Limitations
Although the Custom node is very powerful, it does come with its downsides. In this section, I will go over some of the limitations and caveats when using it.
Rendering Access
Custom nodes cannot access many parts of the rendering pipeline. This includes things such as lighting information and motion vectors. Note that this is slightly different when using forward rendering.
Engine Version Compatibility
HLSL code you write in one version of Unreal is not guaranteed to work in another. As noted in the tutorial, before 4.19, you were able to use a TextureCoordinate to get scene texture UVs. In 4.19, you need to use GetDefaultSceneTextureUV().
Optimization
Here is an excerpt from Epic on optimization:
Using the custom node prevents constant folding and may use significantly more instructions than an equivalent version done with built in nodes! Constant folding is an optimization that UE4 employs under the hood to reduce shader instruction count when necessary.
For example, an expression chain of Time >Sin >Mul by parameter > Add to something can and will be collapsed by UE4 into a single instruction, the final add. This is possible because all of the inputs of that expression (Time, parameter) are constant for the whole draw call, they do not change per-pixel. UE4 cannot collapse anything in a custom node, which can produce less efficient shaders than an equivalent version made out of existing nodes.
As a result, it is best to only use the custom node when it gives you access to functionality not possible with the existing nodes.
Where to Go From Here?
You can download the completed project using the link at the top or bottom of this tutorial.
If you’d like to get more out of the Custom node, I recommend you check out Ryan Bruck’s blog. He has posts detailing how to use the Custom node to create raymarchers and other effects.
If there are any effects you’d like to me cover, let me know in the comments below!
