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Denoiser doesn't work with this emissive material
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Description

System Information
Win10 64bit

Blender Version
Broken: 2.79

Short description of error
This is a simple example of material where baked part via Emission shader mixes with small part of Diffuse shader.
Baked texture is represented by Generated Color Grid.
Material absorbs some negative emission from the star (that is why I have Diffuse here). But in final render Denoiser removes star emission completely.

Exact steps for others to reproduce the error
Open and render

Details

Type
Bug

Event Timeline

I wouldn't consider this a bug.

What happens here is that the emission component of the cube gets stored in the clean pass while the diffuse component (containing the negative values due to the star) end up in the noisy pass. The denoiser then clamps the negative values in the noisy pass (which is intentional), filters and adds the clean pass back in the end.

In this case, the negative clamping changes the result, but I don't really think this is something we should be supporting - negative lights are tricky anyways, and negative render results even more so.