Create a V-Ray Render Elements node as specified above;
Add the render_elements node to V-Ray Renderer > Images > Render Elements > Network field;
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If you are adding more than one render channel, make sure to connect the render element nodes to the input of the channelsContainer node:
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Notes
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Fancy Bullets
type
circle
V-Ray always anti-aliases with respect to the RGB Color channel. Therefore, other channels may appear jagged or noisy in areas where V-Ray has placed fewer samples.
Render channels will render in Distributed Rendering mode.
Render channels require additional storage when rendering and thus increase the amount of RAM used during rendering. When rendering to the V-Ray VFB, you can reduce this by rendering directly to a .vrimg file on disk and not storing the final image in memory.
Render channels may slow down the final image rendering a little bit, depending on the number of elements that are selected. They have no effect on GI/reflection, etc. calculations.