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Subject: Re: how to find pixels inside ellipse ?
Date: Sun, 25 May 2008 17:12:02 +0000 (UTC)
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"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

```