Technical problem

I want to know how to erase the sand when the mouse moves and the background becomes something else instead of this white picture. What should I do?

Hi @shiqi17,

the basis is there - you have a feedback loop that creates a mask - now you can use this mask with the Matte TOP to reveal a different texture. Have a look at the Matte TOP’s OP Snippets for how to use this operator.

cheers
Markus