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How to detect transparent/coloured parts of a mask?

I’m making a top-down 2D map and figured it’d be much faster to paint a collision mask to determine where the player can and can’t go. I could then trace from the cam to the mask to determine whether the area was walk-able or not. However I can’t work out how to find the opaque/transparent parts of a material/mask. I could cut around in a 3D modelling program but that would take much longer and would need to be re-modeled after every change.

If not by alpha, is there a way to get the colour of a material from a location or line trace?

Here’s a visual representation of what I want to do. The pink represents where the mask would be opaque:

Any suggestions?