点と線分の距離を算出． 直線の描画に使える

GLSL-segmentDistance

float lineSegDistance( vec2 p, vec2 l1, vec2 l2) {

    float a2 = pow( distance(l1, p), 2.0 );
    float b2 = pow( distance(l2, p), 2.0 );
    float c  = abs( distance(l1, l2) );
    float c2 = c * c;

    if ( a2 + c2 < b2 || b2 + c2 < a2 ) {
        return 10000000000000000000.0;
    }

    float x = ( a2 - b2 + c2) / ( 2.0 * c );

    return sqrt( a2 - x*x );
}