AFAIK, the GPU will call this material / shader for a given TexCoord (UV) that will be input into this custom node. It will change but only on the GPU as it is making calls in the scene. I think you are wanting to iterate over the entire screen and while you could do that you should consider a much better method as it will run very slowly… but I’m not a shader programmer or technical artist by any means and my knowledge is limited in this area.