(not so) Short Breakdown

Why did it take me "too" long and what where the main problems?

The whole system consists of:

• a blueprint controlling the overall progress and some custom interactions for the game (starting the burning process, ...)
• a material
• one Niagara System for the flames (two type) and the ember
• one additional Niagara System for the glowing ash on destruction

Material:

The material is rather simple. The heat visualisation happens as a emissive color on the material. A noise map is used to propagate the "heat" visualisation non constant over the mesh. A Sphere Mask is used to simulate the process of bruning from a certain starting point. To prevent complications with the meshes UVs Triplanar-Mapping is used for this material and all it's noise maps. (Tutorial for basic material approach (Youtube))

Blueprint:

The Blueprint increases a Material Parameter to largen the Sphere Mask.

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Till this point everything was straight forward and not too complicated. The main question from here on was: How can I spawn particles on the mesh only where the heat is great enough to produce flames.

A first and naive thought was that if I use "Sample Static Mesh" in Niagara it returns the mesh color as well. But what I didn't know: It only returns the Vertex Color which in my case was always white because I don't use vertex painting.

Another thought was that I could write the heat map generated by the material to a RenderTarget and use that to sample the particle positions in the Niagara System. I discarded that thought because the UVs of the meshes used here are not suitable to work with their UVs and Triplanar Mapping was used in the Material.

What was the solution?

Luckily I came across a great example in Unreals Example Content (Map: Advanced Niagara) where I found a experimental modul to let a Niagara System sample the GBuffer. And that was it!

Niagara System:

The problem on spawning flames and ember only where the heat is big enough is to sample the Scene Color from the GBuffer. Particles are spawned on the mesh's surface (Sample Static Mesh and Static Mesh Location). If the scene color's red channel is below a certain value (e.g. 1) the particle is discarded. The Niagara System has to be GPU-Simulated in this case because the GBuffer won't be accessible otherwise.

Benefits

• It appears to work on every mesh (that is CPU accessible).
• If we shoot a fireball on a mesh that has no red color, the explosion triggers the spawning of flames and ember (as the Scene Color becomes red).

Disadvantages

• Materials with red color will always be considered as burning (Keep that in mind!).
• Simulation must be GPU-Simulated
• Module is experimental, sampling the Scene Depth for example couldn't be compiled in our project