Detail Normals are a easy win when it comes to adding fidelity to your work. I speak about it in this write up 
Pushing Fidelity. This page however is breaking down how to set it up for use in your unreal project. Its pretty straight forward but this will make sure you get a good understanding of how to use it to benefit you the most.
The Node Structure
This everyone I'm sure is familiar with. Getting a exposed parameter is really good for tiling the detail normal since a detail normal might need to tile more or less on a specific prop.
A Detail Normal is usually 512x but can also be 256x if needed.
This is the node that does all the work for you when it comes to the strength of the detail normal. Also extremely useful to have exposed. Flatness input is explained below.
This is a exposed parameter that is connected to the Flatten Normal Node mentioned just above. By default the way it works is 1 in the value is completely flat but I like to put a OneMinus after it so that from a end user perspective it makes more sense that 0 if flat and 1 is full strength. Don’t forget that you can scale this past 1 when you need.
Image Comparisons
Expanding The Usage
We can also apply masks to control the flatness of a detail normal. In this example you can use the same mask you have for metallic to mask out detail normals. That way the stone detail normal doesn’t show up on the gold surfaces. This is more useful when your working on baked props with no tiling materials on them.
Blending with Baked Normals
You can take the same setup with your detail normals from above and blend it to a baked normal using the BlendAngleCorrectedNormals Node.
Hope This was useful and helps you understand detail normals that much more.
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