A few years ago, I wrote an example on how to port GLSL shader code from Shadertoy.com to Processing. Since then, Shadertoy has changed quite a lot (adding a lot of new uniforms and input methods) so an update was long overdue.
You can find it here: https://github.com/SableRaf/Shadertoy2Processing
There is still work to be done, but it should be a good starting point for anyone wanting to learn about shaders from the great material available at Shadertoy, and experiment with it in Processing.
I hope that will be helpful to some of you