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https://enzocontini.blog/2017/12/01/how-to-let-an-app-be-displayed-in-mirror-mode-so-that-you-can-see-through-a-mirror-i-e-hud-the-image-in-the-right-way/
How to let an app be displayed in mirror mode, so that you can see through a mirror (i.e. HUD) the image in the "right" way