# How to use a Custom Stencil to mask Exponential Height Fog?

**URL:** <https://forums.unrealengine.com/t/how-to-use-a-custom-stencil-to-mask-exponential-height-fog/489091>\
**Category:** Rendering\
**Tags:** Post-Processing, stencil, unreal-engine, mask, volumetric-fog, question\
**Created:** [September 22, 2021, 8:50pm UTC](https://forums.unrealengine.com/t/how-to-use-a-custom-stencil-to-mask-exponential-height-fog/489091 "2021-09-22T20:50:28Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![TheHammerSV](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@TheHammerSV](https://forums.unrealengine.com/u/TheHammerSV)\
**Post date:** [September 22, 2021, 8:50pm UTC](https://forums.unrealengine.com/t/how-to-use-a-custom-stencil-to-mask-exponential-height-fog/489091/1 "2021-09-22T20:50:28Z")

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The game I am working on involves ghosts, and I am working on an effect where if an invisible ghost steps into a volumetric light, the exponential height fog is affected by a mask so the model makes the room appear unlit within the boundaries of its silhouette. I know I will need to use a Custom Stencil to do this, but I don’t know how the post process material should work.