Arguments & time

What it does

Every numeric parameter accepts four kinds of value:

Form	Example	Evaluated
number	osc(20)	written into the shader as a constant (changing it recompiles)
array	osc([10, 20, 40])	every frame, stepping through the values (modifiers)
function	osc(() => 20 + mouse.x / 10)	every frame
texture/chain	.modulate(noise(3)), .blend(o1)	per pixel (texture inputs)

Functions are called each frame as fn({ time, bpm, mouse, resolution }) and must return a number; otherwise hydra logs function does not return a number and uses the parameter’s default. You can also just read the globals time, mouse, width, height inside the function.

Time

• time is seconds since start, advanced each frame by $dt \cdot \text{speed}$. Set the global speed = 0.5 to slow everything that depends on time — osc’s sync, rotate’s speed, arrays, and your own functions that read time.
• bpm (default 30) only affects arrays: index $= \text{time}\cdot\text{fast}\cdot\text{bpm}/60$, so at the default each array value lasts 2 s. bpm = 120 makes it 0.5 s.
• update = (dt) => { … } runs once per frame before rendering, with dt in milliseconds. Use it to accumulate your own state (e.g. an angle that changes pace smoothly).

Examples

Function argument reading the clock

shape(4, () => 0.3 + 0.1 * Math.sin(time * 2)).out()

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Mouse-driven frequency

osc(() => mouse.x / 20, 0.1, 1).out()

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Smooth pace changes with update

var angle = 0
update = (dt) => { angle += dt * 0.001 * (1 + Math.sin(time)) }
shape(3).rotate(() => angle).out()

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Arrays follow bpm

bpm = 120
shape([3, 4, 5, 6]).out()

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Gotchas

Quirk: arrays and functions have no slider

The playground’s sliders edit literal numbers in your code. An array or function is a running program, not a single value, so the slider for that argument is disabled; the plot above shows what an array does instead.

• Changing a rate like rotate(0, speed) live multiplies the whole elapsed time, so the result jumps; accumulate in update for smooth changes (third example).
• Code runs through a global eval: top-level let/const are private to that one evaluation, while var (or a bare assignment) creates a real global that later evaluations can read. Don’t reuse hydra’s global names: a is the audio object, s0–s3/o0–o3 are sources/outputs, time/speed/bpm are the clock.
• A function that returns a non-number (undefined, a string) silently falls back to the parameter’s default (a warning appears in the console). NaN is a number and is passed through. Array NaN/undefined values render as 0.

Related

• Array modifiers
• Coordinate space