Draw Ellipse four control points

Hi ,
Please help me to draw an ellipse with known 2 diameters and 4 control points(start and end points of the biggest and smallest diameters).

I implemented some algorithm, but I have a problem that in some cases it doesn't work correctly.(example : some of the diameters equal to zero.)


for(float j=-a; j<=a; j=j+1.f) //use the formula for the ellipse to generate the points and draw them on the screen
      {
         float x0 = center_x;
         float y0 = center_y;
      
         float x1 = center_x + (j);
            
         float y1 =0.f;
         if (b==0 || a==0)
                  y1 = center_y;
         else
              y1 = center_y + (float)(sqrt((b*b)-((b*b*j*j)/(a*a))));
       
       float deltax = x1 - x0;
         float deltay = y0 - y1;

         float dis = (float)sqrt((deltax*deltax)+(deltay*deltay));
            
         float c_dis = (float)dis;
         float s_dis = (float)dis;
            
       float pi = (float)acos(-1);
            
       
         //-----------------------------------------------------------------------            
         if(deltay !=0 || deltax!=0)
         {
                    
            float angle;
              if(deltay ==0)
                    angle = atanf(deltax);
              else            
                  angle = (float)atan(deltax/deltay);

            if(deltay <0  && deltax >=0)
              {
                  if (deltax == 0)
                              angle = (float)(pi/2.0) + (float)fabs((atan(deltay)) ) ;
                        else
                              angle = (float)(pi/2.0) + (float)fabs((atan(deltay/deltax)) ) ;
              }

         
              if(deltay <0 && deltax <=0)
              {
                angle = (float)fabs(atan(deltax/deltay)) + (float)(pi);
               
              }
        
         if(deltay >0 && deltax <=0)
             {
                   angle = (float)atan(deltax/deltay) ;
                         
             }

           //---------------------------------------------------------------------

         x1 = (float)x0 + (float)(dis*sin(angle+orient+angle_90));
             y1 = (float)y0 - (float)(dis*cos(angle+orient+angle_90));

         }//end of if (deltay !=0 || deltax!=0)
             
       if(count>0)
         {
              pDC->MoveTo(endpoint.x,endpoint.y);
              find_max_min_x_y_point(endpoint.x,endpoint.y);
            pDC->LineTo((int)(x1+0.5),(int)(y1+0.5));
                      }

       endpoint.x = (int)(x1+0.5);
         endpoint.y = (int)(y1+0.5);
                  count++;

         x0 = center_x;
         y0 = center_y;

         x1 = center_x + (j);
             
         if (b==0||a==0)
               y1= center_y;
         else
             y1 = center_y - (float)(sqrt((b*b)-((b*b*j*j)/(a*a))));
       
       deltax = x1 - x0;
         deltay = y0 - y1;

         dis = (float)sqrt((deltax*deltax)+(deltay*deltay));
            
         c_dis = (float)dis;
         s_dis = (float)dis;
            
       pi = (float)acos(-1);
            
       
         //-----------------------------------------------------------------------            
         if(deltay !=0 || deltax!=0)
         {
                    
             float angle =0.f;
               if(deltay ==0)
                    angle = atanf(deltax);
               else            
                        angle = (float)atan(deltax/deltay);

             if(deltay <0  && deltax >=0)
               {
                   if (deltax == 0)
                              angle = (float)(pi/2.0) + (float)fabs((atan(deltay)) ) ;
                        else
                              angle = (float)(pi/2.0) + (float)fabs((atan(deltay/deltax)) ) ;
               }

           if(deltay <0 && deltax <=0)
               {
                       angle = (float)fabs(atan(deltax/deltay)) + (float)(pi);
               }
        
       
               if(deltay >0 && deltax <=0)
               {
               
                     angle = (float)atan(deltax/deltay) ;
               }

             //---------------------------------------------------------------------

               x1 = (float)x0 + (float)(dis*sin(angle+orient+angle_90));
               y1 = (float)y0 - (float)(dis*cos(angle+orient+angle_90));

         }//end of if (deltay !=0 || deltax!=0)
             
         if(count == 1)
         {
                  endpoint2.x = (int)x1;
                  endpoint2.y = (int)y1;
                  
         }
       if(count>0)
         {
             pDC->MoveTo(endpoint2.x,endpoint2.y);
               find_max_min_x_y_point(endpoint2.x,endpoint2.y);
             pDC->LineTo((int)(x1+0.5),(int)(y1+0.5));
                        }

       endpoint2.x = (int)(x1+0.5);
         endpoint2.y = (int)(y1+0.5);
                  count++;


      }//end of for(j=-a....

Thank you