user18155: In the xfade filter, the speed of transition may be controlled by the duration= parameter – longer duration implies slower transition and vice versa.
So why are there – beside the fade value for the transition= parameter – other 2 values for the same effect, fadefast and fadeslow? What is the difference?

user1871: They differ in rate of progression, i.e. fade is a linear progression, so that at half the duration, fade status is 50% i.e. both inputs are equally mixed. For fadefast, it starts fast and then slows down. Opposite for fadeslow.

18155: <p>In the <code>xfade</code> filter, the speed of transition may be controlled by the <code>duration=</code> parameter – longer duration implies slower transition and vice versa.</p>
<p>So why are there – beside the <code>fade</code> value for the <code>transition=</code> parameter – other 2 values for the same effect, <code>fadefast</code> and <code>fadeslow</code>? What is the difference?</p>
 1871: <p>They differ in rate of progression, i.e. fade is a linear progression, so that at half the duration, fade status is 50% i.e. both inputs are equally mixed. For fadefast, it starts fast and then slows down. Opposite for fadeslow.</p>
