hen implementing post processing of ones 3d scene such as bloom or AO does one discard the rendered 3d scene and just use a rendered quad as what the final image that will be seen or does one overlay those affects and blend them with the actual rendered scene? I imagine there are multiple ways of doing this but i would just like to know what is typically the industry standard.
It depends on what operations you're doing in that final pass where you combine all your post-effects into a final buffer. If you're doing something simple like adding in bloom effects, then you could just blend them additively onto the existing buffer. Or, you might pass a pointer to that buffer as a texture into the final shader and render the final pixel color back to the buffer. This should be ok if you're just reading and writing only within that single pixel. If you're reading multiple texels around the target UV coordinates for a blur or whatever, then you'll be reading some before and some after the render operation, which wont be correct. Any such operation should happen beforehand into another buffer, which you will then source as a texture in the final shader.