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Imagine a car headlamp rendered with the lights off. Using these mattes, one could extract the matte from the light from underneath the headlamp cover/lens and use a composting compositing package to "turn on" the light by using the mask to control glow and color and even animate this without re-rendering. Below we use a very simple example to show the results and some example Light Path Expressions to help you create your own. An example RIB file is included at the bottom of the page, please map your own HDRI to the Dome Light, the one used in the examples is the Griffith Observatory shipped in the Preset Browser assets. The scene has a pane of glass (with thickness) and a mirror.

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