Category : C Source Code
Archive   : GRAFGEM3.ZIP
Filename : PARELARC.C

/*****************************************************************
Plot a series of points along a PI/2-radian arc of an ellipse.
The arc is specified in terms of a control polygon (a triangle)
with vertices P, Q and K. The arc begins at P, ends at Q, and is
completely contained within the control polygon. The draw_point
function plots a single pixel at display coordinates (x,y).

Entry:
xP, yP, xQ, yQ, xK, yK -- coordinates of P, Q and K. These
are 32-bit fixed-point values with 16 bits of fraction.
m -- nonnegative integer that controls spacing between points.
The angular increment between points is 1/2^m radians.
Exit:
The number of points plotted is 1 + floor((PI/2)*2^m).
*****************************************************************/

#define PIV2 102944 /* fixed point PI/2 */
#define TWOPI 411775 /* fixed point 2*PI */
#define HALF 32768 /* fixed point 1/2 */
typedef long FIX; /* 32-bit fixed point, 16-bit fraction */

qtr_elips(xP, yP, xQ, yQ, xK, yK, m)
FIX xP, yP, xQ, yQ, xK, yK;
int m;
{
int i, x, y;
FIX vx, ux, vy, uy, w, xJ, yJ;

vx = xK - xQ; /* displacements from center */
ux = xK - xP;
vy = yK - yQ;
uy = yK - yP;
xJ = xP - vx + HALF; /* center of ellipse J */
yJ = yP - vy + HALF;
ux -= (w = ux >> (2*m + 3)); /* cancel 2nd-order error */
ux -= (w >>= (2*m + 4)); /* cancel 4th-order error */
ux -= w >> (2*m + 3); /* cancel 6th-order error */
ux += vx >> (m + 1); /* cancel 1st-order error */
uy -= (w = uy >> (2*m + 3)); /* cancel 2nd-order error */
uy -= (w >>= (2*m + 4)); /* cancel 4th-order error */
uy -= w >> (2*m + 3); /* cancel 6th-order error */
uy += vy >> (m + 1); /* cancel 1st-order error */
for (i = (PIV2 << m) >> 16; i >= 0; --i) {
x = (xJ + vx) >> 16;
y = (yJ + vy) >> 16;
draw_point(x, y);
ux -= vx >> m;
vx += ux >> m;
uy -= vy >> m;
vy += uy >> m;
}
}