Revelations Using 4 Nested For Loops

I've been working on a Processing sketch to send pixel brightness information from a video to a Max/Msp sketch - doing a kind of reverse audio visualizer.  Originally, I'd set out scanning pixel by pixel until Zach L. showed me how to use 4 ' for loops ' to move around the image, by superimposing a larger 2D array over a smaller 2D array.

  I spent the morning working through it and now am in love with printing out code.. 

photo (6).JPG