diff --git a/doc/rst/source/grdimage.rst b/doc/rst/source/grdimage.rst
index 5d287a8b417..5b640d4b402 100644
--- a/doc/rst/source/grdimage.rst
+++ b/doc/rst/source/grdimage.rst
@@ -255,6 +255,14 @@ have chosen (perhaps implicitly) with **-nn+a** that turns *on* nearest neighbor
 gridding and turns *off* anti-aliasing.  Alternatively, use |-T|
 instead to plot individual polygons centered on each node.
 
+Imaging Categorical Images
+--------------------------
+
+If a 1-byte single layer image is given and the file has no color map then we will
+interpret the byte values as categories and a categorical CPT is required via |-C|.
+If no |-C| is given then we assume the image is a grayscale image with values in the
+0-255 range.
+
 Image formats recognized
 ------------------------
 
diff --git a/src/grdimage.c b/src/grdimage.c
index 8b098200708..d13730e0aab 100644
--- a/src/grdimage.c
+++ b/src/grdimage.c
@@ -1005,6 +1005,42 @@ GMT_LOCAL void grdimage_img_gray_no_intensity (struct GMT_CTRL *GMT, struct GRDI
 	}
 }
 
+GMT_LOCAL void grdimage_img_byte_index (struct GMT_CTRL *GMT, struct GRDIMAGE_CTRL *Ctrl, struct GRDIMAGE_CONF *Conf, unsigned char *image, unsigned char *rgb_used) {
+	/* Function that fills out the image in the special case of 1) 1-byte image, 2) no colormap, 3) external CPT given */
+	int64_t srow, scol;	/* Due to OPENMP on Windows requiring signed int loop variables */
+	uint64_t byte, kk_s, node_s, start;
+	int index, k;
+	struct GMT_GRID_HEADER *H_s = Conf->Image->header;	/* Pointer to the active data header */
+	gmt_M_unused (GMT);
+	gmt_M_unused (Ctrl);
+
+	if (gmt_M_is_dnan (Conf->Image->header->nan_value)) /* Nodata not set, offset to 1 if CPT indicates first key is 1 */
+		start = (irint (Conf->P->data[0].z_low) == 1) ? 1 : 0;	/* No "0" key in CPT so we skip it */
+	else	/* Check if the nan_value is 0 or 255 */
+		start = (irint (Conf->Image->header->nan_value) == 0 || irint (Conf->P->data[0].z_low) == 1) ? 1 : 0;	/* No "0" key in CPT so we skip it */
+
+#ifdef _OPENMP
+#pragma omp parallel for private(srow,byte,kk_s,scol,node_s,k) shared(GMT,Conf,Ctrl,H_s,image)
+#endif
+	for (srow = 0; srow < Conf->n_rows; srow++) {	/* March along scanlines in the output bitimage */
+		byte = (uint64_t)(3 * srow * Conf->n_columns);
+		kk_s = gmt_M_ijpgi (H_s, Conf->actual_row[srow], 0);	/* Start pixel of this image row */
+		for (scol = 0; scol < Conf->n_columns; scol++) {	/* Compute rgb for each pixel along this scanline */
+			node_s = kk_s + Conf->actual_col[scol];	/* Start of current pixel node */
+			index = (int)Conf->Image->data[node_s];
+			if (index < start) {	/* E.g., data is 0 for Nodata */
+				for (k = 0; k < 3; k++)
+					image[byte++] = (unsigned char)gmt_M_s255 (Conf->P->bfn[GMT_NAN].rgb[k]);
+			}
+			else {
+				for (k = 0; k < 3; k++)
+					image[byte++] = (unsigned char)gmt_M_s255 (Conf->P->data[index-start].rgb_low[k]);
+				rgb_used[index] = true;
+			}
+		}
+	}
+}
+
 GMT_LOCAL void grdimage_img_c2s_with_intensity (struct GMT_CTRL *GMT, struct GRDIMAGE_CTRL *Ctrl, struct GRDIMAGE_CONF *Conf, unsigned char *image) {
 	/* Function that fills out the image in the special case of 1) image, 2) color -> gray via YIQ, 3) with intensity */
 	bool transparency = (Conf->Image->header->n_bands == 4);
@@ -1209,7 +1245,7 @@ EXTERN_MSC int gmtlib_ind2rgb (struct GMT_CTRL *GMT, struct GMT_IMAGE **I_in);
                                   (h->wesn[YHI] > 90.0 || h->wesn[XHI] > 720.0))
 
