vipsthumbnail copies metadata to embedded thumb

when working from the embedded thumbnail, vipsthumbnail now copies the
metadata from the main image onto the thumbnail

see

https://github.com/jcupitt/libvips/issues/109

thanks ottob

TODO

@@ -1,12 +1,4 @@
 
-
-- vipsthumbnail needs to attach main image metadata to the jpeg header 
-  thumbnail, see:
-
-	  https://github.com/jcupitt/libvips/issues/109
-
-
-
 - think of a better way to support skipahead
 
   extract needs to hint to it's image sources what line it will start reading

tools/vipsthumbnail.c

@@ -189,6 +189,37 @@ thumbnail_find_jpegshrink( VipsImage *im )
 
 #define THUMBNAIL "jpeg-thumbnail-data"
 
+/* Copy a blob from one image to another.
+ *
+ * We don't make a private copy of the blob, we just copy pointers. Therefore
+ * @from must stay alive as long as @to is alive. 
+ */
+static int
+copy_blob( VipsImage *from, VipsImage *to, const char *field )
+{
+	if( vips_image_get_typeof( from, field ) ) {
+		void *blob;
+		size_t blob_length;
+
+		if( vips_image_get_blob( from, field, &blob, &blob_length ) )
+			return( -1 );
+		vips_image_set_blob( to, field, NULL, blob, blob_length ); 
+	}
+
+	return( 0 ); 
+}
+
+static void *
+copy_exif_field( VipsImage *from, const char *field, GValue *value, void *a )
+{
+	VipsImage *to = (VipsImage *) a;
+
+	if( vips_isprefix( "exif-", field ) ) 
+		vips_image_set( to, field, value ); 
+
+	return( NULL ); 
+}
+
 /* Try to read an embedded thumbnail. 
  */
 static VipsImage *
@@ -228,6 +259,16 @@ thumbnail_get_thumbnail( VipsImage *im )
 		return( NULL ); 
 	}
 
+	/* Copy over the metadata from the main image. We want our thumbnail
+	 * to have the orientation, profile etc. from there.
+	 */
+	if( copy_blob( im, thumb, VIPS_META_EXIF_NAME ) || 
+		copy_blob( im, thumb, VIPS_META_XMP_NAME ) ||
+		copy_blob( im, thumb, VIPS_META_IPCT_NAME ) ||
+		copy_blob( im, thumb, VIPS_META_ICC_NAME ) )
+		return( NULL ); 
+	(void) vips_image_map( im, copy_exif_field, thumb );
+
 	vips_info( "vipsthumbnail", "using %dx%d jpeg thumbnail", 
 		thumb->Xsize, thumb->Ysize );