darshana patel

Midpoint circle algorithm

In computer graphics, the midpoint circle algorithm is an algorithm used to determine the points needed for drawing a circle. The algorithm is a variant of Bresenham's line algorithm, and is thus sometimes known as Bresenham's circle algorithm, although not actually invented by Bresenham.

The algorithm starts accordingly with the circle equation x2 + y2 = r2. So, the center of the circle is located at (0,0). We consider first only the first octant and draw a curve which starts at point (r,0) and proceeds upwards and to the left, reaching the angle of 45°.
The "fast" direction here is the y direction. The algorithm always does a step in the positive y direction (upwards), and every now and then also has to do a step in the "slow" direction, the negative x direction.
The frequent computations of squares in the circle equation, trigonometric expressions or square roots can again be avoided by dissolving everything into single steps and recursive computation of the quadratic terms from the preceding ones.
From the circle equation we obtain the transformed equation x2 + y2 − r2 = 0, where r2 is computed only a single time during initialization:

and accordingly for the x-coordinate. Additionally we need to add the midpoint coordinates when setting a pixel. These frequent integer additions do not limit the performance much, as we can spare those square (root) computations in the inner loop in turn. Again the zero in the transformed circle equation is replaced by the error term.
The initialization of the error term is derived from an offset of ½ pixel at the start. Until the intersection with the perpendicular line, this leads to an accumulated value of r in the error term, so that this value is used for initialization.
A possible implementation of the Bresenham Algorithm for a full circle in C. Here another variable for recursive computation of the quadratic terms is used, which corresponds with the term 2n + 1 above. It just has to be increased by 2 from one step to the next:
void rasterCircle(int x0, int y0, int radius)
{
int f = 1 - radius;
int ddF_x = 1;
int ddF_y = -2 * radius;
int x = 0;
int y = radius;

setPixel(x0, y0 + radius);
setPixel(x0, y0 - radius);
setPixel(x0 + radius, y0);
setPixel(x0 - radius, y0);

while(x < y)
{
// ddF_x == 2 * x + 1;
// ddF_y == -2 * y;
// f == x*x + y*y - radius*radius + 2*x - y + 1;
if(f >= 0)
{
y--;
ddF_y += 2;
f += ddF_y;
}
x++;
ddF_x += 2;
f += ddF_x;
setPixel(x0 + x, y0 + y);
setPixel(x0 - x, y0 + y);
setPixel(x0 + x, y0 - y);
setPixel(x0 - x, y0 - y);
setPixel(x0 + y, y0 + x);
setPixel(x0 - y, y0 + x);
setPixel(x0 + y, y0 - x);
setPixel(x0 - y, y0 - x);
}
}