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This is a paid asset, but now you can download Portal Effect Free.
Portal Effect v1.1 (Latest version)
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This asset allows you to create portals that players can see and walk through as if space itself has been warped. Players can see and step seamlessly across space through portals.
– Game-ready VFX (open, idle and close states)
– Seamless camera transition
– Rigidbodies save their velocities
– Optimization (frustum culling)
In Unity 2017.4.13f1 found bug with CullingGroup API, frustum culling of portals is not recommended to use in that Unity version.