// distancia mínima
// perpendicular

int dim=500;
int center=dim/2;

float lineOx; // x origin
float lineOy; // y origin
float lineEx; // x end
float lineEy; // y end

float intersecx,intersecy; // coord intersection
float ax,ay;
float ff;
float lineMag;

void setup()
{
 size(dim,dim);
 background(255);

 lineOx=random(dim-50);
 lineOy=random(dim-50);
 lineEx=random(dim-50);
 lineEy=random(dim-50);

}

void draw()
{
 background(255);
 doIt();
}

float d(float _x1, float _y1, float _x2, float _y2)
{
 return sqrt(sq(_x2-_x1)+sq(_y2-_y1));
}

void doIt()
{
 ax=mouseX;
 ay=mouseY;
 
 stroke(190);
 fill(255);
 
 lineMag = d(lineOx,lineOy,lineEx,lineEy);
 line(lineOx,lineOy,lineEx,lineEy);
 
 ellipseMode(CENTER);
 ellipse(lineOx,lineOy,10,10);
 ellipse(lineEx,lineEy,10,10);
 ellipse(ax,ay,10,10);
 
 ff =  (( ( ax - lineOx ) * ( lineEx - lineOx ) ) + ( ( ay - lineOy ) * ( lineEy - lineOy ) )) / ( lineMag * lineMag );
 
 if( ff < 0.0 || ff > 1.0 ){
   return;  // doesn't fall within the line: line (if you want) to the nearest vertex
 } else {
   intersecx = lineOx + int(ff * (lineEx - lineOx));
   intersecy = lineOy + int(ff * (lineEy - lineOy));
   line(ax,ay,intersecx,intersecy);
       fill(255);
   ellipse(ax,ay,10,10);
   noStroke();
   fill(255,0,0);
   ellipse(intersecx,intersecy,2,2);
 }

}