normals

Software:
Unreal Engine 4.24

The TeoSidedSign node let’s the shader “know” if a rendered polygon is facing the camera or not by outputting a value of 1 for facing polys and -1 for back-facing polys.
This is useful for creating materials that have different properties when seen front-facing or back-facing.

Example 1:
Blending two different colors based on face direction:

1. Check the Two Sided material attribute.
   * Needed so that the engine will render the polygons beck sides.
2. In the material blueprint, create a blend of to colors using a Lerp (LinearInterpolate) node and connect it to the material’s Base Color input.
3. Add a TwoSidedSign node to get polygon facing input (1,-1).
4. Connect the TwoSidedSign node’s output to a Clamp node to clamp the values to (1,0).
5. Connect the  Clamp node’s output to the Lerp node’s Alpha input so that the polygon’s facing direction will control the Lerp blend.

Note:
You can use this method to blend any other material attribute based on polygon facing direction.

Example 2:
Create an “inwards facing” flipped normal material:

1. Set the material’s Blend Mode to Masked.
   * Needed for being able to make areas parts of the mesh invisible.
2. Check the Two Sided material attribute.
   * Needed so that the engine will render the polygons beck sides.
3. Add a TwoSidedSign node to get polygon facing input (1,-1).
4. Connect the TwoSidedSign node’s output to a Clamp node to clamp the values to (1,0).
5. Connect the Clamp node’s output to a 1-X node to invert the facing input.
6. Connect the 1-X node’s output to the material’s Opacity Mask input so that polygons facing the camera will be invisible.

Related:

1. Blending material using Paint
2. Material Functions
3. UE4 Bump Map