`fade_in`

Smooth amplitude ramp at the start of a source. Complementary to fade_out.

Signature

signal.fade_in(seconds) -> Signal
fade_in(signal, seconds) -> Signal

seconds — duration of the fade-up ramp at the start of the signal. After the ramp the source is passed through unchanged.

The ramp is a sin² curve over [0, 1]:

env(t) = sin²(π/2 · t)

This is the exact complement of fade_out’s cos² shape. A fade_in(d) on layer A combined with a fade_out(d) on layer B at the same offset reconstructs to constant power — useful for crossfading between two textures without a perceptible level dip.

Example

A torpedo emerging from underneath a bubble wash:

patch("torpedo_emerge", "one_shot",
    mix([
        sample("samples/bubbles.wav").pitch(0.6).env(0.05, 1.5).gain(2.0),
        torpedo_layer.fade_in(0.8).fade_out(1.0),   // ramps in over 0.8 s
    ])
    .gain(0.55));

The torpedo rises smoothly out of silence while the bubbles play, then fades out cleanly at the end.

Why decouple fade-in from `env`?

env(attack, decay) already does a linear attack ramp and pairs it with an exponential decay. That’s fine when you want both shaped together — strikes, bell-like tones — but bad when you want a long graceful entry and either no decay or a controlled tail (fade_out) shape.

Want	Use
Hammered onset → exponential ring	`env(0.005, 2.0)`
Slow rise → constant body → smooth tail	`fade_in(1.0).fade_out(1.0)`
Crossfade between two layers	`fade_in(d)` on one, `fade_out(d)` on the other

Notes

Works on bounded and unbounded sources. For an ambient, fade_in shapes only the opening of the stream and then passes through forever.
Zero or negative duration produces no ramp (pass-through).
Constant-power crossfade. Pairing fade_in(d) and fade_out(d) on two layers whose sum you want flat works because sin²(x) + cos²(x) = 1.

Errors

None at construction.

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