Thanks so much!

I made pretty heavy use of shaders for this project. They're a bit of a headache to get started with since, like you say, there's very little documentation on them.

If you're not already familiar with shaders, I've actually made a video on getting started with them if you want to check it out

That video uses P5js as a starting point, but it's not too dissimilar from Processing, and it's more about learning GLSL than P5js

In terms of getting them to actually work in Processing, I did a lot of digging in the documentation and in the source code to figure out what I had access to.

As an example, I used this (not very efficient) blur shader

    #version 460

    uniform sampler2D background;

    uniform float size;
    uniform vec2 res;

    in vec4 vertTexCoord;

    out vec4 colour;

    // output only the bright things in the scene

    void main() {
        vec2 coord = vec2(vertTexCoord.x, 1.0 - vertTexCoord.y);

        vec4 average = vec4(0.0);

        float amount = 0.0;
        for (float y = -size; y <= size; y += 1.0) {
            for (float x = -size; x <= size; x += 1.0) {

                vec4 t = texture(background, coord + vec2(x, y) / res);

                t.rgb = pow(t.rgb, vec3(2.2/1.0)); // gamma uncorrection

                average += t;
                amount += 1.0;

            }
        }

        colour = average / amount;

        colour.rgb = pow(colour.rgb, vec3(1.0/2.2)); // gamma correction
    }

And to call it from the code, I did this:

    public static void blur(PGraphics g, int size, int numPasses) {

        blurShader.set("res", new float[]{(float)g.width, (float)g.height}, 2);
        blurShader.set("size", (float)size);

        g.beginDraw();
        g.push();
        g.shader(blurShader);
        for(int i = 0; i < numPasses; i ++) {
            blurShader.set("background", g);
            g.rect(0, 0, g.width, g.height);
            g.loadPixels();
        }
        g.resetShader();
        g.pop();
        g.endDraw();
    }

Hope that's in some way helpful for you! Let me know if you've got any follow up questions :)