image transformation using perspective transform

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C++

sir

i m working on the image transformation project in visual c++ with the help of anti grain geometry api

http://www.antigrain.com/[^]

on this link there is api and in that api in example folder i got image_perspective transform.
it is four point transform so i am able to strech image using for corners as map_matrix can take only 4 points.
now i want to strech image from the 4 point wich are the mid point of four edges of image. as well as from the centre point of image.

plz let me know how to add more than 4 point in transformation