small fixes

libvips/colour/colour.c

@@ -835,6 +835,7 @@ vips_colour_operation_init( void )
 	extern GType vips_icc_export_get_type( void ); 
 	extern GType vips_icc_transform_get_type( void ); 
 #endif
+	extern GType vips_dE76_get_type( void ); 
 
 	vips_colour_convert_get_type();
 	vips_Lab2XYZ_get_type();
@@ -861,4 +862,5 @@ vips_colour_operation_init( void )
 	vips_icc_export_get_type();
 	vips_icc_transform_get_type();
 #endif
+	vips_dE76_get_type(); 
 }

libvips/colour/dE76.c

@@ -90,6 +90,9 @@ vips_dE76_class_init( VipsdE76Class *class )
 static void
 vips_dE76_init( VipsdE76 *dE76 )
 {
+	VipsColourDifference *difference = VIPS_COLOUR_DIFFERENCE( dE76 ); 
+
+	difference->interpretation = VIPS_INTERPRETATION_LAB;
 }
 
 /**

libvips/iofuncs/operation.c

@@ -130,6 +130,8 @@ vips_operation_usage( VipsOperationClass *class, VipsBuf *buf )
 			buf, &usage );
 	vips_buf_appends( buf, "\n" );
 
+	vips_buf_appendf( buf, "%s\n", object_class->description );
+
 	/* Show required types.
 	 */
 	usage.message = "where:";