First, a few observations:
- Your light has 2048 triangles, that’s probably unnecessary since it’s just a square.
- In your SPPM configuration, you disabled caustics. That’s unusual since SPPM is typically used to render caustics. In any case, that means the fireflies aren’t related to caustics.
- In fact the fireflies also occur in pure unidirectional path tracing mode, that means they aren’t related to SPPM.
It turns out that fireflies are caused by the ground plane being partially (and rough) specular: that’s a difficult situation for any unidirectional path tracer.
I made the ground plane purely diffuse (but still rough) and I increased the frame resolution. Here’s first a render with the Unidirectional Path Tracer, caustics disabled:
Here’s now a render with SPPM with your original settings but with caustics enabled:
Finally, here’s the same render but this time using RT Direct Lighting instead of SPPM Direct Lighting:
Here is a version of your scene with all the fixes applied: AreaLight - repaired.appleseed (21.1 KB)
You can render it in appleseed.studio.
Finally, as I said in my previous post, your scene revealed a small bug that will be fixed in the version of appleseed (or one of the upcoming development build).