III.2.4 Image-source Method in an Arbitrary Geometry
The simple case of a rectangular room is an exceptionally easy one. The rectangular room is an idealistic case where all of the image sources produce a realizable reflection path to all possible listener locations inside the room. However, in practice the geometries are often more complex and there is need for a separate reflection-path validation. In the following, we go through such cases that need more attention. First, let us see a case in which the reflecting surface is of finite length instead of being infinite or ending in a right-angle corner.

- You can move the receiver around and see that at some locations the reflection path is not realizable.
- If you move the receiver below the reflector there is not valid reflection either.

Show image sources: OK On wrong side of reflector Out of reflector

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.

- In the initial situation, the upper surface blocks the reflection from the lower one. But if you move the listener, you can find positions in which the reflection path is valid.
- By moving both the sound source and the listener you can make reflections from both of the surfaces audible at the same time.

Show image sources: OK On wrong side of reflector Out of reflector Obstructed

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).