- * Callback called before passing the first slice of a new frame. If
- * NULL, the filter layer will default to storing a reference to the
- * picture inside the link structure.
- *
- * The reference given as argument is also available in link->cur_buf.
- * It can be stored elsewhere or given away, but then clearing
- * link->cur_buf is advised, as it is automatically unreferenced.
- * The reference must not be unreferenced before end_frame(), as it may
- * still be in use by the automatic copy mechanism.
- *
- * Input video pads only.
- *
- * @return >= 0 on success, a negative AVERROR on error. picref will be
- * unreferenced by the caller in case of error.