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The Coat Filter Render Element stores reflection information calculated from the VRayMtl Coat Layer. Surfaces with no reflection values set in its material(s) the Coat Amount parameter set to 0 will contain no information in the render pass and will therefore render as black.

While the Raw Coat Reflection Render Element gives the full reflection of objects reflecting in the scene, Coat Filter RE shows how much of of that reflection should come through in the composite. In other words, the filter defines the strength of the reflection, while the raw reflection channel defines what is being reflected in the image. When these two elements are multiplied, the true level of reflection is given and produces Coat RE. Using these component parts of the reflection, you can better fine tune the reflection in your final composite.

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