Intersection of two line segments

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Little John
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Intersection of two line segments

Post by Little John »

Hi,

calculating the intersection of two (infinite) lines or (finite) line segments is a problem which is pretty easy to understand, and so seems simple at the first glance. However, some special cases have to be taken into account, and "out there on the internet" I saw code that did not do so properly. In order to realize some typical cases, firstly I show several figures. These figures give the expected results, so that we can check whether the results of our code are correct.

If you are only interested in the code, go directly to the second post.

//Edit 2016-05-21:
  • Extended the above text.
  • Extended figure b.
  • Changed the captions of the figures.

Image     Image
Fig. a                                                                                                     Fig. b
The lines and the segments intersect.                                                     The lines intersect;
                                                                                                             the segments do not intersect.


Image     Image
Fig. c                                                                                                    Fig. d
The lines intersect;                                                                                The lines intersect;
an end point of one segment is on the other segment.                             an end point of one segment is on the other line,
                                                                                                             but not on the other segment.


Image     Image
Fig. e                                                                                                    Fig. f
The lines are parallel.                                                                            The lines are coincident,
                                                                                                             the segments are not in contact.


Image     Image
Fig. g                                                                                                     Fig. h
The lines are coincident;                                                                        The lines are coincident;
the segments share one end point.                                                          the segments share multiple points.


Image     Image
Fig. i                                                                                                     Fig. j
The lines are coincident;                                                                        The lines and the segments are coincident.
one segment contains the other segment.


The figures were created with the excellent open source program GeoGebra.
Last edited by Little John on Fri May 20, 2016 11:07 pm, edited 1 time in total.
Please excuse my flawed English. My native language is PureBasic.
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Re: Intersection of two line segments

Post by Little John »

Works also with PB 5.20

The following procedure is based on C code from http://paulbourke.net/geometry/pointlineplane/. Thanks to Paul Bourke for the code, and thanks to IdeasVacuum for pointing to that website here on the forum.
My code is a corrected and extended version. The original code returns wrong results for examples f and g above. And it always returns only 1 point as result, whereas I decided to return 2 points as result if applicable, in order not to lose any information.

See also Intersection of two circles.

//Edit 2016-05-21:
  • Changed the parameters that take the output (no linked list needed anymore).
  • By means of the new optional parameter 'checkLines', now it's not only possible to check the intersection of two (finite) line segments, but also of two (infinite) lines.

Code: Select all

EnableExplicit

#Epsilon = 0.000001

Structure PointD
   x.d
   y.d
EndStructure

Macro IsBetween (_x_, _a_, _b_)
   ; Is _x_ between _a_ and _b_?
   ; It doesn't matter whether _a_ is the minimum and _b_ is the maximum, or vice versa.
   (((_a_) <= (_x_) And (_x_) <= (_b_)) Or ((_b_) <= (_x_) And (_x_) <= (_a_)))
EndMacro


Procedure.i Intersect_Segments (*p1.PointD, *p2.PointD, *p3.PointD, *p4.PointD,
                                *ps1.PointD, *ps2.PointD, checkLines.i=#False)
   ; -- Determine the intersection of two lines or two line segments (in 2 dimensions).
   ; in : *p1, *p2  : points on line a or segment a
   ;      *p3, *p4  : points on line b or segment b
   ;      checkLines: #True  for checking the intersection of two lines,
   ;                  #False for checking the intersection of two segments.
   ; out: *ps1        : intersection point (if existing), or
   ;      *ps1, *ps2  : endpoints of overlapping part of the segments (if existing)
   ;      return value: 0 if there is no intersection point
   ;                    1 if there is exactly one intersection point
   ;                    2 if the segments overlap (more than one point)
   ;                   -1 if the lines are coincident
   ;
   ; [http://www.purebasic.fr/english/viewtopic.php?f=12&t=53070]
   Protected.d numera, numerb, denom, mua, mub
   Protected.i ret=0
   
   numera = (*p4\x-*p3\x)*(*p1\y-*p3\y) - (*p4\y-*p3\y)*(*p1\x-*p3\x)
   numerb = (*p2\x-*p1\x)*(*p1\y-*p3\y) - (*p2\y-*p1\y)*(*p1\x-*p3\x)
   denom  = (*p4\y-*p3\y)*(*p2\x-*p1\x) - (*p4\x-*p3\x)*(*p2\y-*p1\y)
   
   If Abs(denom) >= #Epsilon                                        ; if the lines are not parallel
      mua = numera / denom
      mub = numerb / denom
      
