Radius, Center of Circle Given 3 PointsDate: 8/28/96 at 17:41:17 From: Stuart Simmons Subject: Radius, Center of Circle Given 3 Points I'm writing a small piece of software for personal use and I have come across a problem. I need an easy formula to calculate the centre point and radius of a circle given three points around the circumference of the circle. (x1,y1),(x2,y2),(x3,y3) Regards Stuart Date: 8/30/96 at 13:27:38 From: Doctor Tom Subject: Re: Radius, Center of Circle Given 3 Points Here's how I'd do it: The perpendicular bisector of the segment between two points is the set of all points equidistant from both. So if you take the perpendicular bisector of (x1,y1) and (x2,y2) and the perpendicular bisector of the segment from (x2,y2) to (x3,y3) and find the intersection of those lines, that point will be the center. To find the radius, find the distance from that point to any of the three points. To find the equation of the perpendicular bisector of (x1,y1) to (x2,y2), you know that it passes through the midpoint of the segment: ((x1+x2)/2,(y1+y2)/2), and if the slope of the line connecting (x1,y1) to (x2,y2) is m, the slope of the perpendicular bisector is -1/m. Work out the equations for the two lines, find their intersection, and bingo! You've got the coordinates of the center. An error should occur if the three points lie on a line, and you'll need special code to check for the case where one of the slopes is zero. Otherwise it's pretty straightforward. By a miracle, I wrote some code (in C++) to do something like this once. I'll include it, and hope you can make some use of it. Don't worry about the "w" coordinates; assume they're all 1.0. I wanted my code to work in the perspective case and I had to worry about "points at infinity": void circle_vvv(circle *c) { c->center.w = 1.0; vertex *v1 = (vertex *)c->c.p1; vertex *v2 = (vertex *)c->c.p2; vertex *v3 = (vertex *)c->c.p3; float bx = v1->xw; float by = v1->yw; float cx = v2->xw; float cy = v2->yw; float dx = v3->xw; float dy = v3->yw; float temp = cx*cx+cy*cy; float bc = (bx*bx + by*by - temp)/2.0; float cd = (temp - dx*dx - dy*dy)/2.0; float det = (bx-cx)*(cy-dy)-(cx-dx)*(by-cy); if (fabs(det) < 1.0e-6) { c->center.xw = c->center.yw = 1.0; c->center.w = 0.0; c->v1 = *v1; c->v2 = *v2; c->v3 = *v3; return; } det = 1/det; c->center.xw = (bc*(cy-dy)-cd*(by-cy))*det; c->center.yw = ((bx-cx)*cd-(cx-dx)*bc)*det; cx = c->center.xw; cy = c->center.yw; c->radius = sqrt((cx-bx)*(cx-bx)+(cy-by)*(cy-by)); } -Doctor Tom, The Math Forum Check out our web site! http://mathforum.org/dr.math/ |
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