| Re: Routine to return indices for circle in R2 [message #9927] |
Tue, 09 September 1997 00:00  |
John Votaw
Messages: 6
Registered: June 1997
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Junior Member |
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David Foster wrote:
>
> Has someone written, seen or heard of a routine that will return
> the indices of a circle within an image of given dimensions,
> given the radius and center coordinate of the circle? I vaguely
> remember seeing this in a newsgroup post some time ago.
>
> For an image of size NX x NY, with the circle at coordinate (Cx,Cy)
> and radius R, I came up with:
>
> indices = lindgen(long(NX)*NY)
> yc = indices / NY
> xc = indices - (yc * NY)
> circle = where( sqrt( (xc - Cx)^2 + (yc-Cy)^2 ) le R )
>
> Can anyone suggest a method that is (a) faster, (b) more clever,
> (c) more elegant, or (d) uses less memory. I'd be willing to settle
> for just one of the above!
>
> Thanks!
>
Here is one idea: define the edge of the circle and then use polyfillv
to return all indicies inside the circle. It definately uses less
memory than the above and depending on the image size, could run much
faster.
Nang=20 ;number of verticies
ang=findgen(nang)*!pi*2./nang
x=r*cos(ang)+cx+.5 ;.5 needed for rounding
y=r*sin(ang)+cy+.5
circle=polyfillv(x,y,Sx,Sy)
where Sx and Sy are defined in the polyfillv manual entry.
Nang determines how smooth the circle is. Larger circles require larger
Nang.
Good Luck,
---
John R. Votaw, Ph.D.
Associate Professor of Radiology
Emory Center for Positron Emission Tomography
votaw@commander.eushc.org
voice: (404)712-7954 FAX: (404)712-7962
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