I want to implement Zoom in and Zoom out in my OpenGL application.

Currently I only find from the document that I need to implement call back function for below events:

EVAS_CALLBACK_MULTI_DOWN

EVAS_CALLBACK_MULTI_UP

EVAS_CALLBACK_MULTI_MOVE

But who can help to describe detail how to use these events to realize Zoom in/out like already done in photocam.

How can I recogonize the zoom in/out gesture from multitouch events?