I've been working on a deferred decal system. So far I have finished the projection part, meaning I can click something in the scene and it will properly project a decal onto the surface of the object.

There are a couple other things I wish to add to this system: normal clipping and normal mapped decals. Normal clipping is when I discard fragments that are stretched across a sharp angle. Seen here. A way to solve this is to discard fragments that have a large difference between the normal in the G-Buffer and the decal's normal. This would require reading the G-buffer's normal texture.

In order to do decal normal mapping, I need to blend the decal's normal (from a normal map) with the G-buffer's normal. The requires writing to the G-buffer's normal texture.

I hope it's clear that in order to support these two features, I need to read and write to the normal texture in the same shader pass. Sadly, this is undefined behavior, so I'm wondering if there is some other way to implement these features.