Name

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PIXWT

Purpose

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Circle-rectangle overlap area computation.

Description

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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

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CCD data processing

Calling Sequence

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

Inputs

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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

Optional Input Parameters

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None.

Keyword Parameters

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None.

Outputs

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Function value: Computed overlap area.

Example

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      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

Common Blocks

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    None.

Procedure

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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

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      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

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