monolith

// Coverage of the fragment under the push constant clip, a rounded rect in

// framebuffer pixels with radii TL, TR, BR, BL. A ring keeps only the band

// that wide inside the edge, for borders.

float rounded_box(vec2 p, vec2 half_size, float r) {

    vec2 q = abs(p) - half_size + r;

    return length(max(q, 0.0)) + min(max(q.x, q.y), 0.0) - r;

}

float clip_alpha() {

    if (pc.clip.z <= 0.0) {

        return 1.0;

    }

    vec2 half_size = pc.clip.zw * 0.5;

    vec2 p = gl_FragCoord.xy - (pc.clip.xy + half_size);

    float r = p.x < 0.0 ? (p.y < 0.0 ? pc.radii.x : pc.radii.w)

                        : (p.y < 0.0 ? pc.radii.y : pc.radii.z);

    float a = clamp(0.5 - rounded_box(p, half_size, r), 0.0, 1.0);

    float ring = pc.ring.x;

    if (ring > 0.0) {

        float inner = rounded_box(p, half_size - ring, max(r - ring, 0.0));

        a *= clamp(0.5 + inner, 0.0, 1.0);

    }

    return a;

}