Pure javascript / HTML5 canvas bilinear image interpolation

This code is written to be easy to read, there’s plenty of room for optimization. First, the inner function call in the inner part of the two-levels pixel processing loop is probably a performance killer.
http://www.philou.ch/pic/fear_makes_the_wolf_look_bigger.jpg
http://www.philou.ch/playground.html

http://www.philou.ch/js-bilinear-interpolation.html