Question: "Adding" Density Functions via Convolution

Greetings,

I have generated two random points in the unit square.  Each point has an x and y coordinate.  The density function for the (absolute) distance between each point is (2-2d) in both the x and y direction.  These distances are shown by the "f" and "g" functions attached.

The overall (rectilinear) difference is what I am after.  The attached worksheet shows the rectilinear distance as "h".  

My question is this...how can I obtain this rectilinear distance by using convolution and the "starting" density function of (2-2d) ? 

This is probably more of a mathematics function than a Maple question, but the "rules" of convolution, particularly setting the upper and lower boundaries of the integrals confuse me.

DensityFunction.mw