How to write 2D filters in UPBGE?

How do I write 2D filters? They’re a bit different from the normal GLSL shaders, so how would I go around doing that. While I have been using Blender for 11+ years, I never quite got into 2D filters, as the language was too advanced for me to comprehend.

I would prefer a guide on how to do 2D filters, which needs to be simple and easy to follow yet with good instructions for both beginners and intermediates.