ANYWHERE | GMT_OBL_ANNOT_LON_HORIZONTAL | GMT_OBL_ANNOT_LAT_HORIZONTAL | GMT_OBL_ANNOT_EXTEND_TICKS;
 	/* MAP_ANNOT_MIN_ANGLE */
 	GMT->current.setting.map_annot_min_angle = 20;
 	/* MAP_ANNOT_MIN_SPACING */

src/gmt_plot.c

@@ -1837,11 +1837,12 @@ GMT_LOCAL void gmtplot_map_symbol (struct GMT_CTRL *GMT, struct PSL_CTRL *PSL, s
 	double line_angle, text_angle, div, tick_length, o_len, len, ca, sa, boost;
 	double *xx = xings->xx, *yy = xings->yy, *line_angles = xings->angle;
 	unsigned int i, annot_type, justify, *sides = xings->sides, nx = xings->nx;
-	bool flip;
+	bool flip, no_tick;
 	len = gmtplot_get_annot_offset (GMT, &flip, level);	/* Get annotation offset, and flip justification if "inside" */
 	annot_type = 2 << type;		/* 2 = NS, 4 = EW */
 
 	for (i = 0; i < nx; i++) {
+		no_tick = false;
 		if (!(GMT->current.setting.map_annot_oblique & GMT_OBL_ANNOT_ANYWHERE) && ((type == 0 && (sides[i] % 2)) || (type == 1 && !(sides[i] % 2)))) continue;
 
 		if (gmtlib_prepare_label (GMT, line_angles[i], sides[i], xx[i], yy[i], type, &line_angle, &text_angle, &justify)) continue;
@@ -1853,12 +1854,28 @@ GMT_LOCAL void gmtplot_map_symbol (struct GMT_CTRL *GMT, struct PSL_CTRL *PSL, s
 		if (!flip && GMT->current.setting.map_annot_oblique & annot_type) o_len = tick_length;
 		if (GMT->current.setting.map_annot_oblique & GMT_OBL_ANNOT_EXTEND_TICKS) {
 			div = ((sides[i] % 2) ? fabs (ca) : fabs (sa));
-			o_len /= div;
+			/* We only ride out to the tip of the extended tick if there actually is one: gmtplot_map_tick skips
+			 * ticks whose gridline meets the border at less than MAP_ANNOT_MIN_ANGLE, and dividing by that same
+			 * tiny div here would fling the annotation far along the border, away from the crossing it belongs
+			 * to and with no tick to connect the two.  Below that angle we step straight out from the border
+			 * instead, so the annotation stays where its gridline meets the frame [issue #8418] */
+			if (div > sind (GMT->current.setting.map_annot_min_angle))
+				o_len /= div;
+			else
+				no_tick = true;
 		}
 		else
 			o_len += copysign (boost, o_len);
-		xx[i] += o_len * ca;
-		yy[i] += o_len * sa;
+		if (no_tick) {	/* Offset normal to the border since there is no oblique tick to follow */
+			if (sides[i] % 2)
+				xx[i] += (sides[i] == 1) ? o_len : -o_len;
+			else
+				yy[i] += (sides[i] == 2) ? o_len : -o_len;
+		}
+		else {
+			xx[i] += o_len * ca;
+			yy[i] += o_len * sa;
+		}
 		if (!flip && (GMT->current.setting.map_annot_oblique & annot_type) && GMT->current.setting.map_annot_offset[level] > 0.0) {
 			if (sides[i] % 2)
 				xx[i] += (sides[i] == 1) ? GMT->current.setting.map_annot_offset[level] : -GMT->current.setting.map_annot_offset[level];

test/psbasemap/oblique_annot.sh

@@ -0,0 +1,34 @@
+#!/usr/bin/env bash
+#
+# Oblique frames must annotate a gridline where it actually meets the border.
+#
+# With MAP_ANNOT_OBLIQUE's tick_extend the annotation goes at the tip of the tick that is extended
+# along the gridline.  gmtplot_map_tick draws no such tick once the gridline meets the border at
+# less than MAP_ANNOT_MIN_ANGLE, but the annotation code used to extend regardless, dividing its
+# offset by a vanishing sine and flinging the label far along the border, detached from the crossing
+# it belongs to.  Here the 175E meridian meets the N border at about 6 degrees: its annotation
+# belongs at x = 3.1 cm and used to be drawn at x = 2.0 cm on a map only 6 cm wide.  See issue #8418.
+#
+# The positions below were checked against the crossings computed independently with gmt mapproject.
+
+# Pin everything the extracted table depends on, so that it is the annotation placement being tested
+gmt set FORMAT_GEO_MAP ddd:mm:ssF MAP_ANNOT_MIN_SPACING 0 \
+	MAP_ANNOT_OBLIQUE anywhere,lon_horizontal,lat_horizontal,tick_extend
+
+gmt psbasemap -JOb168/51/172/32.5/6c -R-500/2300/-400/400+uk -BeNsW -Bxa5f1 -Bya5f1 -P > map.ps
+
+# Pull the annotations out of the PostScript as "label border position_along_that_border_in_cm".
+# PSL units are 1/1200 inch measured from the lower left map corner, hence the 2.54/1200 scaling.
+tr '\r' '\n' < map.ps | awk '
+	{ for (i = 3; i <= NF; i++) if ($i == "M") { x = $(i-2); y = $(i-1) } }	# Remember latest moveto
+	/\([0-9]+.[EN]\) [a-z][a-z] Z/ {
+		for (i = 1; i <= NF; i++) if ($i ~ /^\(/) { lab = $i; just = $(i+1) }
+		if      (just == "bc") printf "%s N %.1f\n", lab, x*2.54/1200
+		else if (just == "tc") printf "%s S %.1f\n", lab, x*2.54/1200
+		else if (just == "mr") printf "%s W %.1f\n", lab, y*2.54/1200
+		else if (just == "ml") printf "%s E %.1f\n", lab, y*2.54/1200
+	}' | sort > result.txt
+
+printf '%b' '(165\260E) W 0.7\n(170\260E) W 1.4\n(175\260E) N 3.1\n(35\260N) N 5.1\n(40\260N) N 3.9\n(45\260N) N 2.7\n(50\260N) N 1.5\n(55\260N) N 0.3\n(55\260N) W 0.4\n' > answer.txt
+
+diff -q --strip-trailing-cr answer.txt result.txt