Home › Forums › Problems and solutions in GDL › Object design › Detecting Line Intersection
Tagged: intersection, line, unit vector, direction vector
 This topic has 5 replies, 2 voices, and was last updated 2 years, 8 months ago by Péter Baksa.

AuthorPosts


March 8, 2019 at 12:32 #4588James GoodeParticipant
Hello,
I’ve recently received this bit of code regarding line intersection but I’m not quite sure how to implement it into my object.
(px1, py1, px2, py2 are coordinates of points on each segments, vx1, vy1, vx2, vy2 are the direction vectors for the segments)
"LineLineIntersection": Scalar = ( (vx1*vx2) + (vy1*vy2) ) if abs(abs(Scalar)  1) < EPS then ! no intersection, parallel lines ipx = 0 ipy = 0 state = 0 else ! Calculate A1, B1 and C1 parameter ! A1, B1 are equal to normal vector A1 = vy1 B1 = vx1 C1 = (A1*px1)  (B1*py1) ! Calculate A2, B2 and C2 parameter ! A2, B2 are equal to normal vector A2 = vy2 B2 = vx2 C2 = (A2*px2)  (B2*py2) ! Calculate coordinates of intersection point ipy = ((A1*C2)  (C1*A2)) / ((B1*A2)  (A1*B2)) if abs(A1) < EPS then ipx = (B2*ipy  C2) / A2 else ipx = (B1*ipy  C1) / A1 endif state = 1 endif return' Specific to my object my line points would be something along the lines of
line2 x[1], y[1], x[2], y[2]
line2 x[2], y[2], x[3], y[3]`How can I implement my parameters into the example code I’ve been given

March 11, 2019 at 09:31 #4593Péter BaksaKeymaster
Hi James,
The direction vectors are unit vectors along the lines, their length have to be exactly 1 (meter). Compute the length of the vector between the two points on the line, and to set the vector length to 1, multiply its end coordinates by 1/length.
If you use an EPS value that is common in GS libparts, this code has a small bug, the lines are treated parallel (no intersections) when their angle is smaller than 0.81°, which is still quite noticable. Using EPS = 0.00000001 will set the limit to 0.0081°
Péter Baksa
Library Platform, Software Engineer
GRAPHISOFT SE 
March 11, 2019 at 11:49 #4600James GoodeParticipant
Do you have any sort of examples for this by any chance?
Thanks

March 12, 2019 at 17:16 #4603Péter BaksaKeymaster
Yes, have a look at “basicStairTread_m” 3D script.
Péter Baksa
Library Platform, Software Engineer
GRAPHISOFT SE 
March 13, 2019 at 11:41 #4611James GoodeParticipant
So I’ve taken a look at the basicStairTread_m 3d script but I don’t really understand what I’m supposed to be looking at. I wish there was an object which was an example purely for line intersection like there are for a few other objects in the help centre. Without being able to detect and stop line intersections in my object, it becomes unusable.

March 21, 2019 at 09:31 #4618Péter BaksaKeymaster
Hi,
There is a call (gosub) to “LineLineIntersection” in the 3D script. The input variables are set up before the call.
vx, vy is computed from two points (_intersectEdge[1] and _intersectEdge[2]) on the line, divided by the distance of the two points:
! line 1 vx1 = (_intersectEdge[2][1]  _intersectEdge[1][1]) / _intersectEdgeLength vy1 = (_intersectEdge[2][2]  _intersectEdge[1][2]) / _intersectEdgeLength
px, py is any point on the line:
px1 = _intersectEdge[1][1] py1 = _intersectEdge[1][2]
This is done again for the second line:
! line 2 vx2 = (_intersectEdgeLeft[2][1]  _intersectEdgeLeft[1][1]) / _intersectEdgeLeftLength vy2 = (_intersectEdgeLeft[2][2]  _intersectEdgeLeft[1][2]) / _intersectEdgeLeftLength px2 = _intersectEdgeLeft[1][1] py2 = _intersectEdgeLeft[1][2]
The result intersection is ipx, ipy, when state = 1 (otherwise the lines are parallel):
gosub "LineLineIntersection" ! returns ipx, ipy, state if state <> 0 then _intersectPtLeft[1] = ipx _intersectPtLeft[2] = ipy endif
Péter Baksa
Library Platform, Software Engineer
GRAPHISOFT SE


AuthorPosts
 The forum ‘Object design’ is closed to new topics and replies.