1 #ifndef lint 2 static char sccsid[] = "@(#)arc.c 4.1 (Berkeley) 11/10/83"; 3 #endif 4 5 #include "hp7221.h" 6 7 /* 8 * 7221 requires knowing the anlge of arc. To do this, the triangle formula 9 * c^2 = a^2 + b^2 - 2*a*b*cos(angle) 10 * is used where "a" and "b" are the radius of the circle and "c" is the 11 * distance between the beginning point and the end point. 12 * 13 * This gives us "angle" or angle - 180. To find out which, draw a line from 14 * beg to center. This splits the plane in half. All points on one side of the 15 * plane will have the same sign when plugged into the equation for the line. 16 * Pick a point on the "right side" of the line (see program below). If "end" 17 * has the same sign as this point does, then they are both on the same side 18 * of the line and so angle is < 180. Otherwise, angle > 180. 19 */ 20 21 #define side(x,y) (a*(x)+b*(y)+c > 0.0 ? 1 : -1) 22 23 arc(xcent,ycent,xbeg,ybeg,xend,yend) 24 int xcent,ycent,xbeg,ybeg,xend,yend; 25 { 26 double radius2, c2; 27 double a,b,c; 28 int angle; 29 30 /* Probably should check that this is really a circular arc. */ 31 radius2 = (xcent-xbeg)*(xcent-xbeg) + (ycent-ybeg)*(ycent-ybeg); 32 c2 = (xend-xbeg)*(xend-xbeg) + (yend-ybeg)*(yend-ybeg); 33 angle = (int) ( 180.0/PI * acos(1.0 - c2/(2.0*radius2)) + 0.5 ); 34 35 a = (double) (ycent - ybeg); 36 b = (double) (xcent - xbeg); 37 c = (double) (ycent*xbeg - xcent*ybeg); 38 if (side(xbeg + (ycent-ybeg), ybeg - (xcent-xbeg)) != side(xend,yend)) 39 angle += 180; 40 41 move(xcent, ycent); 42 /* Not quite implemented... 43 printf("C(A%d c)[%d,%d]", angle, xbeg, ybeg); 44 */ 45 } 46