[LeadcodeDev]

Severity Medium, category correctness. Location: crates/rustmotion/src/engine/render/background.rs:806

Impact

tile_spacing returns one scalar that line 830 applies to both axes, and the Heropattern arm never reads d.height. The function's own doc comment states the invariant it relies on — "since every tiled pattern is exactly periodic on spacing, translating by any offset congruent mod spacing produces byte-identical pixels" — which is false for any non-square pattern. I counted the table programmatically: 41 of the 87 entries in crates/rustmotion-core/src/engine/heropatterns.rs have width != height (architect 100x199, aztec 32x64, bamboo 16x32, bank_note 100x20, death_star 80x105, yyy 60x96...). Concrete case: {"animated-background":{"preset":"heropattern","heropattern":{"pattern":"aztec"},"direction":"down","speed":60}}. The shader tiles vertically on 64px but the offset wraps at 32px, so every 32px of travel the pattern snaps by half a period — a periodic jolt through the whole scene. The .max(20.0) clamp breaks it a second way for the 8 patterns narrower than 20px: bamboo (16 wide) wraps at 20, so even horizontal scroll jumps 4px per wrap. draw_world_bg_with_parallax reuses the same tile_spacing for the camera-pan modulo (line 89), so world views inherit it.

Fix

Make tile_spacing return a (f32, f32) period and have the Heropattern arm emit (d.width * cfg.scale, d.height * cfg.scale); wrap raw_x on the x period and raw_y on the y period at line 830, and do the same for the camera modulo at line 89. Drop the .max(20.0) floor or replace it with a multiple-of-period ceiling (period * (20.0 / period).ceil()) so the clamp preserves periodicity.

Evidence the audit read

BackgroundPreset::Heropattern(cfg) => {
            let def = crate::engine::heropatterns::find_pattern(&cfg.pattern);
            def.map(|d| d.width * cfg.scale).unwrap_or(60.0).max(20.0)
        }
...
pub(super) fn compute_scroll_offset(bg: &AnimatedBackground, time: f32) -> (f32, f32) {
    let (raw_x, raw_y) = raw_scroll_offset(bg, time);
    let spacing = tile_spacing(&bg.preset);
    (raw_x % spacing, raw_y % spacing)

Based directly on the chantier branch.

Part of the September 2026 audit remediation chantier. Refs #220 (RM-19).