Opengl es 2.0 curl effect


I am new to opengl es 2.0
the task i want to perform is a curl effect. its basically shown in this video
i could produce the curl as shown in the image
. I want to reduce the degree of curl and get back to original image in a step manner.

I assume that i have to modify the vertices positions in a step manner and update the texture in new frame.

I am not knowing how to processed. Can you please help me.

Thank you everyone