int age_index;
/**
- * Frame rate of the stream on the link, or 1/0 if unknown;
- * if left to 0/0, will be automatically be copied from the first input
+ * Frame rate of the stream on the link, or 1/0 if unknown or variable;
+ * if left to 0/0, will be automatically copied from the first input
* of the source filter if it exists.
*
* Sources should set it to the best estimation of the real frame rate.
+ * If the source frame rate is unknown or variable, set this to 1/0.
* Filters should update it if necessary depending on their function.
* Sinks can use it to set a default output frame rate.
* It is similar to the r_frame_rate field in AVStream.