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float line(vec2 p, vec2 a,vec2 b) { // --- distance to segment with caps
    p -= a, b -= a;
    float h = clamp(dot(p, b) / dot(b, b), 0., 1.);// proj coord on line
    return length(p - b * h);                      // dist to segment
    // We might directly return smoothstep( 3./R.y, 0., dist),
    //     but its more efficient to factor all lines.
    // We can even return dot(,) and take sqrt at the end of polyline:
    // p -= b*h; return dot(p,p);
}