Implementing Page Turn Effect

#page_turn #objective_c

This article was carelessly deleted by me. So let me rewrite it again.

When I worked on a productivity app - an electronic notebook - for iOS, the customer requested a feature "page turn effect", which is an animation like this:

Page Turn Effect

There is a common rule of thumb for this kind of problems: do NOT start coding until you have thought it over and everything is clear on paper. Therefore, let's do some high school geometry:

Fig. 1: when A is a single point

Take a look at figure 1. The user's finger is at point P. She sees page 1's front, page 2's front, page 2's back, and page 3's front, among which, the triangle "page 2 back" and "page 3 front" are identical. That is the first situation.

The second situation is as below:

Fig. 2: when "A" is a group of points

We have 3 point As: A, A1, and A2. This is a little bit complicated, but we can prove that if we extend line CA, BA1, and PA2, they will converge, making two identical triangles.