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Saturday, 4 February 2017

Mid point ellipse drawing algorithm in c++

In Mid-point Ellipse drawing algorithm we use 4 way symmetry of the ellipse to generate it.We perform calculations for one part and the other three parts will be drawn by using 4-way symmetry.


For given parameter Rx and Ry (radii of ellipse) and (Xc,Yc)-center of ellipse, we determine the point (x,y) for an ellipse in standard position centered on the origin and then we shift the points so the ellipse is centered at (Xc,Yc).

We can see the image given below, how we choose next point. 


Source-www.slideshare.net

Algorithm-

  • Read the two radii Rx and Ry
  • Initialize starting point x=0,y=Ry
  • Calculate initial decision variable for region 1                   d1=Ry*Ry-Rx*Rx*Ry-(Rx*Rx)/4
  • Region 1
     do{
                  plot(x,y) and other 3 symmetric points
                  if(d1<0)
                   {
                           x=x+1;
                           dx=dx=ry*ry*(2*x+3);
                           d1=d1+dx;
                   }
                 else
                 {
                        y=y-1;x=x+1;
                       dx=rx*rx*(2-2*y)+ry*ry*(3+2*x);
                       d1=d1+dx;
                  }

         } while((ry*ry*x)<=(rx*rx*y));
  • Calculate initial decison variable for region 2
       d2=ry*ry*(x+1/2)*(x+1/2)+rx*rx*(y-1)*(y-1)- rx*rx*ry*ry;
  •  Region 2
      do{
              plot(x,y) and other 3 symmetric points
             if(d2>0.0)
             {
                     y=y-1;
                     dy=rx*rx*(3-2*y);
                    d2=d2+dy;
               }
          else
        {
            y=y-1;x=x+1;
            dy=ry*ry*(2+2*x)+rx*rx*(3-2*y);
            d2=d2+dy;
        }
    }while(y>0);

  •  stop


C++ program for mid-point ellipse drawing algorithm- 

#include<bits/stdc++.h>
#include<graphics.h>
using namespace std;
int main()
{
 int rx,ry,gm,gd;
 float x,y;
 float d1,d2,dx,dy;
 //take ellipse radius as input we will not take center as it is default (0,0)
 cout<<"enter radii rx,ry:"<>rx>>ry;
 //initialize starting point
 x=0;y=ry;
 //initialize graph
 detectgraph(&gd,&gm);
 initgraph(&gd,&gm," ");
 //initial decision variable for region 1
 d1=ry*ry-rx*rx*ry+(rx*rx)/4;
 do{
  //draw all four symmetric points
  putpixel(x+300,y+300,1);
  putpixel(-x+300,y+300,2);
  putpixel(-x+300,-y+300,3);
  putpixel(x+300,-y+300,4);
  //change in coordinates based on decision variable
  if(d1<0 .0="" 2="" all="" based="" change="" coordinates="" d1="d1+dx;" d2="" decision="" delay="" do="" draw="" dx="rx*rx*(2-2*y)+ry*ry*(3+2*x);" else="" for="" four="" if="" in="" initial="" on="" points="" putpixel="" region="" rx="" ry="" symmetric="" variable="" while="" x="" y="">0.0)
  {
   y=y-1;
   dy=rx*rx*(3-2*y);
   d2=d2+dy;
  }
  else
  {
   y=y-1;x=x+1;
   dy=ry*ry*(2+2*x)+rx*rx*(3-2*y);
   d2=d2+dy;
  }
  delay(10);
 }while(y>0);
 
 getch();
return 0;
}

OUTPUT-

  • To execute the program save the above given file with .cpp extension i.e.(filename.cpp)
  • open your terminal and type
    g++ filename.cpp -lgraph 
    and click enter.
  • type ./a.out and see the output.
  • Colors are different because we are using different color codes in our program.
mid point ellipse drawing

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