lib/mgCircle.c

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/* mgCircle.c - Simple stepping draw a circle algorithm.  
        Don't even correct aspect ratio. */

#include "common.h"
#include "memgfx.h"

void mgCircle(struct memGfx *mg, int xCen, int yCen, int rad, 
        Color color, boolean filled)
/* Draw a circle using a stepping algorithm.  Doesn't correct
 * for non-square pixels. */
{
int err;
int derr, yerr, xerr;
int aderr, ayerr, axerr;
register int x,y;
int lasty;

if (rad <= 0)
    {
    mgPutDot(mg, xCen, yCen, color);
    return;
    }
err = 0;
x = rad;
lasty = y = 0;
for (;;)
    {
    if (filled)
        {
        if (y == 0)
            mgLineH(mg, yCen, xCen-x, xCen+x, color);
        else
            {
            if (lasty != y)
                {
                mgLineH(mg, yCen-y, xCen-x, xCen+x, color);
                mgLineH(mg, yCen+y, xCen-x, xCen+x, color);
                lasty = y;
                }
            }
        }
    else
        {
        /* draw 4 quadrandts of a circle */
        mgPutDot(mg, xCen+x, yCen+y, color);
        mgPutDot(mg, xCen+x, yCen-y, color);
        mgPutDot(mg, xCen-x, yCen+y, color);
        mgPutDot(mg, xCen-x, yCen-y, color);
        }
    axerr = xerr = err -x-x+1;
    ayerr = yerr = err +y+y+1;
    aderr = derr = yerr+xerr-err;
    if (aderr < 0)
        aderr = -aderr;
    if (ayerr < 0)
        ayerr = -ayerr;
    if (axerr < 0)
        axerr = -axerr;
    if (aderr <= ayerr && aderr <= axerr)
        {
        err = derr;
        x -= 1;
        y += 1;
        }
    else if (ayerr <= axerr)
        {
        err = yerr;
        y += 1;
        }
    else
        {
        err = xerr;
        x -= 1;
        }
    if (x < 0)
        break;
    }
}

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