4x Animals

fn smoothstep(edge0: f32, edge1: f32, t: f32) -> f32 {
    let x = ((t - edge0) / (edge1 - edge0)).clamp(0.0, 1.0);
    x * x * (3.0 - 2.0 * x)
}

/// Hornea un splatmap 64x64: 0 = material-A puro, 255 = material-B puro.
fn bake_splatmap(mask: NeighborMask) -> Vec<u8> {
    let size = SPLATMAP_SIZE as usize;
    let mut pixels = vec![0u8; size * size];
    for pv in 0..size {
        for pu in 0..size {
            let u = (pu as f32 + 0.5) / size as f32;
            let v = (pv as f32 + 0.5) / size as f32;
            let mut blend = 0.0_f32;
            if mask.north { blend = blend.max(smoothstep(0.55, 0.85, v)); }
            if mask.south { blend = blend.max(smoothstep(0.45, 0.15, v)); }
            if mask.east  { blend = blend.max(smoothstep(0.55, 0.85, u)); }
            if mask.west  { blend = blend.max(smoothstep(0.45, 0.15, u)); }
            pixels[pv * size + pu] = (blend * 255.0) as u8;
        }
    }
    pixels
}

@fragment
fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
    let uv_t    = in.uv * 4.0; // tiling del material
    let blend_t = smoothstep(0.3, 0.7,
                      textureSample(splatmap, splat_sampler, in.uv).r);

    let alb = mix(
        textureSample(albedo_a, sampler_a, uv_t).rgb,
        textureSample(albedo_b, sampler_b, uv_t).rgb,
        blend_t);
    let ao = mix(
        textureSample(ao_a, sampler_a, uv_t).r,
        textureSample(ao_b, sampler_b, uv_t).r,
        blend_t);

    let nl = max(dot(in.world_normal, camera.light_direction), camera.ambient);
    return vec4<f32>(alb * nl * ao, 1.0);
}

/// Color por LCG sembrado por 'tick' -> determinista y reproducible.
fn pick_color(tick: u64) -> MeteoriteColor {
    const A: u64 = 6_364_136_223_846_793_005; // multiplicador de Knuth
    const C: u64 = 1_442_695_040_888_963_407;
    const M: u64 = 10_000;

    let hash = tick.wrapping_mul(A).wrapping_add(C);
    let bucket = hash % M; // 0..9999

    if bucket < 5_000 {
        MeteoriteColor::Green // 50%
    } else if bucket < 7_500 {
        MeteoriteColor::Blue  // 25%
    } else if bucket < 9_500 {
        MeteoriteColor::Red   // 20%
    } else {
        MeteoriteColor::Gold  // 5%
    }
}