| Re: Finding the intersection of 2 short lines [message #79206] |
Mon, 13 February 2012 08:50 |
ben.bighair
Messages: 221
Registered: April 2007
|
Senior Member |
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On Feb 13, 10:15 am, Jimmy <jimmyb...@gmail.com> wrote:
> 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
Hi,
Yikes! I don't remember it very well, but I think that is what this
line does:
onSegment = [ (ua GE ZERO) AND (ua LE ONE), (ub GE ZERO) AND (ub LE
ONE)]
Cheers,
Ben
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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 |
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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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| Re: Finding the intersection of 2 short lines [message #79208 is a reply to message #79207] |
Mon, 13 February 2012 07:18 |
jimmybobs
Messages: 9
Registered: January 2009
|
Junior Member |
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On Feb 13, 1:42 pm, David Fanning <n...@idlcoyote.com> wrote:
> ben.bighair writes:
>> 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
>
> You might need to put a Forward_Function command
> in the test programs (or just compile the program
> code twice) to get the test programs to run.
>
> Cheers,
>
> David
>
> --
> David Fanning, Ph.D.
> Fanning Software Consulting, Inc.
> Coyote's Guide to IDL Programming:http://www.idlcoyote.com/
> Sepore ma de ni thui. ("Perhaps thou speakest truth.")
Thanks David - when compiling it in the IDE I did need to do it twice
as you said. Funnily enough when it was tucked away in the IDL lib
folder and called from my code in the working directory it seemd to
work unaltered.
Jimmy
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| Re: Finding the intersection of 2 short lines [message #79213 is a reply to message #79212] |
Mon, 13 February 2012 04:48 |
ben.bighair
Messages: 221
Registered: April 2007
|
Senior Member |
|
|
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
|
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| Re: Finding the intersection of 2 short lines [message #84656 is a reply to message #79213] |
Mon, 13 February 2012 07:51 |
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,
My original reply seems to have been eaten. Your PB_LINES_INTERSECTION
routine worked like a charm, thank you.
I had read that page you linked to, but had missed the vita 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. "
Thanks,
Jimmy
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