How can I transform an object/sprite smoothly? (like growing a button or rotating smoothly)
Increment by dt every tick is a general answer.
Set Sprite scale to 1+dt
Set Sprite angle to Sprite.Angle+dt
Use a multiplier of dt to change the rate.
Can you help with one more thing?
how do I make smooth opacity fade?
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Same thing. Set opacity to sprite.opacity-dt
If you want to constrain it so it doesn't go negative, you would do max(0,sprite.opacity-dt)
Alternatively, use the fade behavior.