I am drawing a 3D line using a succession of transformations (rotateX , rotateY). I then want to use a point along the line to begin drawing another line (branch) - but am having difficulty in finding the starting co-ordinate for the new line to begin from (relative to the main line). I have tried using screenX, screenY and modelX, modelY to get the co-ordinates at which the new line should start from - but with no success.

I have a small example of what I am having problems with here: (a red ellipse is drawn at (0,0,0) using the current transformation matrix and have tried to  'get' the screen co-ordinates of this position using screenX , screenY - and represented this by drawing a blue ellipse - which seems to be way off where it should be.

Any help would be great : )

Code:

import processing.opengl.*;

// a test to replicate the 'branching 3d line problem
// create an object which is translated and re-draws itself.

float angle = 1;
int segmentCount = 2;
int branchNum = 10;
void setup () {
 frameRate (10);
 size (800, 800, OPENGL);
 background (256, 256, 256);
}


void draw () {
 background (256, 256, 256);
 /////////////////////////////////////////////////////////////
 // position / draw entire line on the stage //
 translate (width/2, height/2); // this is the starting point for the entrire environment
 rotateX (radians (angle));  // this applys to the ENTIRE line
 rotateZ(radians (angle));  // this applys to the ENTIRE line
 ////////////////////////////////////////////////////////
 ///// DRAW LINE SEGMENTS //////////////
 pushMatrix ();
 for (int i=0; i<segmentCount; i++) {
   rotateZ (radians (7)); // this applies WITHIN the line
   rotateX (radians (12));
   //rotateY (radians (angle));
   stroke (0, 0, 0);
   line (0, 0, 0, 10, 10, 0);
   fill (0,0,0);
   ellipse (0, 0, 10, 10);
   if (i == 10) {
     // CREATE A STARTING POINT FOR A BRANCH
     //  createBranch (screenX (0,0,0), screenY (0, 0, 0), modelZ (0, 0, 0));
     //  createBranch (modelX (0,0,0), modelY (0, 0, 0), modelZ (0, 0, 0));
     createBranch ();
   }

   translate (10, 10, 0); // this translation allows the next segment to be drawn
 }
 popMatrix () ;
 /////////////////////////////////////////////////////////////////////
 angle +=0.5;
 segmentCount ++;
}

void createBranch ( ) {
 //////////////////////////////////////
 // Draw red circle at position along line ///
 fill (256, 0, 0);
 stroke (256, 0, 0);
 ellipse (0, 0, 15, 15);
 ////////////////////////////////////
 float xpos = (screenX (0,0,0));
 float ypos = (screenY (0,0,0));  
 float zpos = (screenZ (0,0,0));
 pushMatrix();
 translate (xpos, ypos, zpos);
 fill (0, 0, 256);
 ellipse (0, 0, 15, 15);
 popMatrix();
}