| Re: Is there a quick way to find the intersection of two lines? [message #58607] |
Tue, 05 February 2008 06:00 |
ben.bighair
Messages: 221
Registered: April 2007
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Senior Member |
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On Feb 5, 3:24 am, eyuc...@gmail.com wrote:
> On Feb 4, 4:33 pm, "ben.bighair" <ben.bigh...@gmail.com> wrote:
>
>> On Feb 4, 7:08 pm, eyuc...@gmail.com wrote:
>
>>> Hi there,
>
>>> I have two sets of x-y data:
>>> x1=[1,2,3,4,5] y1=[3.2,7.4,8.2,9.3,7.9];
>>> x2=[1.2,1.4,2.3,2.8,3.3,3.9,4.1,4.5,5.2]
>>> y2=[3.1,5.2,6.2,7.3,7.5,8.6,9.6,8.7,7.4];
>
>>> By running:
>>> plot, x1, y1
>>> oplot, x2, y2
>>> we can clearly see that there are four intersections, but it is not
>>> clear what are the x,y coordinates of these points.
>
>>> Is there an easy way to do it? Thank you very much.
>
>> Hi,
>
> Thank you very much. I think I do understand how to find the
> intersection of two lines in principle, but actually doing it requires
> some details such as narrowing down the interval that the two lines
> intersect, counting intersection points, allocating memory to store
> the points and finally solving them.
>
> I just wonder if there are pre-made subroutines, since this is really
> a job we often do. If there are none, I'll try to make one...
>
>> You might want to check out Paul Bourke's great online tidbits about
>> geometry.
>
>> http://local.wasp.uwa.edu.au/~pbourke/geometry/
>
>> I have been chipping away at coding some of the algorithms he
>> describes into IDL, but have been easily sidetracked. You're welcome
>> to use what I have (mostly documented) as a starting point. My
>> implementations come with zero warranty...
>
>> www.tidewater.net/~pemaquid/pb.zip
>
Hi,
I think the routines you are looking for might be found in the pb.zip
collection I shared. You might need to work them in a sequence
testing each segment of polyline A against polyline B, but the
functions are all there.
Cheers,
Ben
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| Re: Is there a quick way to find the intersection of two lines? [message #58609 is a reply to message #58607] |
Tue, 05 February 2008 02:45  |
Wox
Messages: 184
Registered: August 2006
|
Senior Member |
|
|
On Mon, 4 Feb 2008 16:08:16 -0800 (PST), eyuchen@gmail.com wrote:
> Hi there,
>
> I have two sets of x-y data:
> x1=[1,2,3,4,5] y1=[3.2,7.4,8.2,9.3,7.9];
> x2=[1.2,1.4,2.3,2.8,3.3,3.9,4.1,4.5,5.2]
> y2=[3.1,5.2,6.2,7.3,7.5,8.6,9.6,8.7,7.4];
>
> By running:
> plot, x1, y1
> oplot, x2, y2
> we can clearly see that there are four intersections, but it is not
> clear what are the x,y coordinates of these points.
>
> Is there an easy way to do it? Thank you very much.
>
> Eugene
I got carried away and made something with Ben's suggestion. Maybe
there's a better way, but it's a start.
