What an expression cannot do: shape a value over time

smoothstep eases a value as its input crosses a threshold. It cannot ease a trajectory, because an expression is a pure function of the variables it is handed — there is no previous frame inside it to ease away from. Anything time-shaped therefore lives in a table beside [params], not in the expression. For a trajectory that is [smoothing]:

[params]
thickness = "0.9 + beat * 0.6"           # the target: snaps on every beat

[smoothing]
thickness = { attack = 0.02, release = 0.7 }   # the trajectory: hit, then glide

attack is the time constant while the value is rising, release while it is falling or level; a plain thickness = 0.3 means the same constant both ways. That is what makes a percussive accent land in a frame or two and then decay over most of a second — a single constant slows the rise exactly as much as the fall, so there is no value that gives both.

A two-constant entry stops being a low-pass and becomes a rectifier. Because rise and fall are treated differently, a symmetric input comes out with a DC offset: the parameter rides above its input’s mean under sustained material. On a percussive accent that is the point. On a continuous parameter it is a surprise — a fast-attack hue drifts upward instead of tracking. Use the pair where you want a hit; leave anything continuous symmetric. Full table reference: Parameter roster.

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