how player collision？

[zhaishengfu]

wonderful projects， thanks。
I have seen noita gdc talk and your answer for "rigidbody collision", and you said you have "pixel perfect player collision" . i wonder how you do that for realistic sand. for example:

1. when people falling to sand, it will trap in it, and some sand will fly to the air
2. when people move in sand, the speed will slow, and sand will also change
3. when people swim in water, water will move

the way i can think is: make a mask image for the player, and when the pixel is player mask, it will do the logic, but how to do the logic(for example, player move in sand)