Another challenge is caused by occlusion. In the rectangular geometry all the image sources are visible to all the listener locations inside the room but this does not generalize to any geometry. Let us start by looking a simple geometry of two surfaces such that the higher one blocks the visibility of the sound source to a section on the lower one.
It is important to note that the ray-path validation required to handle the cases discussed above is a separate process from the image-source construction. The validity of a given path depends on the location of the listener and it does not affect the actual image source. Especially, this means that if a reflection path from a given order image source is invalid for a specific receiver location it still needs to be reflected further to produce higher order image sources as a higher order path utilizing this image source may still be realizable for the same receiver location. In the image source tree this is seen such that a node can be valid for a given listener location (red in the tree illustration) although none of its ancestors is valid for that specific listener (black node in the tree).
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