How to apply a shader on the whole screen?

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:bust_in_silhouette: Asked By JulioYagami

I need to make a game pause and I want to apply a shader that lets the screen with gray scales

:bust_in_silhouette: Reply From: SIsilicon

Take a look at the official Godot demo that shows off screen-space shaders.
https://godotengine.org/asset-library/asset/122