• The effect will render no matter what (through walls and when visible). As of now with the current API I attempted to make it work only in certain conditions but don't think it is possible.
• Currently there is a problem with alpha which I cannot figure out so for now alpha will be forced to one. If you try to decrease the alpha in the color picker nothing will happen. It is intended.
• If you are curious on what the issue is it is lack of access to stencil buffer. You can only write it to render targets and have no way of passing written stencils from one render target to another without setting the render target. However, the issue comes with the composite shader as you are not setting a render target at the end since you want to render. I have spent quite some time trying to find any workaround but couldn't. The issue can be fixed if I could just write to a global stencil buffer or maybe have a way to just be able to pass the stencil buffer to any shader.

Github: