December 03, 2020, 07:27:08 PM

### Author Topic: [bb] Lines_Intersect() by sswift [ 1+ years ago ]  (Read 370 times)

#### BlitzBot

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•  • Posts: 1 ##### [bb] Lines_Intersect() by sswift [ 1+ years ago ]
« on: June 29, 2017, 12:28:39 AM »
Title : Lines_Intersect()
Author : sswift
Posted : 1+ years ago

Description : This function determines if two line segments intersect in 2D, and if so, where they intersect.

Code :
Code: BlitzBasic
1. Global Intersection_X#                                                                  ; Values returned by the Lines_Intersect() function.
2. Global Intersection_Y#
3. Global Intersection_AB#
4. Global Intersection_CD#
5.
6.
7. ; -------------------------------------------------------------------------------------------------------------------
8. ; This function determines if two lines in intersect in 2D.
9. ;
10. ; A & B are the endpoints of the first line segment.  C & D are the endpoints of the second.
11. ;
12. ;
13. ; If the lines DO NOT instersect, the function returns FALSE.
14. ;
15. ; If the lines DO intersect, the point of intersection is returned in the global variables:
16. ; Intersection_X#, Intersection_Y#, Intersection_AB#, and Intersection_CD#
17. ;
18. ;
19. ; Those last two variables indicate the location along each line segment where the point of intersection lies.
20. ;
21. ; For example:
22. ;
23. ; If Intersection_AB# is 0, then the point of intersection is at point A.  If it is 1, then it is at point B.
24. ; If it is 0.5, then it is halfway between the two.  And if it is less than 0 or greater than 1, then the point lies
25. ; on the line but outside of the specified line segment.
26. ;
27. ;
28. ; Because you can determine if the intersection point lies within both line segments, you can also use this function
29. ; to check to see if the line segments themselves intersect.
30. ;
31. ; Also, if these line segments indicate vectors of motion, then if either of the location values returned is negative
32. ; then you know that the objects paths intersected in the past, and will not intersect in the future.
33. ;
34. ; And finally, please note that segments which are coincident (lie on the same line) are considered to be
35. ; non-intersecting, as there is no single point of intersection.  You can easily detect this condition by changing
36. ; the code below slightly as indicated.
37. ; -------------------------------------------------------------------------------------------------------------------
38. Function Lines_Intersect(Ax#, Ay#, Bx#, By#, Cx#, Cy#, Dx#, Dy#)
39.
40.
41.         Rn# = (Ay#-Cy#)*(Dx#-Cx#) - (Ax#-Cx#)*(Dy#-Cy#)
42.         Rd# = (Bx#-Ax#)*(Dy#-Cy#) - (By#-Ay#)*(Dx#-Cx#)
43.
44.         If Rd# = 0
45.
46.                 ; Lines are parralel.
47.
48.                 ; If Rn# is also 0 then lines are coincident.  All points intersect.
49.                 ; Otherwise, there is no intersection point.
50.
51.                 Return False
52.
53.         Else
54.
55.                 ; The lines intersect at some point.  Calculate the intersection point.
56.
57.                 Sn# = (Ay#-Cy#)*(Bx#-Ax#) - (Ax#-Cx#)*(By#-Ay#)
58.
59.                 Intersection_AB# = Rn# / Rd#
60.                 Intersection_CD# = Sn# / Rd#
61.
62.                 Intersection_X# = Ax# + Intersection_AB#*(Bx#-Ax#)
63.                 Intersection_Y# = Ay# + Intersection_AB#*(By#-Ay#)
64.
65.                 Return True
66.
67.         EndIf
68.
69.
70. End Function

AntonyWells(Posted 1+ years ago)

Doesn't work, always returns true intersection and treats the second line as if it were infinitely long. I.e it's accurate but not usable.

RemiD(Posted 6 months ago)

@SSwift>>thanks for sharing !