Adding an Outline to a ViewportFrame

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I have a container with a viewportframe inside displaying an model. How would I add an outline/stroke to the model? Highlight and Stroke Features have not worked.

Bit of a hacky solution, but maybe you could have a darkened duplicate of the ViewportFrame and place it behind the colored one to make it look like an outline?

Although it would be a pain, you could make a new version of the mesh in Blender that has that “cartoony” outline.

That being said, I don’t know how many models are in your game, and doing this for every weapon could take a while.