function segmentintersect,L1x,L1y,L2x,L2y,xy=xy
; code:
; 0: not intersecting
; 1: intersect in 1 point
; 2: parallel
; 3: coincident
denom=float(L2y[1]-L2y[0])*(L1x[1]-L1x[0])-(L2x[1]-L2x[0])*( L1y[1]-L1y[0])
numa=(L2x[1]-L2x[0])*(L1y[0]-L2y[0])-(L2y[1]-L2y[0])*(L1x[0] -L2x[0])
numb=(L1x[1]-L1x[0])*(L1y[0]-L2y[0])-(L1y[1]-L1y[0])*(L1x[0] -L2x[0])
if denom eq 0 then code= (numa eq 0 and numb eq 0)+2 $
else begin
ua = numa / denom
ub = numb / denom
code= ua ge 0 and ua le 1 and ub ge 0 and ub le 1
if code then
xy=[L1x[0]+ua*(L1x[1]-L1x[0]),L1y[0]+ua*(L1y[1]-L1y[0])]
endelse
return,code
end;function segmentintersect
;%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%
pro segtest
x1=[1,2,3,4,5]
y1=[3.2,7.4,8.2,9.3,7.9];
x2=[1.2,1.4,2.3,2.8,3.3,3.9,4.1,4.5,5.2]
y2=[3.1,5.2,6.2,7.3,7.5,8.6,9.6,8.7,7.4]
window
plot,x1,y1,psym=-2
oplot,x2,y2,psym=-2
n=n_elements(x2)
y2_1=interpol(y1,x1,x2)
b=y2_1 gt y2
interval=where(b[0:n-2]-b[1:*],ct)
if ct ne 0 then begin
xy=fltarr(2,ct)
for i=0,ct-1 do begin
j=interval[i]
L2x=x2[j:j+1]
L2y=y2[j:j+1]
j=value_locate(x1,L2x)
k=0
repeat begin
L1x=x1[j[k]:j[k]+1]
L1y=y1[j[k]:j[k]+1]
code=segmentintersect(L1x,L1y,L2x,L2y,xy=tmp)
b=code eq 1
if b then begin
xy[*,i]=tmp
plots,[tmp[0],tmp[0]],[0,tmp[1]],/data
endif
k++
endrep until b or (k eq 2)
endfor
endif
end;pro segtest
;%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%
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| Re: Is there a quick way to find the intersection of two lines? [message #58610 is a reply to message #58609] |
Tue, 05 February 2008 00:24 |
eyuchen
Messages: 8
Registered: February 2008
|
Junior Member |
|
|
On Feb 4, 4:33 pm, "ben.bighair" <ben.bigh...@gmail.com> wrote:
> On Feb 4, 7:08 pm, eyuc...@gmail.com wrote:
>
>> Hi there,
>
>> I have two sets of x-y data:
>> x1=[1,2,3,4,5] y1=[3.2,7.4,8.2,9.3,7.9];
>> x2=[1.2,1.4,2.3,2.8,3.3,3.9,4.1,4.5,5.2]
>> y2=[3.1,5.2,6.2,7.3,7.5,8.6,9.6,8.7,7.4];
>
>> By running:
>> plot, x1, y1
>> oplot, x2, y2
>> we can clearly see that there are four intersections, but it is not
>> clear what are the x,y coordinates of these points.
>
>> Is there an easy way to do it? Thank you very much.
>
> Hi,
>
Thank you very much. I think I do understand how to find the
intersection of two lines in principle, but actually doing it requires
some details such as narrowing down the interval that the two lines
intersect, counting intersection points, allocating memory to store
the points and finally solving them.
I just wonder if there are pre-made subroutines, since this is really
a job we often do. If there are none, I'll try to make one...
> You might want to check out Paul Bourke's great online tidbits about
> geometry.
>
> http://local.wasp.uwa.edu.au/~pbourke/geometry/
>
> I have been chipping away at coding some of the algorithms he
> describes into IDL, but have been easily sidetracked. You're welcome
> to use what I have (mostly documented) as a starting point. My
> implementations come with zero warranty...
>
> www.tidewater.net/~pemaquid/pb.zip
>
> Cheers,
> Ben
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| Re: Is there a quick way to find the intersection of two lines? [message #58612 is a reply to message #58610] |
Mon, 04 February 2008 16:33 |
ben.bighair
Messages: 221
Registered: April 2007
|
Senior Member |
|
|
On Feb 4, 7:08 pm, eyuc...@gmail.com wrote:
> Hi there,
>
> I have two sets of x-y data:
> x1=[1,2,3,4,5] y1=[3.2,7.4,8.2,9.3,7.9];
> x2=[1.2,1.4,2.3,2.8,3.3,3.9,4.1,4.5,5.2]
> y2=[3.1,5.2,6.2,7.3,7.5,8.6,9.6,8.7,7.4];
>
> By running:
> plot, x1, y1
> oplot, x2, y2
> we can clearly see that there are four intersections, but it is not
> clear what are the x,y coordinates of these points.
>
> Is there an easy way to do it? Thank you very much.
>
Hi,
You might want to check out Paul Bourke's great online tidbits about
geometry.
http://local.wasp.uwa.edu.au/~pbourke/geometry/
I have been chipping away at coding some of the algorithms he
describes into IDL, but have been easily sidetracked. You're welcome
to use what I have (mostly documented) as a starting point. My
implementations come with zero warranty...
www.tidewater.net/~pemaquid/pb.zip
Cheers,
Ben
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