OnRenderImage はすべてのレンダリングが RenderImage へと完了したときに呼び出されます。
It allows you to modify final image by processing it with shader based filters. The incoming image is
source render texture. The result should end up in
destination render texture. You must always issue a Graphics.Blit() or render a fullscreen quad if you override this method.
When there are multiple image filters attached to the camera, they process image sequentially, by passing first filter's destination as the source to the next filter.
This message is sent to all scripts attached to the camera.
See Also: Writing Post-Processing Effects.