Hello there. I'm out of my depth here but I was wondering if anyone can set me off on where start my research.

I've built an arcade cabinet from scratch powered by a raspberry pi3 using retropie and retroarch. The later uses shaders of the .glslp and .glsl combined type.

I'm looking to make or use shader that my pi recognises but actually does nothing...yes nothing at all!

The reason being I have button wired up to 'next shader' and I want it to cycle between two- active CRT look shader and blank inert shader.

What I'm trying to create is a CRT fx button to press that switches on my favourite 'crt curved' shader and second press is off....no effects blank shader as cool retro a feature for my Cab.

Any suggestions on where to begin learning?

Thank you