Adding API test for plotting a global matrix/grid for different centr…

…al meridians (#3811) (#3812)

* Explore passing matrix to grdimage for different central longitudes

* Update testapi_matrix_360.c

* Try rotating the grid

* Update testapi_matrix_360.c

* Add the test script

* Update gmt_api.c

Co-authored-by: Paul Wessel <pwessel@hawaii.edu>

src/testapi_matrix_360.c

@@ -0,0 +1,34 @@
+#include "gmt_dev.h"
+
+/* Testing the passing of a matrix to grdimage for global projection
+ * when the central meridian changes. This currently fails as in
+ * https://github.com/GenericMappingTools/pygmt/issues/515#issue-655281714
+ */
+
+int main () {
+	unsigned int mode = GMT_SESSION_EXTERNAL;
+	struct GMT_MATRIX *M = NULL;
+	char input[GMT_VF_LEN], args[256] = {""};
+	struct GMTAPI_CTRL *API = NULL;
+
+	API = GMT_Create_Session ("test", 2U, mode, NULL);
+
+	/* Read in earth_relief_01d as a matrix from text file and set correct region, inc, registartion */
+	M = GMT_Read_Data (API, GMT_IS_MATRIX, GMT_IS_FILE, GMT_IS_SURFACE, GMT_READ_NORMAL, NULL, "@earth_relief_01d.txt", NULL);
+	M->range[0] = -180;	M->range[1] = 180; M->range[2] = -90;	M->range[3] = 90.0;
+	M->inc[0] = M->inc[1] = 1.0;
+	M->registration = 1;
+	/* Create a virtual file to pass as a grid */
+	GMT_Open_VirtualFile (API, GMT_IS_GRID|GMT_VIA_MATRIX, GMT_IS_SURFACE, GMT_IN|GMT_IS_REFERENCE, M, input);
+	/* Call grdimage with central longitude 0, which is the center of the grid */
+	sprintf (args, "%s -Rg -JH0/6i -Bg30 -K -Cgeo -P", input);
+	GMT_Call_Module (API, "grdimage", GMT_MODULE_CMD, args);
+	GMT_Init_VirtualFile (API, 0, input);
+	/* Call grdimage with central longitude 180 which means grid needs to be rotated 180 */
+	sprintf (args, "%s -R -JH180/6i -Bg30 -O -Cgeo -Y3.5i", input);
+	GMT_Call_Module (API, "grdimage", GMT_MODULE_CMD, args);
+	GMT_Close_VirtualFile (API, input);
+
+	if (GMT_Destroy_Session (API)) return EXIT_FAILURE;
+	exit (0);
+}

test/api/apimat_360.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+#
+# Test the C API for passing a global grid as a matrix to grdimage
+# and have the central meridian work correctly.
+
+ps=apimat_360.ps
+testapi_matrix_360 > $ps