GLSL math helpers for script.js

The first hour anyone spends in a scene’s script.js involves reaching for mix and finding it isn’t there. script.js is plain JavaScript — it gets Math.* and nothing else. Every interpolation habit you built in GLSL has to be rebuilt by hand, and everyone rebuilds the same three.

Here they are so you can stop.

function mixval(x, y, a) { return x * (1 - a) + y * a; }   // = glsl mix()

function smoothstep(edge0, edge1, x) {
    var t = Math.max(0, Math.min(1, (x - edge0) / (edge1 - edge0)));
    return t * t * (3 - 2 * t);
}

function nsin(x) { return (Math.sin(x) + 1) * 0.5; }        // 0-1 sin

function clamp(x, lo, hi) { return Math.max(lo, Math.min(hi, x)); }

nsin is the interesting one: GLSL scenes have it as the built-in _nsin, and script.js doesn’t. That asymmetry — a helper that exists on one side of the scene and not the other — is most of why this entry exists at all.

Usage

Mostly for shaping a value before it crosses into the shader:

function update(dt) {
    // ease a uniform toward a control instead of snapping to it
    smoothed = mixval(smoothed, my_slider, 0.1);

    // a slow breathing pulse, already in 0-1
    setUniform('breathe', nsin(TIME * 0.5));

    // fade an effect in over the first 2 seconds of a bass hit
    var fade = smoothstep(0.0, 2.0, TIME - hitTime);
    setUniform('intensity', clamp(fade * syn_BassLevel * 2.0, 0.0, 1.0));
}

Notes

• mixval is named oddly on purpose — mix is a perfectly legal JS identifier, but the mismatch with GLSL’s argument semantics has burned people. Keeping the name distinct means you never mistake which side of the scene you’re editing.
• smoothstep here clamps like GLSL’s does. If you skip the clamp you get a cubic that runs away outside the edges, which is occasionally what you want and usually a bug.
• These belong at the top of script.js, above setup(). There’s no include mechanism on the JS side — every scene carries its own copy.
• Doing heavy per-pixel-style math in JS is a mistake; this is for shaping a handful of scalars per frame, not for anything you could do in the shader.