Hello,
I try to find the intersection point between  bisector and the ellipse. In my sketch this point is the intersection of the red line start from the center of the ellipse pass by the red point C to the radius ellipse. I hope is clear.
I find no equation to calculate this point on internet. So if any body have an idea, thanks

Stan

1. PVector pointA, pointB, pointC ;
   
   void setup()
   {
     pointA = pointB = pointC =new PVector (0,0) ;
     size( 400,400) ;
   }
   
   
   void draw()
   {
     background(0) ;
     float size = 200 ;
     noFill() ;
     stroke(255) ;
     ellipse(width/2, height/2, size, size) ;
    
     //point A
     pointA = new PVector (0,0) ;
     pointA.x = AxisRotationPos(mouseX, width, size /2).x + width /2 ;
     pointA.y = AxisRotationPos(mouseX, width, size /2).z +height/2  ;
     ellipse (pointA.x, pointA.y, 10, 10 ) ;
    
     //pointB
     pointB = new PVector (0,0) ;
     pointB.x = AxisRotationPos(mouseY, height, size /2).x + width /2 ;
     pointB.y = AxisRotationPos(mouseY, height, size /2).z +height/2  ;
     ellipse (pointB.x, pointB.y, 10, 10 ) ;
    
    
     //line
     line(pointA.x, pointA.y, pointB.x, pointB.y) ;
     stroke(250,35,35) ;
     line(width/2, height/2, pointC.x, pointC.y) ;
    
     //point C bisector
     pointC = new PVector(0,0) ;
     noStroke() ;
     fill(250,35,35) ;
     pointC.x = bisector(pointA,pointB).x ;
     pointC.y = bisector(pointA, pointB).y ;
     ellipse(pointC.x, pointC.y, 20,20) ;
   }
   
   
   //ANNEXE VOID
   float rotationPlan ;
   
   PVector AxisRotationPos(int posMouse, int dist, float distanceToFocus)
   {
     PVector pos = new PVector (0,0,0) ;
    
     rotationPlan = map(posMouse, 0, dist, 0, TAU)  ;
    
     PVector marge = new PVector((width -distanceToFocus *2.0) *.5,(height -distanceToFocus *2.0) *.5) ;
    
     float dataPosXY = cos(rotationPlan) *distanceToFocus ;
     float dataPosZ =  sin(rotationPlan) *distanceToFocus ;
    
     pos = new PVector ( dataPosXY, dataPosXY, dataPosZ ) ;
     return pos ;
   }
   
   PVector bisector(PVector p1, PVector p2)
   {
     PVector b ;
     b = PVector.add(p1, p2);
     b.div(2) ;
    
     return b ;
   }