ui.wgsl

#import bevy_render::view::View

const TEXTURED = 1u;
const RIGHT_VERTEX = 2u;
const BOTTOM_VERTEX = 4u;
const BORDER: u32 = 8u;

fn enabled(flags: u32, mask: u32) -> bool {
    return (flags & mask) != 0u;
}

@group(0) @binding(0) var<uniform> view: View;

struct VertexOutput {
    @location(0) uv: vec2<f32>,
    @location(1) color: vec4<f32>,

    @location(2) @interpolate(flat) size: vec2<f32>,
    @location(3) @interpolate(flat) flags: u32,
    @location(4) @interpolate(flat) radius: vec4<f32>,    
    @location(5) @interpolate(flat) border: vec4<f32>,    

    // Position relative to the center of the rectangle.
    @location(6) point: vec2<f32>,
    @builtin(position) position: vec4<f32>,
};

@vertex
fn vertex(
    @location(0) vertex_position: vec3<f32>,
    @location(1) vertex_uv: vec2<f32>,
    @location(2) vertex_color: vec4<f32>,
    @location(3) flags: u32,

    // x: top left, y: top right, z: bottom right, w: bottom left.
    @location(4) radius: vec4<f32>,

    // x: left, y: top, z: right, w: bottom.
    @location(5) border: vec4<f32>,
    @location(6) size: vec2<f32>,
) -> VertexOutput {
    var out: VertexOutput;
    out.uv = vertex_uv;
    out.position = view.view_proj * vec4(vertex_position, 1.0);
    out.color = vertex_color;
    out.flags = flags;
    out.radius = radius;
    out.size = size;
    out.border = border;
    var point = 0.49999 * size;
    if (flags & RIGHT_VERTEX) == 0u {
        point.x *= -1.;
    }
    if (flags & BOTTOM_VERTEX) == 0u {
        point.y *= -1.;
    }
    out.point = point;

    return out;
}

@group(1) @binding(0) var sprite_texture: texture_2d<f32>;
@group(1) @binding(1) var sprite_sampler: sampler;

// The returned value is the shortest distance from the given point to the boundary of the rounded 
// box.
// 
// Negative values indicate that the point is inside the rounded box, positive values that the point 
// is outside, and zero is exactly on the boundary.
//
// Arguments: 
//  - `point`        -> The function will return the distance from this point to the closest point on 
//                    the boundary.
//  - `size`         -> The maximum width and height of the box.
//  - `corner_radii` -> The radius of each rounded corner. Ordered counter clockwise starting 
//                    top left:
//                      x: top left, y: top right, z: bottom right, w: bottom left.
fn sd_rounded_box(point: vec2<f32>, size: vec2<f32>, corner_radii: vec4<f32>) -> f32 {
    // If 0.0 < y then select bottom left (w) and bottom right corner radius (z).
    // Else select top left (x) and top right corner radius (y).
    let rs = select(corner_radii.xy, corner_radii.wz, 0.0 < point.y);
    // w and z are swapped so that both pairs are in left to right order, otherwise this second 
    // select statement would return the incorrect value for the bottom pair.
    let radius = select(rs.x, rs.y, 0.0 < point.x);
    // Vector from the corner closest to the point, to the point.
    let corner_to_point = abs(point) - 0.5 * size;
    // Vector from the center of the radius circle to the point.
    let q = corner_to_point + radius;
    // Length from center of the radius circle to the point, zeros a component if the point is not 
    // within the quadrant of the radius circle that is part of the curved corner.
    let l = length(max(q, vec2(0.0)));
    let m = min(max(q.x, q.y), 0.0);
    return l + m - radius;
}

fn sd_inset_rounded_box(point: vec2<f32>, size: vec2<f32>, radius: vec4<f32>, inset: vec4<f32>) -> f32 {
    let inner_size = size - inset.xy - inset.zw;
    let inner_center = inset.xy + 0.5 * inner_size - 0.5 * size;
    let inner_point = point - inner_center;

    var r = radius;

    // Top left corner.
    r.x = r.x - max(inset.x, inset.y);

    // Top right corner.
    r.y = r.y - max(inset.z, inset.y);

    // Bottom right corner.
    r.z = r.z - max(inset.z, inset.w); 

    // Bottom left corner.
    r.w = r.w - max(inset.x, inset.w);

    let half_size = inner_size * 0.5;
    let min_size = min(half_size.x, half_size.y);

    r = min(max(r, vec4(0.0)), vec4<f32>(min_size));

    return sd_rounded_box(inner_point, inner_size, r);
}

fn draw(in: VertexOutput) -> vec4<f32> {
    let texture_color = textureSample(sprite_texture, sprite_sampler, in.uv);

    // Only use the color sampled from the texture if the `TEXTURED` flag is enabled. 
    // This allows us to draw both textured and untextured shapes together in the same batch.
    let color = select(in.color, in.color * texture_color, enabled(in.flags, TEXTURED));

    // Signed distances. The magnitude is the distance of the point from the edge of the shape.
    // * Negative values indicate that the point is inside the shape.
    // * Zero values indicate the point is on on the edge of the shape.
    // * Positive values indicate the point is outside the shape.

    // Signed distance from the exterior boundary.
    let external_distance = sd_rounded_box(in.point, in.size, in.radius);

    // Signed distance from the border's internal edge (the signed distance is negative if the point 
    // is inside the rect but not on the border).
    // If the border size is set to zero, this is the same as as the external distance.
    let internal_distance = sd_inset_rounded_box(in.point, in.size, in.radius, in.border);

    // Signed distance from the border (the intersection of the rect with its border).
    // Points inside the border have negative signed distance. Any point outside the border, whether 
    // outside the outside edge, or inside the inner edge have positive signed distance.
    let border_distance = max(external_distance, -internal_distance);

    // The `fwidth` function returns an approximation of the rate of change of the signed distance 
    // value that is used to ensure that the smooth alpha transition created by smoothstep occurs 
    // over a range of distance values that is proportional to how quickly the distance is changing.
    let fborder = fwidth(border_distance);
    let fexternal = fwidth(external_distance);

    if enabled(in.flags, BORDER) {   
        // The item is a border

        // At external edges with no border, `border_distance` is equal to zero. 
        // This select statement ensures we only perform anti-aliasing where a non-zero width border 
        // is present, otherwise an outline about the external boundary would be drawn even without 
        // a border.
        let t = 1. - select(step(0.0, border_distance), smoothstep(0.0, fborder, border_distance), external_distance < internal_distance);
        return color.rgba * t;
    }

    // The item is a rectangle, draw normally with anti-aliasing at the edges.
    let t = 1. - smoothstep(0.0, fexternal, external_distance);
    return color.rgba * t;
}

@fragment
fn fragment(in: VertexOutput) -> @location(0) vec4<f32> {
    return draw(in);
}