How to add interesting transitions between levels?(Iris, mosaic/pixelization, melting, waves, etc.)

If you want to use this effects in a 3D your best option is to use a Post Process Effect
You have a tutorial explaining how to use this 2D materials into a 3D scene using a post process volume

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I think this method is easier than taking a screenshot and pass it as a parameter to the “2d” Material, so take a look to the tutorial and give me a feedback with your results