// 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);

    var intensity = 0.0;
    for (var i = 0; i < 8; i++) {
        let seed = f32(i) * 17.3 + t;
        // ~25% chance this scratch is visible this frame
        let visible = step(0.75, hash_1f(seed + 3.1));
        // Random vertical position [0, 1]
        let y = hash_1f(seed + 7.9);
        // Thickness in pixels [1, 4]
        let thickness = hash_1f(seed + 13.5) * 3.0 + 1.0;
        // Brightness [0.5, 1.0]
        let brightness = hash_1f(seed + 21.7) * 0.5 + 0.5;
        // Soft linear falloff from line center
        let dy = abs(uv.y - y) * resolution.y;
        let line = clamp(1.0 - dy / thickness, 0.0, 1.0);
        intensity += line * brightness * visible;
    }
    return clamp(intensity, 0.0, 1.0);
}