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enter image description here

None of these Greater Than nodes are working with fire sim, because Alpha socket is empty both in RGBA node and RenderLayer pass. The only way to see "alpha channel" is to uncheck Use Alpha in Viewer or Composite nodes (as on the screenshot).

Material setup for simulation is very basic: Flame/Density Attribute->Emission shader->Volume output

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  • $\begingroup$ What are you trying to do? Fire (in general) doesn't occlude objects and therefore has very little alpha. Could you explain your actual goal? $\endgroup$ – Leander Oct 15 '18 at 12:56
  • $\begingroup$ Nothing particular really. During my last project noticed this thing and it seemed strange to me. So, a question about "actual goal" is more likely asked by me towards Blender. What is a point to have no visual of alpha but still use it (from technical side of view)? $\endgroup$ – Serge L Oct 15 '18 at 13:09

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