opengl - ellipse midpoint algorithm counterclockwise version -

i'm trying convert standart clockwise ellipse midpoint algorithm taken book "computer graphics opengl" work counter-clockwise starting region 2. got work , draw ellipse not identical 1 original algorithm draws assume have small bug in code can't seem find, can please help?

this original algorithm:

void ellipsemidpoint(int xcenter, int ycenter, int rx, int ry) {     int rx2 = rx * rx;     int ry2 = ry * ry;     int tworx2 = 2 * rx2;     int twory2 = 2 * ry2;     int p;     int x = 0;     int y = ry;     int px = 0;     int py = tworx2 * y;     void ellipseplotpoints(int, int, int, int);      /* plot initial point in each quadrant. */     ellipseplotpoints(xcenter, ycenter, x, y);      /* region 1 */     p = round(ry2 - (rx2 * ry) + (0.25 * rx2));     while (px < py) {         x++;         px += twory2;         if (p < 0)             p += ry2 + px;         else {             y--;             py -= tworx2;             p += ry2 + px - py;         }         ellipseplotpoints(xcenter, ycenter, x, y);     }      /* region 2 */     p = round(ry2 * (x + 0.5) * (x + 0.5) + rx2 * (y - 1) * (y - 1) - rx2 * ry2);     while (y > 0) {         y--;         py -= tworx2;         if (p > 0)             p += rx2 - py;         else {             x++;             px += twory2;             p += rx2 - py + px;         }         ellipseplotpoints(xcenter, ycenter, x, y);     } }  void ellipseplotpoints(int xcenter, int ycenter, int x, int y) {     setpixel(xcenter + x, ycenter + y);     setpixel(xcenter - x, ycenter + y);     setpixel(xcenter + x, ycenter - y);     setpixel(xcenter - x, ycenter - y); } 

and version:

void ellipsemidpointcounterclockwise(int xcenter, int ycenter, int rx, int ry) {     int rx2 = rx * rx;     int ry2 = ry * ry;     int tworx2 = 2 * rx2;     int twory2 = 2 * ry2;     int p;     int x = rx;     int y = 0;     int px = twory2 * x;     int py = 0;     void ellipseplotpoints(int, int, int, int);      /* plot initial point in each quadrant. */     ellipseplotpoints(xcenter, ycenter, x, y);      /* region 2 */     p = round(ry2 * (x - 0.5) * (x - 0.5) + rx2 * (y + 1) * (y + 1) - rx2 * ry2);     while (py < px) {         y++;         py += tworx2;         if (p > 0)             p += rx2 - py;         else {             x--;             px -= twory2;             p += rx2 - py + px;         }         ellipseplotpoints(xcenter, ycenter, x, y);     }      /* region 1 */      p = round(ry2 * (x - 1.0) * (x - 1.0) + rx2 * (y + 0.5) * (y + 0.5) - rx2 * ry2);     while (x > 0) {         x--;         px -= twory2;         if (p < 0)             p += ry2 + px;         else {             y++;             py += tworx2;             p += ry2 + px - py;         }         ellipseplotpoints(xcenter, ycenter, x, y);     } } 

i appreciate helping finding did wrong.

the first problem original code not symmetric respect x , y (and ellipsemidpointcounterclockwise replaces x , y).

ellipsemidpoint(0,0,3,2) generates first quadrant

0 2 1 2 2 1 3 0 

whereas ellipsemidpoint(0,0,2,3) generates first quadrant

0 3 1 3 2 2 2 1 2 0 

when exchanging coordinates , reversing order get:

0 2 1 2 2 2 3 1 3 0 

graphically means:

002 | 12 +--0 

where + denotes center, 0 denotes ellipse points in both results, 1 point in first result, 2 points in second result (mirrored).

if want ellipsemidpointcounterclockwise generate same points - instead of replacing x , y - go negative x-direction (i.e. x-- instead of x++ in original code). otherwise have live difference in symmetry.

the second problem did not replace x , y other strange replacements. if replace x , y within regions correct result:

void ellipsemidpointcounterclockwise(int xcenter, int ycenter, int rx, int ry) {   int rx2 = rx * rx;   int ry2 = ry * ry;   int tworx2 = 2 * rx2;   int twory2 = 2 * ry2;   int p;   int x = rx;   int y = 0;   int px = twory2 * x;   int py = 0;   void ellipseplotpoints(int, int, int, int);    /* plot initial point in each quadrant. */   ellipseplotpoints(xcenter, ycenter, x, y);    /* region 2 */   p = round(rx2 - (ry2 * rx) + (0.25 * ry2));   while (py < px) {     y++;     py += tworx2;     if (p < 0)       p += rx2 + py;     else {       x--;       px -= twory2;       p += rx2 + py - px;     }     ellipseplotpoints(xcenter, ycenter, x, y);   }    /* region 1 */   p = round(rx2 * (y + 0.5) * (y + 0.5) + ry2 * (x - 1) * (x - 1) - ry2 * rx2);   while (x > 0) {     x--;     px -= twory2;     if (p > 0)       p += ry2 - px;     else {       y++;       py += tworx2;       p += ry2 - px + py;     }     ellipseplotpoints(xcenter, ycenter, x, y);   } }