 EXTERN_MSC int GMT_grdimage (void *V_API, int mode, void *args) {
-	bool done, need_to_project, normal_x, normal_y, resampled = false, gray_only = false;
+	bool done, need_to_project, normal_x, normal_y, resampled = false, gray_only = false, byte_image_no_cmap;
 	bool nothing_inside = false, use_intensity_grid = false, got_data_tiles = false, rgb_cube_scan;
 	bool has_content, mem_G = false, mem_I = false, mem_D = false, got_z_grid = true;
 	unsigned int grid_registration = GMT_GRID_NODE_REG, try, row, col, mixed = 0, pad_mode = 0;
@@ -1328,6 +1364,11 @@ EXTERN_MSC int GMT_grdimage (void *V_API, int mode, void *args) {
 			}
 		}
 	}
+	byte_image_no_cmap = (I && I->type == GMT_UCHAR && I->n_indexed_colors == 0);
+	if (byte_image_no_cmap && !Ctrl->C.active) {
+		GMT_Report (API, GMT_MSG_ERROR, "Byte image without indexed color requires a CPT via -C\n");
+		Return (GMT_RUNTIME_ERROR);
+	}
 
 	gmt_detect_oblique_region (GMT, Ctrl->In.file);	/* Ensure a proper and smaller -R for oblique projections */
 
@@ -1628,11 +1669,27 @@ EXTERN_MSC int GMT_grdimage (void *V_API, int mode, void *args) {
 	/* Get or calculate a color palette file */
 
 	has_content = (got_z_grid) ? false : true;	/* Images always have content but grids may be all NaN */
-	if (got_z_grid) {	/* Got a single grid so need to convert z to color via a CPT */
+	if (byte_image_no_cmap) {	/* Need external CPT for byte indices */
 		if (Ctrl->C.active) {	/* Read a palette file */
-			double zmin = Grid_orig->header->z_min, zmax = Grid_orig->header->z_max;
-			char *cpt = gmt_cpt_default (API, Ctrl->C.file, Ctrl->In.file, Grid_orig->header);
-			grdimage_reset_grd_minmax (GMT, Grid_orig, &zmin, &zmax);
+			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);
+				gmt_free_header (API->GMT, &header_I);
+				Return (API->error);	/* Well, that did not go so well... */
+			}
+			gray_only = (P && P->is_gray);	/* Flag that we are doing a gray scale image below */
+			Conf->P = P;
+			if (P && P->has_pattern) GMT_Report (API, GMT_MSG_WARNING, "Patterns in CPTs will be ignored\n");
+		}
+	}
+	else if (got_z_grid ) {	/* Got a single grid so need to convert z to color via a CPT */
+		if (Ctrl->C.active) {	/* Read a palette file */
+			double zmin = 0.0, zmax = 0.0;
+			char *cpt = (byte_image_no_cmap) ? strdup (Ctrl->C.file) : gmt_cpt_default (API, Ctrl->C.file, Ctrl->In.file, Grid_orig->header);
+			if (!byte_image_no_cmap) {
+				zmin = Grid_orig->header->z_min;
+				zmax = Grid_orig->header->z_max;
+				grdimage_reset_grd_minmax (GMT, Grid_orig, &zmin, &zmax);
+			}
 			if ((P = gmt_get_palette (GMT, cpt, GMT_CPT_OPTIONAL, zmin, zmax, Ctrl->C.dz)) == NULL) {
 				GMT_Report (API, GMT_MSG_ERROR, "Failed to read CPT %s.\n", Ctrl->C.file);
 				gmt_free_header (API->GMT, &header_G);
@@ -1788,8 +1845,15 @@ EXTERN_MSC int GMT_grdimage (void *V_API, int mode, void *args) {
 	Conf->int_mode  = use_intensity_grid;
 	Conf->nm        = header_work->nm;
 