      If checkLines Or
         (mua >= 0.0 And mua <= 1.0 And mub >= 0.0 And mub <= 1.0)  ; if the intersection is along the segments
         *ps1\x = *p1\x + mua * (*p2\x-*p1\x)
         *ps1\y = *p1\y + mua * (*p2\y-*p1\y)
         ProcedureReturn 1
      Else
         ProcedureReturn 0
      EndIf
      
   ElseIf Abs(numera) < #Epsilon And Abs(numerb) < #Epsilon         ; if the lines are coincident
      If checkLines
         ProcedureReturn -1
         
      Else                                                          ; do the segments share one or more points?
         If IsBetween(*p1\x, *p3\x, *p4\x) And IsBetween(*p1\y, *p3\y, *p4\y)
            *ps1\x = *p1\x
            *ps1\y = *p1\y
            ret + 1
         EndIf
         If IsBetween(*p3\x, *p1\x, *p2\x) And IsBetween(*p3\y, *p1\y, *p2\y)
            If ret = 0
               *ps1\x = *p3\x
               *ps1\y = *p3\y
               ret + 1
            ElseIf *ps1\x <> *p3\x Or *ps1\y <> *p3\y
               *ps2\x = *p3\x
               *ps2\y = *p3\y
               ret + 1
            EndIf
         EndIf
         If IsBetween(*p2\x, *p3\x, *p4\x) And IsBetween(*p2\y, *p3\y, *p4\y)
            If ret = 0
               *ps1\x = *p2\x
               *ps1\y = *p2\y
               ret + 1
            ElseIf ret = 1 And (*ps1\x <> *p2\x Or *ps1\y <> *p2\y)
               *ps2\x = *p2\x
               *ps2\y = *p2\y
               ret + 1
            EndIf
         EndIf
         If IsBetween(*p4\x, *p1\x, *p2\x) And IsBetween(*p4\y, *p1\y, *p2\y)
            If ret = 1 And (*ps1\x <> *p4\x Or *ps1\y <> *p4\y)
               *ps2\x = *p4\x
               *ps2\y = *p4\y
               ret + 1
            EndIf
         EndIf
         ProcedureReturn ret                                        ; 0, 1, or 2
      EndIf
      
   Else                                                             ; if the lines are parallel, but not coincident
      ProcedureReturn 0
   EndIf
EndProcedure


CompilerIf #PB_Compiler_IsMainFile
   ; -- Demo
   
   Macro ShowResult (_label_, _expected_result_, _checkLines_=#False)
      Debug _label_ + ":"
      count = Intersect_Segments(p1, p2, p3, p4, ps1, ps2, _checkLines_)
      If count <> _expected_result_
         Debug "ERROR: count = " + count
      EndIf
      If count = 0
         Debug "no intersection"
      ElseIf count = -1
         Debug "coincident"
      Else
         Debug "(" + StrD(ps1\x,2) + ", " + StrD(ps1\y,2) + ")"
         If count = 2
            Debug "(" + StrD(ps2\x,2) + ", " + StrD(ps2\y,2) + ")"
         EndIf
      EndIf
   EndMacro
   
   
   Define.PointD p1, p2, p3, p4, ps1, ps2
   Define.i count
   
   ; Example a: The lines and the segments intersect.
   p1\x = 1.0 : p1\y = 3.0
   p2\x = 3.0 : p2\y = 2.0
   p3\x = 1.5 : p3\y = 1.0
   p4\x = 3.0 : p4\y = 4.0
   Debug "* Example a *"
   ShowResult("Lines", 1, #True)
   ShowResult("Segments", 1)
   Debug "---------------"
   
   ; Example b: The lines intersect;
   ;            the segments do not intersect.
   p1\x = 1.0 : p1\y = 3.0
   p2\x = 1.8 : p2\y = 2.5
   p3\x = 1.5 : p3\y = 1.0
   p4\x = 3.0 : p4\y = 4.0
   Debug "* Example b *"
   ShowResult("Lines", 1, #True)
   ShowResult("Segments", 0)
   Debug "---------------"
   
   ; Example c: The lines intersect;
   ;            an end point of one segment is on the other segment.
   p1\x = 1.0 : p1\y = 3.0
   p2\x = 3.0 : p2\y = 2.0
   p3\x = 2.0 : p3\y = 2.5
   p4\x = 3.0 : p4\y = 4.0
   Debug "* Example c *"
   ShowResult("Lines", 1, #True)
   ShowResult("Segments", 1)
   Debug "---------------"
   
