{focal11,focal12}.png are taken from http://www.ampercent.com

{focal21,focal22}.png are taken from http://www.cs.toronto.edu/~jepson/csc320/notes/pyramids.pdf

{focal31,focal32,focal41,focal42}.png are taken from Bai et al., Edge preserved image fusion based on multiscale toggle contrast operator, Image and Vision Computing, 29(12), 2011