Only write gray if monochrome CPT (#8196)

* Only write gray if monochrome CPT

Might as well just write gray for r = g = b CPTs.

* Update dump_rgb.rst_

doc/rst/source/dump_rgb.rst_

@@ -30,4 +30,5 @@
 
     **Note**: We append four extra output columns at the end of the output records.  These
     are the red, green, and blue components color components (all in 0-255 range) and the
-    transparency value (0-100% range, with 0 meaning opaque).
+    transparency value (0-100% range, with 0 meaning opaque). If the color table is a gray
+    table, then we output *gray* (0-255) instead of three identical components.

src/grd2xyz.c

@@ -489,11 +489,18 @@ EXTERN_MSC int GMT_grd2xyz (void *V_API, int mode, void *args) {
 	}
 	else if (io.binary) GMT->common.b.active[GMT_OUT] = true;
 
+	if (Ctrl->C.active) {	/* Just sample the CPT for the grid z values and append three columns r g b a to the x y z */
+		if ((P = gmt_get_palette (GMT, Ctrl->C.file, GMT_CPT_OPTIONAL, 0.0, 255.0, Ctrl->C.dz)) == NULL) {
+			GMT_Report (API, GMT_MSG_ERROR, "Failed to read CPT %s.\n", Ctrl->C.file);
+			Return (API->error);	/* Well, that did not go so well... */
+		}
+		Out = gmt_new_record (GMT, out, NULL);	/* Since we only need to worry about numerics in this module */
+	}
 	n_output = (Ctrl->Z.active) ? 1 : ((Ctrl->W.active) ? 4 : ((Ctrl->F.mode == 2) ? 2 : 3));
 	if (Ctrl->Z.active)
 		n_output = 1;
-	else if (Ctrl->C.active)	/* x, y, z, r, g, b, a */
-		n_output = 7;
+	else if (Ctrl->C.active)	/* x, y, z, r[, g, b], a */
+		n_output = (P->is_gray) ? 5 : 7;
 	else {
 		n_output = (Ctrl->F.mode == 2) ? 2 : 3;
 		if (Ctrl->W.active) n_output++;
@@ -513,13 +520,6 @@ EXTERN_MSC int GMT_grd2xyz (void *V_API, int mode, void *args) {
 
 	out[w_col] = Ctrl->W.weight;
 
-	if (Ctrl->C.active) {	/* Just sample the CPT for the grid z values and append three columns r g b a to the x y z */
-		if ((P = gmt_get_palette (GMT, Ctrl->C.file, GMT_CPT_OPTIONAL, 0.0, 255.0, Ctrl->C.dz)) == NULL) {
-			GMT_Report (API, GMT_MSG_ERROR, "Failed to read CPT %s.\n", Ctrl->C.file);
-			Return (API->error);	/* Well, that did not go so well... */
-		}
-		Out = gmt_new_record (GMT, out, NULL);	/* Since we only need to worry about numerics in this module */
-	}
 
 	for (opt = options; opt; opt = opt->next) {	/* Loop over arguments, skip options */
 
@@ -539,16 +539,18 @@ EXTERN_MSC int GMT_grd2xyz (void *V_API, int mode, void *args) {
 		}
 
 		if (Ctrl->C.active) {	/* Just sample the CPT for the z value and append four columns r g b a */
-			unsigned int k, kol, geo = gmt_M_is_geographic (GMT, GMT_IN);
-			static char *coord[2] = {"x\ty\tz\tred\tgreen\tblue\ttransparency", "lon\tlat\tz\tred\tgreen\tblue\ttransparency"};
-			if (first_grd) GMT_Put_Record (API, GMT_WRITE_TABLE_HEADER, coord[geo]);
+			unsigned int k, kol, geo = gmt_M_is_geographic (GMT, GMT_IN), nc = (P->is_gray) ? 1 : 3;
+			static char *rgbcoord[2] = {"x\ty\tz\tred\tgreen\tblue\ttransparency", "lon\tlat\tz\tred\tgreen\tblue\ttransparency"};
+			static char *graycoord[2] = {"x\ty\tz\tgray\ttransparency", "lon\tlat\tz\tgray\ttransparency"};
+			char *this = (nc == 3) ? rgbcoord[geo] : graycoord[geo];
+			if (first_grd) GMT_Put_Record (API, GMT_WRITE_TABLE_HEADER, this);
 			first_grd = false;
 			x = (G->x) ? G->x : gmt_grd_coord (GMT, G->header, GMT_X);
 			y = (G->y) ? G->y : gmt_grd_coord (GMT, G->header, GMT_Y);
 			gmt_M_grd_loop (GMT, G, row, col, ij) {
 				out[GMT_X] = x[col];	out[GMT_Y] = y[row];	out[GMT_Z] = G->data[ij];
 				gmt_get_rgb_from_z (GMT, P, G->data[ij], rgb);
-				for (k = 0, kol = GMT_Z + 1; k < 3; k++) out[kol++] = irint (255.0 * rgb[k]);	/* r/g/b in 0-255 range */
+				for (k = 0, kol = GMT_Z + 1; k < nc; k++) out[kol++] = irint (255.0 * rgb[k]);	/* r/g/b in 0-255 range */
 				out[kol] = irint (100.0 * rgb[k]);	/* Transparency in 0-100% range */
 				write_error = GMT_Put_Record (API, GMT_WRITE_DATA, Out);
 				if (write_error == GMT_NOTSET) n_suppressed++;	/* Bad value caught by -s[r] */

src/sample1d.c

@@ -403,18 +403,19 @@ EXTERN_MSC int GMT_sample1d (void *V_API, int mode, void *args) {
 	if (Ctrl->C.active) {	/* Just sample the CPT for last input column value and append four columns r g b a */
 		struct GMT_PALETTE *P = NULL;
 		double rgb[4] = {0.0, 0.0, 0.0, 0.0};
-		unsigned int k, col, last = Din->n_columns - 1;
+		unsigned int k, col, last = Din->n_columns - 1, nc;
 		if ((P = gmt_get_palette (GMT, Ctrl->C.file, GMT_CPT_OPTIONAL, 0.0, 255.0, Ctrl->C.dz)) == NULL) {
 			GMT_Report (API, GMT_MSG_ERROR, "Failed to read CPT %s.\n", Ctrl->C.file);
 			Return (API->error);	/* Well, that did not go so well... */
 		}
-		gmt_adjust_dataset (GMT, Din, Din->n_columns + 4);	/* Add one more output columns for r, g, b, a */
+		nc = (P->is_gray) ? 1 : 3;
+		gmt_adjust_dataset (GMT, Din, Din->n_columns + nc + 1);	/* Add one more output columns for r[, g, b], a */
 		for (tbl = 0; tbl < Din->n_tables; tbl++) {
 			for (seg = 0; seg < Din->table[tbl]->n_segments; seg++) {
 				S = Din->table[tbl]->segment[seg];	/* Current segment */
 				for (row = 0; row < S->n_rows; row++) {	/* Current point */
 					gmt_get_rgb_from_z (GMT, P, S->data[last][row], rgb);
-					for (k = 0, col = last + 1; k < 3; k++, col++)
+					for (k = 0, col = last + 1; k < nc; k++, col++)
 						S->data[col][row] = irint (255.0 * rgb[k]);	/* r/g/b in 0-255 range */
 					S->data[col][row] = irint (100.0 * rgb[k]);	/* Transparency in 0-100% range */
 				}