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The mental ray renderer does not support G-Buffer options in post processing and image file output. If you attempt to use mental ray to render a scene with a Render Effects or Video Post function that uses the G-Buffer, such as Render Effects > Depth of Field or any of the Lens Effects tools, at render time an alert box appears stating, "One or more requested G-buffer channels are unavailable." Click OK to continue rendering without the G-Buffer options, or Cancel to abort the render.