Implementation of ellipse drawing algorithm

1. #include<iostream.h>
2. #include<conio.h>
3. #include<graphics.h>
4. #include<dos.h>
5. 
   
6. void main()
7. {
8. clrscr();
9. float rx=0,ry=0,x=0,y=0,dx=0,dy=0,d1=0,d2=0; 
10. float rxSqr=0,rySqr=0,rx2Sqr=0,ry2Sqr=0;
11. int x1=0,y1=0; int gd,gm;
12. 
    
13. detectgraph(&gd,&gm); initgraph(&gd,&gm,"c:\\tc\\bgi");
14. 
    
15. //textbackground(15);
16. //textcolor(8);
17. //setbkcolor(15);
18. //setcolor(8);
19. 
    
20. 
    
21. cout<<"Enter pixels for centre of an ellipse (x,y): "; cin>>x1>>y1;
22. 
    
23. cout<<"Enter rx radius: "; cin>>rx;
24. cout<<"Enter ry radius: "; cin>>ry;
25.  
26. line(0,getmaxy()/2,getmaxx(),getmaxy()/2); line(getmaxx()/2,0,getmaxx()/2,getmaxy());
27. 
    
28. x=0;
29. y=ry;
30. 
    
31. rxSqr=rx*rx; rx2Sqr=2*rxSqr; rySqr=ry*ry; ry2Sqr=2*rySqr; dx=ry2Sqr*x; dy=rx2Sqr*y;
32. d1=rySqr-rxSqr*ry+(.25*rxSqr);
33. do
34. {
35. putpixel(getmaxx()/2+(x1+x),getmaxy()/2-(y1+y),8); 
36. putpixel(getmaxx()/2+(x1-x),getmaxy()/2-(y1+y),8); 
37. putpixel(getmaxx()/2+(x1+x),getmaxy()/2-(y1-y),8); 
38. putpixel(getmaxx()/2+(x1-x),getmaxy()/2-(y1-y),8); 
39. if(d1<0)
40. {
41. x+=1;
42. y=y; dx+=ry2Sqr; d1+=dx+rySqr;
43. }
44. else
45. {
46. x+=1;
47. y-=1;
48. dx+=ry2Sqr; dy-=rx2Sqr;
49. d1+=dx-dy+rySqr;
50. }
51. delay(30);
52. }while(dx<dy);
53. d2=rySqr*((x+(.5))*(x+(.5)))+rxSqr*((y-1)*(y-1))-rxSqr*rySqr; do
54. {
55. putpixel(getmaxx()/2+(x1+x),getmaxy()/2-(y1+y),8); 
56. putpixel(getmaxx()/2+(x1-x),getmaxy()/2-(y1+y),8);
57.  putpixel(getmaxx()/2+(x1+x),getmaxy()/2-(y1-y),8); 
58. putpixel(getmaxx()/2+(x1-x),getmaxy()/2-(y1-y),8); 
59. if(d2>0)
60. {
61. x=x; y-=1;
62. dy-=rx2Sqr; d2-=dy+rxSqr;
63. }
64. else
65. {
66. x+=1; y-=1;
67. dy-=rx2Sqr; dx+=ry2Sqr; d2+=dx-dy+rxSqr;
68. }
69. delay(10);
70. }while(y>0);
71. getch();
72. }

Output: