google maps with matrix - pixel grid

im new to google maps API, tried searching the web for the answer, but
nothing. Also did some reading, and nothing. I want to color (an
overlay) a region on the map, but i need a way to consider the map as
a pixel grid. So that i could run with a nested loop on the map and
give each pixel a color (some sort of a heat map).
this has to be done dynamicaly (no image overlay) .
Ofcourse the matrix's size can be constant.
Imagine a rect/circle polygon (based on points selected), that i want
to run on each pixel of it and give it a color (lets say one of 5
color based on some math formula).
How can i do this and maybe a link to an example would be great.

P.s
im using PHP and mySQL

thanks!

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