Line Line Intersect

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float ax = 50;   // 1st point x

float ay = 50;   // 1st point y

float bx = 200;  // 2nd point x

float by = 300;  // 2nd point y

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float cx = 350;  // 3rd point x

float cy = 80;   // 3rd point y

float dx;        // 4th point x

float dy;        // 4th point y

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void setup() {

  size(400, 400); 

  ellipseMode(RADIUS);

  smooth();

}

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void draw() {

  background(204);

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  dx = mouseX;

  dy = mouseY;

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  if (lineLineIntersect(ax, ay, bx, by, cx, cy, dx, dy) == true) {

    stroke(255); 

  } else {

    stroke(0); 

  }

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  line(ax, ay, bx, by);

  line(cx, cy, dx, dy);

}

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boolean lineLineIntersect(float x1, float y1, float x2, float y2, float x3, float y3, float x4, float y4 ) {

  boolean over = false;

  float a1 = y2 - y1;

  float b1 = x1 - x2;

  float c1 = a1*x1 + b1*y1;

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  float a2 = y4 - y3;

  float b2 = x3 - x4;

  float c2 = a2*x3 + b2*y3;

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  float det = a1*b2 - a2*b1;

  if(det == 0){

    // Lines are parallel

  } 

  else {

    float x = (b2*c1 - b1*c2)/det;

    float y = (a1*c2 - a2*c1)/det;

    if(x > min(x1, x2) && x < max(x1, x2) && 

       x > min(x3, x4) && x < max(x3, x4) &&

       y > min(y1, y2) && y < max(y1, y2) &&

       y > min(y3, y4) && y < max(y3, y4)){

      over = true; 

    }

  }

  return over;

}

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