tiffsave: sync with jpegsave

libvips/foreign/vips2tiff.c

@@ -682,21 +682,32 @@ wtiff_compress_jpeg_header(Wtiff *wtiff,
 
 	jpeg_set_defaults(cinfo);
 
-	// disable chroma subsample for high Q
+	/* Set compression quality.
+	 */
+	jpeg_set_quality(cinfo, wtiff->Q, TRUE);
+
+	/* Disable chroma subsample for high Q.
+	 */
 	if (wtiff->Q >= 90)
 		for (int i = 0; i < image->Bands; i++) {
 			cinfo->comp_info[i].h_samp_factor = 1;
 			cinfo->comp_info[i].v_samp_factor = 1;
 		}
+	else if (image->Bands > 1) {
+		/* Use 4:2:0 subsampling, but only if the image has more than
+		 * 1 band, since sampling factors are actually meaningless with
+		 * greyscale JPEG files.
+		 */
+		cinfo->comp_info[0].h_samp_factor = 2;
+		cinfo->comp_info[0].v_samp_factor = 2;
 
-	// use RGB mode (no chroma subsample) for high Q RGB images
-	if (wtiff->Q >= 90 &&
-		image->Bands == 3)
-		jpeg_set_colorspace(cinfo, JCS_RGB);
-
-	/* Set compression quality. Must be called after setting params above.
-	 */
-	jpeg_set_quality(cinfo, wtiff->Q, TRUE);
+		/* Rest should have sampling factors 1,1.
+		 */
+		for (int i = 1; i < image->Bands; i++) {
+			cinfo->comp_info[i].h_samp_factor = 1;
+			cinfo->comp_info[i].v_samp_factor = 1;
+		}
+	}
 
 	// Avoid writing the JFIF APP0 marker.
 	cinfo->write_JFIF_header = FALSE;