Path: news.mathworks.com!not-for-mail From: <HIDDEN> Newsgroups: comp.soft-sys.matlab Subject: Re: angle between two vectors Date: Fri, 6 Aug 2010 19:18:06 +0000 (UTC) Organization: The MathWorks, Inc. Lines: 17 Message-ID: <i3hn5d$eh7$1@fred.mathworks.com> References: <i3hlf0$olk$1@fred.mathworks.com> Reply-To: <HIDDEN> NNTP-Posting-Host: webapp-05-blr.mathworks.com Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Trace: fred.mathworks.com 1281122286 14887 172.30.248.35 (6 Aug 2010 19:18:06 GMT) X-Complaints-To: news@mathworks.com NNTP-Posting-Date: Fri, 6 Aug 2010 19:18:06 +0000 (UTC) X-Newsreader: MATLAB Central Newsreader 1187260 Xref: news.mathworks.com comp.soft-sys.matlab:659861 "brian adams" <adamsbriand@yahoo.com> wrote in message <i3hlf0$olk$1@fred.mathworks.com>... > In 2-space, I have a three points: > c, the ctr > p, an arbitrary point, and > v, a point indicating a direction (as a vector from the ctr to v). > > I need to determine whether the point p is in the forward direction of the vector v from the center point, c. > > I have gotten this to work by means of the law of cosines to determine the angle between the the vectors, p-c and v-c. I needed to calculate the distances between the three points, apply acos, and determine if the abs of the angle is <= 90. > > However, this method is computational intensive for the number of points for which I need to perform this operation. > > Is there a less computationally intensive means by which to determine this? - - - - - - - If dot(p-c,v-c) > 0, then p is in the "forward" direction if I interpret you correctly. No need to find the angle or the lengths of the vectors. Roger Stafford