OpenGl ES : Ray Picking for multiple objects drawn using push/pop matrix


i got a question regarding multiple objects drawn using push/ pop stack.something similar to this.


---

开发者_开发技巧

    glMatrixMode(GL_MODELVIEW);  
    glLoadIdentity();  
    glTranslatef(x,y,z);  
    glRotate(r,1,0,0);     
        
    glTranslate(-x,-y,-z);


    for (i=0 to 20) objects    
    
      glpushMatrix();    

          draw_object()    
    
      glpopMatrix();    
   
 end
        
    

---

each object is a unit circle with its own transformation.

In such a case how does ray picking work. How should i keep track of center point of the object to calculate ray intersection. i really appreciate any help.

---

You can get the current matrix for the each object:

glMatrixMode(GL_MODELVIEW);  
glLoadIdentity();  
glTranslatef(x,y,z);  
glRotate(r,1,0,0);     
        
glTranslate(-x,-y,-z);
    

for (i=0 to 20) objects    
    
  glpushMatrix();    
      ... some matrix transformations specific for the object
      ... and get the final matrix and store it to object member
      glGetFloatv(GL_MODELVIEW_MATRIX, (GLfloat*)&object->modelMatrix);
      draw_object()    
    
  glpopMatrix();  

When doing ray intersection just multiply the object matrix with local center point coordinate to get it in absolute space where ray is defined.