point2line
index		  
Name lineSegmentIntersection ( )
Description Calculates the intersection of two line segments.

The code comes from a post on Code & Form by Marius Watz, which led to a response with a more efficient algorithm taken from Graphics Gems. See the original discussion.

I had trouble making the algorithm work with floating point values, so I did a workaround. If you happen to know a better solution I'd love to hear about it. Typically you don't need to change the precision value.
Syntax 
lineSegmentIntersection(p1, p2, p3, p4);
lineSegmentIntersection(p1, p2, p3, p4, precision);
Parameters
p1,p2,p3,p4   two pair of vectors each defining a line (Vect2,Vect2,Vect2,Vect2)
precision.   4 is default (int)
Returns null if there is no intersection, otherwise a vector containing the intersection coordinate (Vect2)
Usage Web & Application
Related
Examples 
import point2line.*;

Vect2[] line1;
Vect2[] line2;

void setup()
{
  size( 200, 200 );
  smooth();
  fill( 255, 0, 0 );
  stroke( 255 );

  line1 = new Vect2[ 2 ];
  line1[ 0 ] = new Vect2( 0, height* 0.4 );
  line1[ 1 ] = new Vect2( width, height* 0.9 );
  
  line2 = new Vect2[ 2 ];
  line2[ 0 ] = new Vect2( width*0.5, 0 );
  line2[ 1 ] = new Vect2(); // will be updateed by the mouse
}


void draw()
{
  line2[ 1 ] = new Vect2( mouseX, mouseY );
  
  Vect2 intersectionPoint = Space2.lineIntersection( line1[ 0 ], line1[ 1 ], line2[ 0 ], line2[ 1 ] );
  
  // display //
  background( 0 );
  if( intersectionPoint != null ){
    ellipse( intersectionPoint.x, intersectionPoint.y, 10, 10 );
  }
  line( line1[ 0 ].x, line1[ 0 ].y, line1[ 1 ].x, line1[ 1 ].y );
  line( line2[ 0 ].x, line2[ 0 ].y, line2[ 1 ].x, line2[ 1 ].y );
}