Detect colour beneath an img object

[Barry Barber]

Hello all,
This question will probably (I hope) be easier for some of you to answer than for me to formulate!

I am trying to detect when the colour changes below the loc of an image (png) object lying 
above another of the same type. The catch is that the top img would "see" the underlying colour 
through a transparent hole in another png which is on an intermediate level between the two of them!

To put it another way:  there are three png images directly superimposed, the one on the middle 
layer has a 'hole' or 'window' in it and I need the top layer to detect when it is over the window (i.e., detect the change in colour beneath it's loc).

Here's hoping!
Barry Barber



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