- * If a request is successful then the filter_frame() function will be called
- * at least once before ff_request_frame() returns
+ * If a request is successful then some progress has been made towards
+ * providing a frame on the link (through ff_filter_frame()). A filter that
+ * needs several frames to produce one is allowed to return success if one
+ * more frame has been processed but no output has been produced yet. A
+ * filter is also allowed to simply forward a success return value.