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A Maxwell Render test trying to hone in on a look for neon.
This test consists of a thin emitter tube within a thicker tube of some roughness, a colored transmittance (in this case red), and short attenuation. I'm trying to make it seem like a hot gas inside a glass tube. This may not be the optimal solution but it's not far off for my liking.
The Photoshop Blend-if sliders are a great way to interactively blend two images based on luminosity ranges.
I sometimes use it to wear away an image of a painted graphic against an underlying wood grain or brick texture, to make it seem as if the paint is very old. When I do this I often wish I had a layer mask to use to control the blend, so I could take it into, say, Maxwell, and make a layered material with the wood rough and the paint smooth.
Here's one way to preview the composite using the Blend-if sliders and extract the underlying luminosity mask into a layer mask.