import java.awt.geom.Point2D; class Line { float x1; float y1; float x2; float y2; public Line(float _x1, float _y1, float _x2, float _y2) { x1 = _x1; y1 = _y1; x2 = _x2; y2 = _y2; } public Point2D.Float intersects(Line otherLine) { float x3 = otherLine.x1; float y3 = otherLine.y1; float x4 = otherLine.x2; float y4 = otherLine.y2; float d = (x1-x2)*(y3-y4) - (y1-y2)*(x3-x4); if (d == 0) return null; float xi = ((x3-x4)*(x1*y2-y1*x2)-(x1-x2)*(x3*y4-y3*x4))/d; float yi = ((y3-y4)*(x1*y2-y1*x2)-(y1-y2)*(x3*y4-y3*x4))/d; Point2D.Float p = new Point2D.Float(xi, yi); if (xi < Math.min(x1, x2) || xi > Math.max(x1, x2)) return null; if (xi < Math.min(x3, x4) || xi > Math.max(x3, x4)) return null; return p; /* float resX = ((x1*y2 - y1*x2)*(x3-x4) - (x1-x2)*(x3*y4 - y3*x4)) / ( (x1-x2)*(y3 -y4) - (y1-y2)*(x3-x4)); float resY = ((x1*y2 - y1*x2)*(y3-y4) - (y1-y2)*(x3*y4 - y3*x4)) / ( (x1-x2)*(y3 -y4) - (y1-y2)*(x3-x4)); return new Point2D.Float(resX, resY); */ } public void draw() { line(x1, y1, x2, y2); } } Line lineA = new Line(0, 0, 400, 300); Line lineB = new Line(500, 100, 100, 500); void setup() { //setup function called initially, only once size(800, 600); frameRate(30); smooth(); background(255); //set background white } private int lineSelected = 1; void keyPressed() { if (key=='1') { print("1 pressed "); lineSelected = 1; } if (key=='2') { print("2 pressed "); lineSelected = 2; } } void mouseClicked() { if (mouseButton == LEFT) { if (lineSelected==1) { lineA.x1 = mouseX; lineA.y1 = mouseY; } else { lineB.x1 = mouseX; lineB.y1 = mouseY; } } else { if (lineSelected==1) { lineA.x2 = mouseX; lineA.y2 = mouseY; } else { lineB.x2 = mouseX; lineB.y2 = mouseY; } } } void draw() { //draw function loops background(255); stroke(100, 150, 250); lineA.draw(); stroke(250, 150, 100); lineB.draw(); Point2D.Float pointIntersection = lineA.intersects(lineB); if ( pointIntersection != null ) { stroke(20, 20, 20); ellipse( (float)pointIntersection.getX(), (float)pointIntersection.getY(), 5, 5); } //line(s1.x,s1.y,s2.x, s2.y); }

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A quick prototype to test a function, that gives, if it exists, the intersection point of 2 lines.

Commands.

key 1 or 2 : select the redLine or the blueLine.

left click : set the new starting position of the selected line.

right click : set the new ending position of the selected line.