[FFmpeg-user] Hello. Gurus. Regarding PNG image overlay with alpha transparency change

[Lee Mitchell]

Hi.

What I know is alpha change for another video clip pon base video like as
"-filter_complex
"[1:v]geq=r='r(X,Y)':a='if(between(t,0,9),alpha(X,Y)*(T/9),0)'[alpha];"
which works very well as time goes by.

But it is not working for a PNG image file as time goes by. It only sets
alpha when the overlay starts which does not change while playing.
How to change PNG alpha by base video clip's time?
Could you guys advise me or hint me on keywords?

Thank you in advance.