how to get the shadow of the page curl

I want to finish the page curl effect,see my output http://i.6.cn/cvbnm/18/43/49/eb2aa5204761ecacee06f10f43036aea.jpg .

I have got the effect of page curl,but i do not know , how to ge开发者_如何学编程t the shadow of the curve page footer , I have to use black pen to mark the shadow that i need to, the shadow is so fuzzy. My question is that how to use maths mathos to solve the shadow problem ,can any one give me the mathematical formula, and how to draw the shadow.

i have sloved the problem: if want to determine the shadow, you need to know the fixed-point of the shadow,i assuming the fixed-point is a1,so triangle abk is similar with triangle a1di,so we can get the coordinates of a1. bu using |bk|/|id| =|b.x-a.x|/|d.x-a1.x| ,|bk| is the distance point b and k.