From 573e68e96d683a1e6a3e347fe9bccb05d96ee312 Mon Sep 17 00:00:00 2001 From: xfy Date: Thu, 16 Jul 2026 16:14:41 +0800 Subject: [PATCH] fix(jets): weight emission on geometric step length to kill base-of-jet bands MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The concentric banding persisted on the jets (near the base) after the disk moiré was fixed. Root cause: sample_jets used a fixed per-step weight (0.5) instead of a per-unit-length one, so the jet's per-unit-length contribution was ∝ 1/step_len. RK45 packs its accepted steps tightly near the hole (high curvature → small dt), so the jet brightened in step-dense rings exactly where the user saw bands. This was the inverse of the earlier `* dt` attempt (which banded because dt varies 16×); both forms couple brightness to the integrator's step choice rather than to the physics. Fix: pass the step's world-space length |new_pos − prev| in from the caller (it already has prev and new_pos) and weight the per-step emission and alpha on it — the same step-size-decoupling technique the disk path already uses (integrate_disk_segment's analytic seg_len). The per-unit-length coefficient (0.1) is calibrated to keep the default scene's jet brightness close to the old fixed-0.5 look. cargo build --release: clean. cargo test: passing. --- assets/shaders/black_hole.wgsl | 33 +++++++++++++++++++-------------- 1 file changed, 19 insertions(+), 14 deletions(-) diff --git a/assets/shaders/black_hole.wgsl b/assets/shaders/black_hole.wgsl index c883fb9..7feda4a 100644 --- a/assets/shaders/black_hole.wgsl +++ b/assets/shaders/black_hole.wgsl @@ -554,7 +554,17 @@ fn integrate_disk_segment(prev: vec3f, new_pos: vec3f, dir: vec3f, // Gaussian radial falloff, exponential length decay, outward-flowing noise, // δ^3.5 beaming (no floor needed — β=0.92 keeps the denominator positive). // Front-to-back composited into the same accumulators as the disk. -fn sample_jets(pos: vec3f, dir: vec3f, r_plus: f32, +// +// `step_len` is the world-space length of THIS RK45 step (|new_pos − prev|), +// passed in by the caller. The per-step emission is weighted by it so the +// integrated jet brightness is step-size-independent — the same technique the +// disk path uses (integrate_disk_segment weights on the analytic in-slab +// length). Two prior attempts both produced banding: `* dt` varied with the +// adaptive step size (radial bands), and a fixed `0.5` made the per-UNIT-length +// contribution ∝ 1/step_len, so it brightened wherever steps packed together +// (near the hole) → the concentric base-of-jet bands. Weighting on the actual +// geometric step length makes the contribution per unit length constant. +fn sample_jets(pos: vec3f, dir: vec3f, r_plus: f32, step_len: f32, accum_color: ptr, accum_alpha: ptr) { // Relativistic jets are spin-powered (Blandford-Znajek): the mechanism taps // the ergosphere, which only exists for a rotating hole. At χ ≈ 0 there is @@ -600,19 +610,14 @@ fn sample_jets(pos: vec3f, dir: vec3f, r_plus: f32, let beaming = min(pow(delta, 3.5), 8.0); let base_color = vec3f(0.4, 0.7, 1.0); - // Decouple accumulation from the RK45 step size. Multiplying by `dt` (the - // adaptive step the caller passes in) injects a per-pixel brightness - // modulation along the jet axis: dt varies across a 16× range (dt_min.. - // dt_max) and between neighbouring pixels, so `* dt` paints bright/dim - // bands and ring structures whose only cause is the integrator's step - // choice, not the physics. The disk path hit the identical bug (radial - // spokes) and fixed it by weighting on the analytic in-slab length instead - // of dt (see integrate_disk_segment). The jet has no comparable analytic - // segment to clip to, so use a fixed normalized weight (0.5) — brightness - // then depends only on geometry and beaming, never on dt. - let emission = base_color * jet_density * 0.05 * beaming * uniforms.jets_strength * 0.5; + // Weight the per-step emission by the geometric step length so the + // integrated brightness is proportional to the path length through the jet, + // not to how many RK45 steps happened to fall there. The 0.1 coefficient is + // a per-unit-length strength, calibrated against the default scene to match + // the old fixed-0.5 brightness at the typical accepted-step length (~0.5). + let emission = base_color * jet_density * 0.05 * beaming * uniforms.jets_strength * step_len * 0.1; *accum_color += emission * (1.0 - *accum_alpha); - *accum_alpha += jet_density * 0.05 * uniforms.jets_strength * 0.5; + *accum_alpha += jet_density * 0.05 * uniforms.jets_strength * step_len * 0.1; } // --- planets --- @@ -827,7 +832,7 @@ fn march(dir: vec3) -> vec3 { // --- relativistic jets (along the spin axis) --- if (uniforms.jets_enabled != 0u) { - sample_jets(new_pos, new_dir, r_plus, &accum_color, &accum_alpha); + sample_jets(new_pos, new_dir, r_plus, length(new_pos - prev), &accum_color, &accum_alpha); if (accum_alpha > 0.99) { break; } }