Light Direction Vector Disabled in Pixel Shaders - Does a Solution Exist?

paradoxical-pixel

I've noticed that UE4 doesn't allow one to access the light direction vector when creating a pixel shader in the material editor.

I understand that we are now in the age of physically-based rendering, but I find this limitation to be quite problematic. I cannot, for instance, recreate the Heidrich–Seidel anisotropic distribution model inside of UE4 as the formula relies upon an incoming light direction vector.

I wonder if anyone has found a way around this restriction. Is it possible to access the light direction vector if one writes and compiles in HLSL, and then integrates the shader instructions into UE4 somehow? Has anyone else hit this problem and found a way around it?

Chariots

There's a hacky solution for directional lights. You can create a Material Parameter Collection, add a new vector, then create a blueprint actor that takes the rotation of the directional light in the level, and updates the MPC vector you've made earlier.

All materials that have that MPC vector will get auto updated as the blueprint updates the vector, without any noticeable performance cost.

More info on MPCs: https://docs.unrealengine.com/latest/INT/Engine/Rendering/Materials/ParameterCollections/index.html

Obviously, this will only work for 1 light, but it very doable if one light is all you need.

Arnage

The best way to implement it would probably be a custom shading model.

If you don't mind diving into the code, the source for the basic shading models is all editable. This just goes beyond what you can do in the shader editor.

Also, if you look around in the shader sources you'll notice that some alternative shading models are already hidden in there, just not exposed by default. Nothing anisotropic as far as I know though.

paradoxical-pixel

I've been looking for a good excuse to properly learn HLSL so this might be it.

Xendance

This is because the renderer is deferred, not because of PBR. You'd have to implement a forward renderer in order to get the light vector.