Hi i’ve ran into an issue that i cannot believe hasn’t popped up before, for me at least. How does the node setup look if you have multiple normal maps?
I have 2 normal maps, one is just scratches and the other a repeating texture. I need to be able to control the strength on both and to feed them into my normal slot of a principled shader.
Im not sure using photoshop to merge them is such a good idea as i want to have more control and have them separate. Any help?
A simple approach is to transform the normal maps into world coordinates with NormalMap nodes, mix both normals with an RGBmixer and normalize the output color with a VectorMath node. Note that this will just output an interpolation from one normal vector to the other.
If you need a more complex approach, you can use this node setup. This one will let you use a base normal map, and add details to it from a secondary normal map, using quaternion rotations. For some setups (like adding scratches to a bumped surface) this will give you better results.
cant you just daisy chain normal map nodes.
I remember seeing a nodegroup for mixing normal maps, let me try and find it real quick.
Here it is! https://www.blendswap.com/blends/view/76176
It’s exactly the same nodegroup! 
Stan_Pancakes is radcapricorn in Blendswap. 
There is another way to do this. All you need is quixel ddo and setup 2 normal maps. THey use different method that needs less computing power and produce similar result.
Fantastic guys, thanks. Got her working now
Out of curiousity… if you were to make something super complex with 4 or 5 normal maps, is this do-able? I assume movie production assets have huge details and multiple maps.
I don’t think normal maps are used much in movie production. Using displacement you get longer renders but avoid all the problems with normal maps.
Yes, it’s doable. But for final assets, you should bake the mixed normal map to speed up renderings.
@CarlG, they are still used everywhere… displacement is something very expensive and difficult to control, specially to represent small details.