+	if (byte_image_no_cmap) {	/* Check if we have a nan_value (No data pixel) */
+		if (gmt_M_is_dnan (Conf->Image->header->nan_value) || irint (Conf->Image->header->nan_value) == 0 || irint (Conf->P->data[0].z_low) == 1) { /* Nodata given as 0 */
+			Ctrl->Q.active = true;
+			rgb_cube_scan = true;
+		}
+	}
+
 	NaN_rgb = (P) ? P->bfn[GMT_NAN].rgb : GMT->current.setting.color_patch[GMT_NAN];	/* Determine which color represents a NaN grid node */
-	if (got_z_grid && Ctrl->Q.active) {	/* Want colormasking via the grid's NaN entries */
+	if ((got_z_grid || byte_image_no_cmap) && Ctrl->Q.active) {	/* Want colormasking via the grid's NaN entries */
 		if (gray_only) {
 			GMT_Report (API, GMT_MSG_INFORMATION, "Your image is gray scale only but -Q requires building a 24-bit image; your image will be expanded to 24-bit.\n");
 			gray_only = false;	/* Since we cannot do 8-bit and colormasking */
@@ -1870,7 +1934,7 @@ EXTERN_MSC int GMT_grdimage (void *V_API, int mode, void *args) {
 			bitimage_24 = gmt_M_memory (GMT, NULL, 3 * header_work->nm + Conf->colormask_offset, unsigned char);
 			if (Ctrl->Q.transp_color)
 				for (k = 0; k < 3; k++) bitimage_24[k] = gmt_M_u255 (Ctrl->Q.rgb[k]);	/* Scale the specific rgb up to 0-255 range */
-			else if (P)	/* Use the CPT NaN color */
+			else if (P && Ctrl->Q.active)	/* Use the CPT NaN color */
 				for (k = 0; k < 3; k++) bitimage_24[k] = gmt_M_u255 (P->bfn[GMT_NAN].rgb[k]);	/* Scale the NaN rgb up to 0-255 range */
 			/* else we default to 0 0 0 of course */
 		}
@@ -1927,6 +1991,8 @@ EXTERN_MSC int GMT_grdimage (void *V_API, int mode, void *args) {
 					grdimage_img_gray_no_intensity (GMT, Ctrl, Conf, bitimage_8);
 				else if (Ctrl->M.active) 	/* Image, color converted to gray, with intensity */
 					grdimage_img_c2s_no_intensity (GMT, Ctrl, Conf, bitimage_8);
+				else if (byte_image_no_cmap)
+					grdimage_img_byte_index (GMT, Ctrl, Conf, bitimage_24, rgb_used);
 				else	/* Image, color, no intensity */
 					grdimage_img_color_no_intensity (GMT, Ctrl, Conf, bitimage_24);
 			}
diff --git a/src/psscale.c b/src/psscale.c
index 9457d34f90f..461993c66ed 100644
--- a/src/psscale.c
+++ b/src/psscale.c
@@ -828,7 +828,7 @@ GMT_LOCAL void psscale_draw_colorbar (struct GMT_CTRL *GMT, struct PSSCALE_CTRL
 
 	if (P->categorical) {	/* For categorical CPTs the default is -L<gap> with sum of all gaps = 15% of bar length  */
 		Ctrl->L.spacing = gap = 0.01 * PSSCALE_GAP_PERCENT * fabs (length) / (P->n_colors - 1);
-		B_set = false;
+		B_set = false;	/* And no -B can be used now */
 	}
 
 	max_intens[0] = Ctrl->I.min;