The [feedback] table — the warp family and the deposit blend

Two structural choices about the same accumulation, in a table rather than as params because each selects a code path rather than a quantity — [curve] family’s rule (and see Structural config for the others). An unknown value is a load error: the preset is rejected with a message naming what was expected, rather than quietly rendering the default.

[feedback]
warp  = "swirl"   # none (default) | swirl | ripple | fisheye
blend = "add"     # max (default)  | add

warp picks a curated procedural distortion that rides on top of the fb_* affine, about the same fb_center_*. Its strength is the bindable fb_warp (default 0, so a selected warp does nothing until you drive it) — and, like every other rate here, it is per second.

warp	what it does to the past	fb_warp reads as
none	nothing — the affine alone	inert
swirl	a vortex: rotates about the centre, strongest in the middle and faded by the rim	rad/s at the centre
ripple	concentric standing waves in radius — the past breathes in rings	frame-heights/s of radial push
fisheye	a radial magnification growing with radius	positive draws the periphery in, negative pushes it out

swirl and fb_rotate are not the same gesture: fb_rotate turns the whole frame rigidly, swirl turns the middle faster than the edge, which is what makes a vortex rather than a spin.

blend picks how this frame’s light lands on the faded past.

• max (default) — max(current, past * trails). A bright head with a fading tail; the accumulation can never exceed the brightest thing in it. This is what the stage did before the choice existed.
• add — current + past * trails. Overlapping echoes sum, so a figure crossing its own tail lights up where they meet. Bounded, despite appearances: the series converges to 1 / (1 - trails), i.e. at most 50x at the 0.98 ceiling, and the extra rolls off through the tonemap instead of clipping — which only works because the composite runs in linear light (see Linear light and exposure).

add wants a dimmer preset than max: with trails = 0.95 a figure is already 20x its own brightness once the tail settles. Start by dropping brightness (or exposure) by roughly 1 - trails and tune up from there.