Question

I have this code to draw an ellipse in the screen but I don’t understand what does it means the long define statement, and I only want to know how to write the same code without all that confuse define statement.

#define incx() x++, dxt += d2xt, t += dxt 
#define incy() y--, dyt += d2yt, t += dyt

void ellipse(int xc, int yc, int rx, int ry, int color)
{
    int x = 0, y = ry;
    long rx2 = (long)rx*rx, ry2 = (long)ry*ry;
    long crit1 = -(rx2/4 + rx%2 + ry2);
    long crit2 = -(ry2/4 + ry%2 + rx2);
    long crit3 = -(ry2/4 + ry%2);
    long t = -rx2*y; // e(x+1/2,y-1/2) - (a^2+b^2)/4
    long dxt = 2*ry2*x, dyt = -2*rx2*y;
    long d2xt = 2*ry2, d2yt = 2*rx2;

    while (y>=0 && x<=rx)
    {
        pixel(xc+x, yc+y, color);
        if (x!=0 || y!=0)
            pixel(xc-x, yc-y, color);
        if (x!=0 && y!=0)
        {
            pixel(xc+x, yc-y, color);
            pixel(xc-x, yc+y, color);
        }
        if (t + ry2*x <= crit1 ||   //e(x+1,y-1/2) <= 0
            t + rx2*y <= crit3)     //e(x+1/2,y) <= 0
            incx();
        else if (t - rx2*y > crit2) //e(x+1/2,y-1) > 0
            incy();
        else
        {
            incx();
            incy();
        }
    }
}

I have tried to remove the define piece by piece and it doesn’t work, any suggestion?

Answer 1

If you just want to remove it, run the code through cpp:

 cpp cpp.c > cppout.c

gives me

# 1 "cpp.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "cpp.c"



void ellipse(int xc, int yc, int rx, int ry, int color)
{
    int x = 0, y = ry;
    long rx2 = (long)rx*rx, ry2 = (long)ry*ry;
    long crit1 = -(rx2/4 + rx%2 + ry2);
    long crit2 = -(ry2/4 + ry%2 + rx2);
    long crit3 = -(ry2/4 + ry%2);
    long t = -rx2*y;
    long dxt = 2*ry2*x, dyt = -2*rx2*y;
    long d2xt = 2*ry2, d2yt = 2*rx2;

    while (y>=0 && x<=rx)
    {
        pixel(xc+x, yc+y, color);
        if (x!=0 || y!=0)
            pixel(xc-x, yc-y, color);
        if (x!=0 && y!=0)
        {
            pixel(xc+x, yc-y, color);
            pixel(xc-x, yc+y, color);
        }
        if (t + ry2*x <= crit1 ||
            t + rx2*y <= crit3)
            x++, dxt += d2xt, t += dxt;
        else if (t - rx2*y > crit2)
            y--, dyt += d2yt, t += dyt;
        else
        {
            x++, dxt += d2xt, t += dxt;
            y--, dyt += d2yt, t += dyt;
        }
    }
}

The problem you may have had is the use of the comma operator in the macro. I recommend replacing the commas with ;, and putting the if parts inside {}, with line breaks. (Here I hand-inserted the { and }, then used M-x replace-string RET , RET ; C-Q C-J in Emacs, followed by C-M-\ to indent the region.)

    if (t + ry2*x <= crit1 ||
        t + rx2*y <= crit3) {

        x++;
        dxt += d2xt;
        t += dxt;

    } else if (t - rx2*y > crit2) {
        y--;
        dyt += d2yt;
        t += dyt;

    } else {
        x++;
        dxt += d2xt;
        t += dxt;

        y--;
        dyt += d2yt;
        t += dyt;
    }