Some day, I started to think: why do I need separate normal and diffuse images?
And I found a solution: I maked a sobel nodegroup for Cycles.
Here it is:
P.S. Replaced the group because it has a tooltip.
P.S. Replaced the group because it has a tooltip.
Any image of usage example? I’m not sure if I understand you 
just from the node tree I can say - image texture for normal map will be faster then this maths
Or…
This node setup does the same thing with only a small fraction of the complexity. The key here is sampled coordinate manipulation (so you use coordinate manipulation with a noise texture to blur a copy of the image texture and get the difference between it and the unaltered texture).