   ; Example d: The lines intersect;
   ;            an end point of one segment is on the other line,
   ;            but not on the other segment.
   p1\x = 1.0 : p1\y = 3.0
   p2\x = 3.0 : p2\y = 2.0
   p3\x = 4.0 : p3\y = 1.5
   p4\x = 3.0 : p4\y = 4.0
   Debug "* Example d *"
   ShowResult("Lines", 1, #True)
   ShowResult("Segments", 0)
   Debug "---------------"
   
   ; Example e: The lines are parallel.
   p1\x = 1.0 : p1\y = 3.0
   p2\x = 3.0 : p2\y = 2.0
   p3\x = 2.0 : p3\y = 4.0
   p4\x = 4.0 : p4\y = 3.0
   Debug "* Example e *"
   ShowResult("Lines", 0, #True)
   ShowResult("Segments", 0)
   Debug "---------------"
   
   ; Example f: The lines are coincident;
   ;            the segments are not in contact.
   p1\x = 1.0 : p1\y = 3.0
   p2\x = 3.0 : p2\y = 2.0
   p3\x = 4.0 : p3\y = 1.5
   p4\x = 5.0 : p4\y = 1.0
   Debug "* Example f *"
   ShowResult("Lines", -1, #True)
   ShowResult("Segments", 0)
   Debug "---------------"
   
   ; Example g: The lines are coincident;
   ;            the segments share one end point.
   p1\x = 1.0 : p1\y = 3.0
   p2\x = 3.0 : p2\y = 2.0
   p3\x = 3.0 : p3\y = 2.0
   p4\x = 5.0 : p4\y = 1.0
   Debug "* Example g *"
   ShowResult("Lines", -1, #True)
   ShowResult("Segments", 1)
   Debug "---------------"
   
   ; Example h: The lines are coincident;
   ;            the segments share multiple points.
   p1\x = 1.0 : p1\y = 3.0
   p2\x = 3.0 : p2\y = 2.0
   p3\x = 2.0 : p3\y = 2.5
   p4\x = 5.0 : p4\y = 1.0
   Debug "* Example h *"
   ShowResult("Lines", -1, #True)
   ShowResult("Segments", 2)
   Debug "---------------"
   
   ; Example i: The lines are coincident;
   ;            one segment contains the other segment.
   p1\x = 1.0 : p1\y = 3.0
   p2\x = 3.0 : p2\y = 2.0
   p3\x = 0.5 : p3\y = 3.25
   p4\x = 5.0 : p4\y = 1.0
   Debug "* Example i *"
   ShowResult("Lines", -1, #True)
   ShowResult("Segments", 2)
   Debug "---------------"
   
   ; Example j: The lines and the segments are coincident.
   p1\x = 1.0 : p1\y = 3.0
   p2\x = 3.0 : p2\y = 2.0
   p3\x = 1.0 : p3\y = 3.0
   p4\x = 3.0 : p4\y = 2.0
   Debug "* Example j *"
   ShowResult("Lines", -1, #True)
   ShowResult("Segments", 2)
CompilerEndIf
Last edited by Little John on Fri May 20, 2016 11:19 pm, edited 2 times in total.
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eddy
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Re: Intersection of two line segments

Post by eddy »

Hi
Is it fast enough for sprite polygon collision ?
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Re: Intersection of two line segments

Post by Little John »

eddy wrote:Hi
Is it fast enough for sprite polygon collision ?
Hi,

I don't know. Please test yourself. :-)

Regards, Little John
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luis
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Re: Intersection of two line segments

Post by luis »

Nice, thank you :)
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STARGÅTE
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Re: Intersection of two line segments

Post by STARGÅTE »

eddy wrote:Hi
Is it fast enough for sprite polygon collision ?
Little John code is for line-line-collision, a polygon is a surface, there you check whether a point is in the triangle.
Or you mean a polyline?

@Little John: line-line-collision is ok, but have you a code for check bézier curve collision?
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Little John
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Re: Intersection of two line segments

Post by Little John »

STARGÅTE wrote:@Little John: line-line-collision is ok, but have you a code for check bézier curve collision?
I don't have such code, sorry.
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Re: Intersection of two line segments

Post by eddy »

Sorry I meant polyline.
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Re: Intersection of two line segments

Post by IceSoft »

You should use the Chipmunk4PB wrapper for collision detection.
Belive!
<Wrapper>4PB, PB<game>, =QONK=, PetriDish, Movie2Image, PictureManager,...
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Re: Intersection of two line segments

Post by said »

Interesting, thanks for sharing and for the link :)
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Re: Intersection of two line segments

Post by Little John »

Code extended and slightly improved.
For details see 1st and 2nd post.
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