1//! The lens distortion postprocessing effect.
2
3/// See `package::effect_stack::LensDistortion` for more
4/// information on these fields.
5struct LensDistortionSettings {
6    intensity: f32,
7    scale: f32,
8    multiplier: vec2<f32>,
9    center: vec2<f32>,
10    edge_curvature: f32,
11    unused: u32,
12}
13
14const VISUAL_THRESHOLD: f32 = 1e-4;
15const MATH_EPSILON: f32 = 1e-6;
16
17/// The settings supplied by the developer.
18@group(0) @binding(5) var<uniform> lens_distortion_settings: LensDistortionSettings;
19
20fn lens_distortion(uv: vec2<f32>) -> vec2<f32> {
21    let intensity = lens_distortion_settings.intensity;
22    if (abs(intensity) < VISUAL_THRESHOLD) {
23        return uv;
24    }
25    let multiplier = lens_distortion_settings.multiplier;
26    let center = lens_distortion_settings.center;
27
28    let uv_centered = uv - center;
29    // Prevent division by zero.
30    let radius = max(length(uv_centered), MATH_EPSILON);
31
32    let direction = uv_centered / radius;
33    let adjust = dot(abs(direction), multiplier);
34
35    // Maintains the correlation between k2 and k1, while ensuring the sign of k2
36    // is determined solely by `edge_curvature` rather than being influenced by intensity.
37    let k1 = intensity * adjust;
38    let k2 = k1 * intensity * lens_distortion_settings.edge_curvature;
39
40    let r2 = radius * radius;
41    let r_distorted = radius * (1.0 + (k1 + k2 * r2) * r2);
42
43    let uv_distorted = direction * r_distorted + center;
44
45    // Compensates for the distortion pushing pixels outside the [0,1] UV bounds.
46    let uv_scaled = (uv_distorted - center) / lens_distortion_settings.scale + center;
47
48    // Discard out-of-bounds pixels to prevent edge bleeding artifacts.
49    let uv_safe = clamp(uv_scaled, vec2<f32>(0.0), vec2<f32>(1.0));
50
51    return uv_safe;
52}