Path: news.mathworks.com!not-for-mail From: <HIDDEN> Newsgroups: comp.soft-sys.matlab Subject: Re: how to find pixels inside ellipse ? Date: Tue, 16 Aug 2011 20:58:30 +0000 (UTC) Organization: The MathWorks, Inc. Lines: 34 Message-ID: <j2ellm$gt5$1@newscl01ah.mathworks.com> References: <g1bsct$ggh$1@fred.mathworks.com> <g1c6l2$b04$1@fred.mathworks.com> Reply-To: <HIDDEN> NNTP-Posting-Host: www-06-blr.mathworks.com Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Trace: newscl01ah.mathworks.com 1313528310 17317 172.30.248.38 (16 Aug 2011 20:58:30 GMT) X-Complaints-To: news@mathworks.com NNTP-Posting-Date: Tue, 16 Aug 2011 20:58:30 +0000 (UTC) X-Newsreader: MATLAB Central Newsreader 2486811 Xref: news.mathworks.com comp.soft-sys.matlab:740212 "Roger Stafford" wrote in message <g1c6l2$b04$1@fred.mathworks.com>... > "elflord woods" <elflord.woods@gmail.com> wrote in message <g1bsct$ggh > $1@fred.mathworks.com>... > > hi all > > > > I have an ellipse region in an image and i know the size of > > the ellipse axes and its orientation > > > > how can i extract all the pixels inside this ellipse region . > > > > thanks > ------------------ > I assume you also know the location of the ellipse's center. Call that (x0,y0). > Let t be the counterclockwise angle the major axis makes with respect to the > x-axis. Let a and b be the semi-major and semi-minor axes, respectively. If > P = (x,y) is an arbitrary point then do this: > > X = (x-x0)*cos(t)+(y-y0)*sin(t); % Translate and rotate coords. > Y = -(x-x0)*sin(t)+(y-y0)*cos(t); % to align with ellipse > > If > > X^2/a^2+Y^2/b^2 > > is less than 1, the point P lies inside the ellipse. If it equals 1, it is right on > the ellipse. If it is greater than 1, P is outside. > > Roger Stafford > I was curious how to do this with lat lon points. I have data describing a rotated ellipse (the center of the ellipse in lat lon coordinates, the lengths of the major and minor axes in kilometers, and the angle that the ellipse is oriented). I do not know the location of the foci, but assume there is a way to figure them out somehow. How can I determine if a specific lat lon point is within this ellipse? Thanks, Cody