Filament Trails

"use client";

import { useEffect, useRef } from "react";
import { mountShaderSurface } from "@/lib/shader-surface";

/**
 * A live WebGL field of long-exposure light filaments — thin warm streaks
 * drifting across a warm near-black ground, near-horizontal and sparse at the
 * top, fanning into a dense crossing weave toward the base, the brightest
 * threads burning to a near-white core. The look of anamorphic light trails
 * caught in a slow shutter.
 *
 * The pointer stirs the trails: threads near the cursor bow toward it, their
 * flow speeds up, and the nearest cores catch a soft warm bloom — as if the
 * light were being dragged through with a finger.
 *
 * One of the reusable background fields (`TileField`, `SilkField`,
 * `AuroraVeil`, `SpineField`, `SlabField`, `RakeField`, `OpalField`). Drop it
 * into any `position: relative`/`isolate` parent — it fills the box. Built on
 * `lib/shader-surface.ts`, so every degradation path is already handled: no
 * WebGL, a blocked or lost context, a hidden tab, or `prefers-reduced-motion`
 * all leave the CSS `.iris-filamentfield__floor` underneath visible (a still
 * composed frame in the same palette, never a blank box).
 *
 * Like `RakeField` / `AuroraVeil` (and unlike `SilkField`), the palette is a
 * fixed warm set passed as uniforms rather than read from the token ramp — the
 * near-white filament core lives nowhere in the accent ramp, so the CSS floor
 * carries the colour rather than a live token read.
 *
 * Reading guard: when `guardSelector` resolves to an element, the field
 * measures that block every frame and clamps its own luminance under a ceiling
 * inside that region (hue and saturation untouched), so copy laid over the
 * field holds WCAG AA with no overlay layer. `null` (the default) turns the
 * guard off — for decorative use where nothing sits on top.
 */

/* Palette, sRGB 0–1. Uniforms, not tokens — see the note above. */
const PALETTE: Record<string, [number, number, number]> = {
  u_ground: [0.023, 0.014, 0.008], // the warm near-black behind everything
  u_ember: [0.223, 0.09, 0.028], // the dim, barely-lit filaments
  u_amber: [0.95, 0.46, 0.12], // the mid filaments and the pointer bloom
  u_flare: [1.0, 0.86, 0.62], // the near-white core riding the hottest threads
};

const FRAG = `
uniform vec2  u_res;
uniform float u_time;
uniform float u_scale;
uniform vec3  u_pointer;   /* x, y (0 bottom .. 1 top), presence 0..1 */

uniform vec3 u_ground;
uniform vec3 u_ember;
uniform vec3 u_amber;
uniform vec3 u_flare;

/* Reading region for the contrast guard: xy = centre (uv, y up),
   zw = half-extent. Measured from the live copy block every frame.
   u_guard is 0 when the guard is off. */
uniform vec4  u_readA;
uniform float u_guard;

const int LAYERS = 6;

/* One drifting set of near-horizontal light trails. Returns the soft body of
   the streak; writes the near-white core into 'core'. */
float trails(vec2 p, float seed, float freq, float speed, float t, out float core) {
  float flow  = p.x - t * speed;
  float lane  = p.y * freq + seed * 7.0;
  float laneId = floor(lane);

  /* the lanes are not ruler-straight — a slow warp bows each one */
  float wob = fbm(vec2(flow * 0.6, laneId * 1.3 + seed)) * 0.34;
  float dy  = (fract(lane) - 0.5) + wob;

  float body = exp(-dy * dy * 240.0);
  core = exp(-dy * dy * 1500.0);

  /* along-trail brightness: long bright runs broken by gaps, per lane */
  float m = fbm(vec2(flow * 0.9 + laneId * 4.0, laneId * 2.0 + seed * 3.0));
  float bright = smoothstep(0.02, 0.55, m);

  /* not every lane is lit */
  bright *= step(0.28, dotHash(vec2(laneId, seed * 13.0)));

  core *= bright;
  return body * bright;
}

void main() {
  vec2 res = u_res / u_scale;
  vec2 uv  = gl_FragCoord.xy / u_scale / res;            /* 0..1, y up */
  float aspect = res.x / res.y;
  vec2 P = (uv - 0.5) * vec2(aspect, 1.0);

  vec2 ptr = (u_pointer.xy - 0.5) * vec2(aspect, 1.0);
  vec2 toP = P - ptr;
  float dP = length(toP);
  float pull = u_pointer.z * exp(-dP * dP * 22.0);

  /* the cursor drags the trails toward it and speeds the local flow up */
  vec2  Pw = P - toP * pull * 0.35;
  float localSpeed = 1.0 + pull * 2.6;

  /* crossing grows toward the base: top trails run flat, lower ones fan */
  float depth = clamp(1.0 - uv.y, 0.0, 1.0);            /* 0 top .. 1 bottom */
  float dens  = smoothstep(1.02, 0.12, uv.y);           /* sparse up top */

  vec3 col = mix(u_ground, u_ember, 0.12 + 0.22 * depth);
  float hot = 0.0;

  for (int i = 0; i < LAYERS; i++) {
    float fi   = float(i);
    float k    = fi / float(LAYERS - 1);                /* 0..1 across layers */
    float seed = fi * 1.618;
    float freq = mix(7.0, 27.0, k);
    float speed = mix(0.045, 0.16, fract(seed * 2.0)) * localSpeed;

