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PIXWT

PIXWT

Purpose

Circle-rectangle overlap area computation.

Description

Compute the fraction of a unit pixel that is interior to a circle.
The circle has a radius r and is centered at (xc, yc). The center of
the unit pixel (length of sides = 1) is at (x, y).

Category

CCD data processing

Calling Sequence

area = Pixwt( xc, yc, r, x, y )

Inputs

xc, yc : Center of the circle, numeric scalars
r : Radius of the circle, numeric scalars
x, y : Center of the unit pixel, numeric scalar or vector

None.

None.

Outputs

Function value: Computed overlap area.

Example

What is the area of overlap of a circle with radius 3.44 units centered
on the point 3.23, 4.22 with the pixel centered at [5,7]
IDL> print,pixwt(3.23,4.22,3.44,5,7) ==> 0.6502

None.

Procedure

Divides the circle and rectangle into a series of sectors and
triangles. Determines which of nine possible cases for the
overlap applies and sums the areas of the corresponding sectors
and triangles. Called by aper.pro

Notes

If improved speed is needed then a C version of this routines, with
notes on how to linkimage it to IDL is available at
ftp://ftp.lowell.edu/pub/buie/idl/custom/

Modification History

Ported by Doug Loucks, Lowell Observatory, 1992 Sep, from the
routine pixwt.c, by Marc Buie.

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