1*15484Sralph #ifndef lint 2*15484Sralph static char sccsid[] = "@(#)arc.c 4.1 (Berkeley) 11/10/83"; 3*15484Sralph #endif 4*15484Sralph 5*15484Sralph #include "gigi.h" 6*15484Sralph 7*15484Sralph /* 8*15484Sralph * gigi requires knowing the anlge of arc. To do this, the triangle formula 9*15484Sralph * c^2 = a^2 + b^2 - 2*a*b*cos(angle) 10*15484Sralph * is used where "a" and "b" are the radius of the circle and "c" is the 11*15484Sralph * distance between the beginning point and the end point. 12*15484Sralph * 13*15484Sralph * This gives us "angle" or angle - 180. To find out which, draw a line from 14*15484Sralph * beg to center. This splits the plane in half. All points on one side of the 15*15484Sralph * plane will have the same sign when plugged into the equation for the line. 16*15484Sralph * Pick a point on the "right side" of the line (see program below). If "end" 17*15484Sralph * has the same sign as this point does, then they are both on the same side 18*15484Sralph * of the line and so angle is < 180. Otherwise, angle > 180. 19*15484Sralph */ 20*15484Sralph 21*15484Sralph #define side(x,y) (a*(x)+b*(y)+c > 0.0 ? 1 : -1) 22*15484Sralph 23*15484Sralph arc(xcent,ycent,xbeg,ybeg,xend,yend) 24*15484Sralph int xcent,ycent,xbeg,ybeg,xend,yend; 25*15484Sralph { 26*15484Sralph double radius2, c2; 27*15484Sralph double a,b,c; 28*15484Sralph int angle; 29*15484Sralph 30*15484Sralph /* Probably should check that this is really a circular arc. */ 31*15484Sralph radius2 = (xcent-xbeg)*(xcent-xbeg) + (ycent-ybeg)*(ycent-ybeg); 32*15484Sralph c2 = (xend-xbeg)*(xend-xbeg) + (yend-ybeg)*(yend-ybeg); 33*15484Sralph angle = (int) ( 180.0/PI * acos(1.0 - c2/(2.0*radius2)) + 0.5 ); 34*15484Sralph 35*15484Sralph a = (double) (ycent - ybeg); 36*15484Sralph b = (double) (xcent - xbeg); 37*15484Sralph c = (double) (ycent*xbeg - xcent*ybeg); 38*15484Sralph if (side(xbeg + (ycent-ybeg), ybeg - (xcent-xbeg)) != side(xend,yend)) 39*15484Sralph angle += 180; 40*15484Sralph 41*15484Sralph move(xcent, ycent); 42*15484Sralph printf("C(A%d c)[%d,%d]", angle, xbeg, ybeg); 43*15484Sralph } 44