Module lsystem 

L-system scene: expensive to build, cheap to animate (ADR-0007 generator build model). At preset load (configure, off the hot path) the grammar is expanded and turtle-walked into one cached segment buffer per depth 1..=max_depth. Per frame the scene only picks the visible depth and applies a rotation / scale / colour / draw-on transform into the draw buffer — no expansion, no allocation.

That is growth = "fixed". Under growth = "endless" nothing is cached: the derivation is streamed through Endless (a lazy depth-first walk of the grammar, see grammar::Stream) and turtle-walked a few draw steps every frame into a fixed ring of trail segments. Every buffer it touches is reserved at configure, so the per-frame walk still allocates nothing.

Beat accents advance visible_depth (grow one iteration); continuous motion drives rotation, hue, draw_progress, etc.

The colour axis: generation depth (ADR-0059)

This scene honours [palette] / [palette_b] / palette_mix / hue_spread / saturation, sampled on the CPU exactly as spectrum does. Each line scene walks hue_spread along the axis its own generator makes meaningful, and for an L-system that axis is generation depth: the branch-nesting level the turtle drew a segment at, 0 on the trunk and one more for every open [. Colouring by it makes an older branch read as older, which is what the whole subject of a rewriting system is.

The ramp is normalized over the figure’s own deepest generation, not over visible_depth. ADR-0059 wrote the latter; it is wrong in both directions and the code follows the measurement instead. A grammar can open more than one branch per rewrite — lsystem_fern’s X -> F+[[X]-X]-F[-FX]+X opens two, so its deepest generation runs 1, 3, 5, 7, 9, 11 over visible_depth 1 to 6, and dividing by 6 would leave five sixths of the figure clamped at the palette’s far end — while a grammar with no brackets at all (lsystem_arrowhead, deepest generation 0 at every one of its seven depths) has no range for the divisor to describe. Normalizing over the built figure’s own maximum makes hue_spread = 1 span the palette exactly once on any grammar, and it is a load-time quantity, so an eased visible_depth cannot sweep the divisor through fractional values mid-fall.

A bracket-free grammar therefore has exactly one generation and colours flat — that is a property of such a figure (every segment of a Sierpinski arrowhead genuinely sits at the same recursion level), not a gap. Such a preset still reaches the palette; what it cannot reach is a ramp across a figure that has no depth to ramp along.

hue_spread = 0 collapses the ramp to the single hue the scene has always drawn, so the surface is a strict superset.

Structs

LSystemScene

A generator scene driven by an L-system grammar.

Enums

Growth

How the figure is grown: [generator] growth.

Constants

DEFAULT_FOLLOW_WINDOW

[generator] follow_window’s default: how many of the newest segments the follow point is the centroid of.

DEFAULT_TRAIL

[generator] trail’s default: how many segments an endless figure keeps.

FAMILY_PARAMS

Every parameter whose reading depends on the turtle or the growth (ADR-0258, ADR-0180 rule 4). The generated reference prints each as inert on the modes that ignore it.

MODES

The modes FAMILY_PARAMS is written against: each turtle, fixed and endless, in that order.

PARAMS

Parameter vocabulary — see fragment_field::PARAMS. Keep in sync with set_param below.