src/shaders/render/scratch_lines.wgsl


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// Horizontal film scratch lines utility
// Returns scratch intensity [0,1] to additively overlay on a color buffer.
//
// Parameters:
//   uv         - normalized texture coordinates [0,1]
//   resolution - screen size in pixels (used for sub-pixel thickness)
//   time       - physical time in seconds
//
// Simulates aged/damaged film: ~8 random horizontal lines per frame,
// each with independent position, thickness, brightness and on/off state.
// Time is quantized to 24 buckets/sec so scratches flicker like film frames.
#include "math/noise"

fn scratch_lines(uv: vec2f, resolution: vec2f, time: f32) -> f32 {
  // Quantize to ~24fps to simulate film frame rate
  let t = floor(time * 24.0);

  // Scanlines
  let scan = abs(sin(uv.y * uniforms.resolution.y * 3.14159265));
  var intensity = 0.2 * scan;

  // Per-pixel temporal noise
  let pixel = floor(uv * uniforms.resolution * .4);
  let n = hash_2f(pixel + vec2f(time * 47.3, time * 31.7)) * 0.04 - 0.02;
  intensity += n;

  // Horizontal jitter line (random scanline rolling artifact)
  let jitter_y = hash_1f(fract(t * 0.37));
  let jitter_band = abs(uv.y - jitter_y);
  let scratch = smoothstep(5. / uniforms.resolution.y, 0., jitter_band);
  intensity += scratch * 0.11 * hash_1f(uv.x * 100.0 + t);
  return clamp(intensity, 0.0, 1.0);
}