note and use res unit on jpg load/save

previously, jpeg save always used pixels/inch and jpeg load converted to
vips pixels/mm

now on jpg load the image's res unit is recorded in
VIPS_META_RESOLUTION_UNIT, and on jpg save the res unit is set from
VIPS_META_RESOLUTION_UNIT (or defaults to inches).

you can now copy a cm-preferring tiff to a jpg and the unit is preserved

ChangeLog

@@ -6,6 +6,7 @@
 - so affinei and affinei_all appear in Python
 - be more cautious enabling YCbCr mode in tiff write
 - add "DEPRECATED" flag to arguments
+- jpeg load/save note and use the preferred resolution unit
 
 20/7/12 started 7.30.0
 - support "rs" mode in vips7

libvips/foreign/jpeg2vips.c

@@ -45,6 +45,8 @@
  * 	- read jfif resolution as well as exif
  * 19/2/12
  * 	- switch to lazy reading
+ * 7/8/12
+ * 	- note EXIF resolution unit in VIPS_META_RESOLUTION_UNIT
  */
 
 /*
@@ -493,6 +495,8 @@ set_vips_resolution( VipsImage *im, ExifData *ed )
 		 */
 		xres /= 25.4;
 		yres /= 25.4;
+		vips_image_set_string( im, 
+			VIPS_META_RESOLUTION_UNIT, "in" );
 		break;
 
 	case 3:
@@ -500,6 +504,8 @@ set_vips_resolution( VipsImage *im, ExifData *ed )
 		 */
 		xres /= 10.0;
 		yres /= 10.0;
+		vips_image_set_string( im, 
+			VIPS_META_RESOLUTION_UNIT, "cm" );
 		break;
 
 	default:

libvips/foreign/vips2jpeg.c

@@ -48,6 +48,8 @@
  * 	- rebuild exif tags from coded metadata values 
  * 24/11/11
  * 	- turn into a set of write fns ready to be called from a class
+ * 7/8/12
+ * 	- use VIPS_META_RESOLUTION_UNIT to select resoltuion unit
  */
 
 /*
@@ -345,13 +347,33 @@ static int
 set_exif_resolution( ExifData *ed, VipsImage *im )
 {
 	double xres, yres;
+	char *p;
 	int unit;
 
-	/* Always save as inches - more progs support it for read.
+	/* Default to inches, more progs support it.
 	 */
-	xres = im->Xres * 25.4;
-	yres = im->Yres * 25.4;
 	unit = 2;
+	if( vips_image_get_typeof( im, VIPS_META_RESOLUTION_UNIT ) &&
+		!vips_image_get_string( im, VIPS_META_RESOLUTION_UNIT, &p ) &&
+		vips_isprefix( "cm", p ) ) 
+		unit = 3;
+
+	switch( unit ) {
+	case 2:
+		xres = im->Xres * 25.4;
+		yres = im->Yres * 25.4;
+		break;
+
+	case 3:
+		xres = im->Xres * 10.0;
+		yres = im->Yres * 10.0;
+		break;
+
+	default:
+		vips_warn( "VipsJpeg", 
+			"%s", _( "unknown EXIF resolution unit" ) );
+		return;
+	}
 
 	if( write_tag( ed, EXIF_TAG_X_RESOLUTION, EXIF_FORMAT_RATIONAL, 
 		vips_exif_set_double, (void *) &xres ) ||