Rotating meshes using Vertex Shaders

Rotating (ambient) meshes in your world adds a dynamic element, but doing this on the CPU and having to pass it to the GPU each frame is a relatively slow operation. Without realizing you may be updating your collision every tick too, causing overlap updates and hurting performance even more.

There is an old trick to instead rotate all vertices on the GPU directly using the vertex shader, completely omitting the CPU. Keep in mind that this does not update the rotation of the mesh collision. You should either disable collision or consider using a simple collision shape that doesn’t need to rotate and handles approximate collision regardless of the rotation.

While this trick is pretty common I was surprised to not find clear information on it for UE4. Especially on fixing the normals after moving the vertices around. So this post should serve as a complete tutorial on how to rotate meshes on the GPU within Unreal Engine.

In case you’re wondering, the planet’s mesh in the background isn’t rotating since it’s just a sphere. Instead, you simply add a UV-panner to the texture.

Rotating Vertices on Axis

Rotating the vertices is luckily very easy as we have a ready-to-use material node available: RotateAboutAxis. We plug this into World Position Offset with some simple inputs like Time to determine rotation angle, and the object’s orientation for its rotation Axis.

Note: I used the ObjectPivotPoint, but keep in mind that its not available in the Pixel-shader. The “Normal”-pin runs on the pixel-shader while the “World Position Offset”-pin runs on the vertex shader.

You may notice your lighting doesn’t correctly update now that your mesh rotates on the GPU. We’ll need to fix the vertex normals next.

Fixing Vertex Normals

With the vertices moved around we need to fix the vertex normals as they still point the original direction, causing lighting issues. Again, we have a Material Function already available: FixRotateAboutAxisNormals.

I was able to directly plug this into the pixel shader which is the “Normal” pin. The recommended way (as mentioned in the Material Function’s description) is to add CustomizedUVs (option in the Material properties itself) to the material and instead re-calculate the normals in the vertex shader and pass it as UVs to the pixel shader. This can heavily reduce the number of times you need to re-calculate normals. (equal to the number of vertices in your mesh instead of the number of pixels on-screen for that mesh) This is especially useful if your meshes have low vertex counts.

Closing

Hopefully, you’ve found this helpful. I’m using this ancient trick to rotate ambient meshes such as the space station and asteroids. But don’t forget your collision doesn’t update, so if you want players to interact with the rotated mesh, you might need to alter the collision or use the CPU after all.