Re: Intersecting geometric shapes

[m.hadfield(at)niwa.cri.nz (Mark Hadfield)]

From: "Rick Towler" <rtowler@u.washington.edu>
Newsgroups: comp.lang.idl-pvwave
Sent: Friday, June 29, 2001 6:00 PM
Subject: Re: Intersecting geometric shapes


> I'm sure there is a more elegant way of doing this but since
> your post has sat here all day without a response...

I was reticent for exactly the same reason, but since you've poked your head
above the parapet...

> Fire up your favorite drawing program and create a new
> image, say 200x200. Draw your circle white on black.  Now
> create another 200x200 image this time drawing your rectangle.
> Save as an indexed color image (.gif (pre IDL 5.4)
> or .png should work)
>
> load these images into IDL, add the arrays, and do a where on
> the new array for values that are double your white color palette
> index value.  The number of elements in the result of your where will
> give you the pixel area of your intersection.  Then all you have to do
> is convert pixel area back to your measure of area.

I'd like to point out that you can do essentially the same thing without
actually drawing anything using the POLYFILLV function.

Of course drawing the shapes out with POLYFILL to check what you're doing
isn't a bad idea either...

---
Mark Hadfield
m.hadfield@niwa.cri.nz  http://katipo.niwa.cri.nz/~hadfield
National Institute for Water and Atmospheric Research




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