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Re: Finding the intersection of 2 short lines [message #79207 is a reply to message #79206]

Mon, 13 February 2012 07:15

jimmybobs
Messages: 9
Registered: January 2009

Junior Member

On Feb 13, 12:48 pm, "ben.bighair" <ben.bigh...@gmail.com> wrote:
> On Feb 13, 5:39 am, Jimmy <jimmyb...@gmail.com> wrote:
>
>> Hi everyone.
>
>> I'm trying to find a way of detecting the crossing point of two short
>> lines (as part of a larger problem with detecting overlapping
>> polygons). I've used intersect.pro which gives me the general
>> intersect, but then I have to use the mother of all ugly if statements
>> (which I'm having trouble getting working) to detect whether the
>> intersect is actually on my short lines or extended away somewhere
>> else.
>
>> Is there a more elegant way of doing this? The code needs to find
>> whether the two lines cross, and return the intersection point.
>
> Hi,
>
> I have often used Paul Bourke's geometry webpages as a good starting
> point, like this one for line intersections (2d)...
>
> http://paulbourke.net/geometry/lineline2d/
>
> I used that stuff to develop these (you'll need David Fanning's cg*
> routines)
>
> http://dl.dropbox.com/u/8433654/pbourke_lines.zip
>
> Cheers,
> Ben

Hi Ben,

Thanks very much for this, works like a charm. I had looked at the
website you linked, but missed the crucial line at the bottom: "The
equations apply to lines, if the intersection of line segments is
required then it is only necessary to test if ua and ub lie between 0
and 1. Whichever one lies within that range then the corresponding
line segment contains the intersection point. If both lie within the
range of 0 to 1 then the intersection point is within both line
segments". Reading your code, this appears to be how you find it?

Thanks again,
Jimmy

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[Message index]

		Re: Finding the intersection of 2 short lines By: ben.bighair on Mon, 13 February 2012 08:50
		Re: Finding the intersection of 2 short lines By: jimmybobs on Mon, 13 February 2012 07:15
		Re: Finding the intersection of 2 short lines By: jimmybobs on Mon, 13 February 2012 07:18
		Re: Finding the intersection of 2 short lines By: David Fanning on Mon, 13 February 2012 05:42
		Re: Finding the intersection of 2 short lines By: ben.bighair on Mon, 13 February 2012 04:48
		Re: Finding the intersection of 2 short lines By: jimmybobs on Mon, 13 February 2012 07:51

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