    /* per-layer shear, opened up lower in the frame. Driven by uv.x (0..1,
       independent of the box's aspect ratio) rather than the aspect-scaled
       P.x a rotation matrix would use — a rotation's vertical displacement
       grows with how far a pixel sits from centre, so on a short wide card
       (large aspect, P.x reaching much further from 0) the same angle flung
       trails through several lanes across the width, reading as stretched
       diagonal smears instead of a gentle fan. Shearing by uv.x keeps the
       maximum displacement the same shape at any aspect. */
    float shearAmt = (fract(seed * 3.0) - 0.5) * (0.55 + depth * 1.7);
    vec2 rp = Pw;
    rp.y += (uv.x - 0.5) * shearAmt;

    float core;
    float s = trails(rp, seed, freq, speed, u_time, core);

    /* upper layers weigh toward the top, lower layers toward the bottom */
    float band = mix(1.0 - depth, depth, k);
    float w = mix(0.3, 1.15, band) * dens;

    vec3 tint = mix(u_ember, u_amber, smoothstep(0.0, 0.62, s));
    col += tint * s * w * 1.7;
    col += u_amber * s * s * w * 0.7;                   /* a little bloom off the body */
    hot += core * w;
  }

  col += u_flare * hot * 1.1;

  /* the touch: a warm radial bloom, and the nearest cores flare harder */
  col += u_amber * pull * 0.5;
  col += u_flare * pull * hot * 0.7;

  col = max(col, 0.0);

  /* ---- the reading guard (see TileField for the full rationale) ---- */
  vec2 rd = abs(uv - u_readA.xy) / max(u_readA.zw, vec2(0.02));
  float md = mix(max(rd.x, rd.y), length(rd), 0.4);
  float guardBand = 1.0 - smoothstep(0.72, 2.1, md);
  col = mix(col, holdUnder(col, 0.09), guardBand * u_guard);

  col += (bayer8(gl_FragCoord.xy) - 0.5) * (2.2 / 255.0);

  gl_FragColor = vec4(col, 1.0);
}
`;

export interface FilamentFieldProps {
  className?: string;
  /**
   * CSS selector for the block the reading guard should keep readable,
   * resolved against `document`. `null` (the default) turns the guard off.
   */
  guardSelector?: string | null;
}

export function FilamentField({ className, guardSelector = null }: FilamentFieldProps) {
  const canvasRef = useRef<HTMLCanvasElement>(null);

  useEffect(() => {
    const canvas = canvasRef.current;
    if (!canvas) return;

    const guardOn = guardSelector != null;

    /* The block the reading guard protects. Looked up once, lazily. */
    let guardEl: Element | null | undefined;
    const readGuardEl = () => {
      if (guardEl === undefined) {
        guardEl = guardOn
          ? document.querySelector(guardSelector as string)
          : null;
      }
      return guardEl;
    };

    return mountShaderSurface(canvas, {
      fragment: FRAG,
      uniforms: [...Object.keys(PALETTE), "u_readA", "u_guard"],
      onInit: (gl, u) => {
        for (const name of Object.keys(PALETTE)) {
          if (u[name]) gl.uniform3fv(u[name], PALETTE[name]);
        }
        if (u.u_readA) gl.uniform4f(u.u_readA, 0.5, 0.5, 0.44, 0.32);
        if (u.u_guard) gl.uniform1f(u.u_guard, guardOn ? 1 : 0);
      },
      onFrame: (gl, u, s) => {
        if (!guardOn || !u.u_readA) return;
        let cx = 0.5, cy = 0.5, hw = 0.44, hh = 0.32;
        const el = readGuardEl();
        const { rect } = s;
        if (el && rect.width > 0 && rect.height > 0) {
          const r = el.getBoundingClientRect();
          const padX = rect.width * 0.09;
          const padY = rect.height * 0.11;
          cx = (r.left + r.width / 2 - rect.left) / rect.width;
          cy = 1 - (r.top + r.height / 2 - rect.top) / rect.height;
          hw = (r.width / 2 + padX) / rect.width;
          hh = (r.height / 2 + padY) / rect.height;
        }
        gl.uniform4f(u.u_readA, cx, cy, hw, hh);
      },
      onPainted: () => canvas.setAttribute("data-shader", "on"),
      /* No onIdle — same contract as SilkField / RakeField: once painted, the
         last frame stays on screen while the surface is parked off-view, so
         scrolling away and back does not crossfade to the static floor. A lost
         context still clears it below. */
      onLost: () => canvas.removeAttribute("data-shader"),
      maxPixels: 2_200_000,
      dprCap: 1.5,
    });
  }, [guardSelector]);

  return (
    <div
      className={`absolute inset-0 overflow-hidden${className ? ` ${className}` : ""}`}
      aria-hidden="true"
      data-print="hide"
    >
      <div className="absolute inset-0 [background:radial-gradient(_120%_80%_at_60%_108%,oklch(0.7_0.19_55)_0%,transparent_52%_),radial-gradient(_90%_60%_at_20%_96%,oklch(0.6_0.14_48)_0%,transparent_50%_),radial-gradient(_120%_120%_at_50%_0%,oklch(0.06_0.02_40)_0%,transparent_60%_),oklch(0.055_0.018_42)] after:content-[''] after:absolute after:inset-0 after:[background:repeating-linear-gradient(-4deg,transparent_0_5px,oklch(0.86_0.16_62_/_0.5)_5px_6px),repeating-linear-gradient(6deg,transparent_0_9px,oklch(0.78_0.15_50_/_0.32)_9px_10px)] after:[mask-image:linear-gradient(180deg,transparent_0_22%,oklch(0_0_0_/_0.35)_55%,#000_100%)] after:[-webkit-mask-image:linear-gradient(180deg,transparent_0_22%,oklch(0_0_0_/_0.35)_55%,#000_100%)]" />
      <canvas
        ref={canvasRef}
        className="absolute inset-0 w-full h-full block opacity-0 transition-opacity duration-[--duration-slow] ease-[--ease-standard] motion-reduce:transition-none data-[shader=on]:opacity-100"
      />
    </div>
  );
}