This routine was made by Chen Qing Jun, qjchen and is found at http://www.theswamp.org/index.php?topic=40494.msg457913#msg457913

It is quite handy if you happen to need to break a line that passes through a “plane.” I am using the term “plane” loosely because it doesn’t have to be a 3d object like a face or a solid. You define the “plane” by picking 3 points without having to use a different UCS…

Here’s how:

Thank you Chen!!

;;;Break 3d lines by a 3d Plane ;;;Author: Chen Qing Jun, qjchen ;;;Programed by: South China University of Technology ;;;date: 2011.12.19 ;;;Note: the intersection function of Line and Plane, has the parameter of nil and T ;;; For T, the line segment should pass through the plane ;;; but it is not necessary for the nil parameter. ;;; The plane defined by 3p is infinite. ;;; ;;; http://www.theswamp.org/index.php?topic=40494.msg457913#msg457913 ;;; ;;;_Some vector function, some get from gile's great function________________ ;;; (defun q:geo:is-3p-plane(p1 p2 p3) (> (q:vec:Len (q:vec:Norm (q:vec:cross* (q:vec:- p2 p1) (q:vec:- p3 p1)))) 1e-6) ) (defun q:geo:is-samepoint(p1 p2) (< (distance p1 p2) 1e-5) ) (defun q:vec:+(v1 v2) (mapcar '+ v1 v2) ) (defun q:vec:-(v1 v2) (mapcar '- v1 v2) ) (defun q:vec:*c(v a) (mapcar '(lambda(x) (* x a)) v) ) (defun q:vec:dot*(v1 v2) (apply '+ (mapcar '* v1 v2)) ) (defun q:vec:cross*(v1 v2) (list (q:det:2 (cadr v1) (caddr v1) (cadr v2) (caddr v2)) (q:det:2 (caddr v1) (car v1) (caddr v2) (car v2)) (q:det:2 (car v1) (cadr v1) (car v2) (cadr v2))) ) ;;;;cal determinant ;;;;|a1 a2| ;;;;|b1 b2| (defun q:det:2(a1 a2 b1 b2) (- (* a1 b2) (* a2 b1))) ;;;;Normalize a vector (defun q:vec:Norm(v / l) (if (not (zerop (setq l (distance '(0 0 0) v)))) (mapcar '(lambda(x) (/ x l)) v)) ) ;;;;Vector Length (defun q:vec:Len(v / l) (distance '(0 0 0) v)) ;;;;a normal to a plane, (defun q:geo:normal.to.3p(p1 p2 p3) (if (q:geo:is-3p-plane p1 p2 p3)(q:vec:cross* (q:vec:- p2 p1) (q:vec:- p3 p1))) ) ;;p1 p2 are two points,V is the normal of the plane, VP is one point on the plane ;;ref: http://en.wikipedia.org/wiki/Line-plane_intersection ;; http://softsurfer.com/Archive/algorithm_0104/algorithm_0104B.htm (defun q:geo:line-intersect-plane-1(P1 P2 V VP F / d l n) (setq n (q:vec:Norm V) l (q:vec:Norm (q:vec:- P2 P1))) (if (not (zerop (q:vec:dot* l n))) (progn (setq d (/ (q:vec:dot* (q:vec:- VP P1) n) (q:vec:dot* l n))) (setq res (q:vec:+ P1 (q:vec:*c l d))) (setq temp (q:vec:Len (q:vec:- P2 P1))) (if (and F (or (< d 0) (> d (q:vec:Len (q:vec:- P2 P1))))) (setq res nil)) ) ) res ) ;;;;;;______end of Vector function__________________________________;;; (defun q:entmake:point(pt layer) (entmake (list (cons 0 "POINT")(cons 8 layer)(cons 10 pt))) ) (defun q:mulentmod (ent numlst contentlst / i x) (setq i 0) (foreach x numlst (if (/= (assoc x ent) nil) (setq ent (subst (cons x (nth i contentlst)) (assoc x ent) ent ) ) (setq ent (append ent (list (cons x (nth i contentlst))) ) ) ) (setq i (1+ i)) ) (entmod ent) ;(entupd ent) ) ;;;;;stdlib from Reini Urban (defun std-sslist (ss / n lst) (if (eq 'pickset (type ss)) (repeat (setq n (fix (sslength ss))) ; fixed (setq lst (cons (ssname ss (setq n (1- n))) lst)) ) ) ) ;;;???______by qjchen____________________________;;; (defun c:PBL() (setq p1 (getpoint "\nThe first point:") p2 (getpoint "\nThe second point:") p3 (getpoint "\nThe third point:")) (if (q:geo:is-3p-plane p1 p2 p3) (progn (setq c (std-sslist(ssget '((0 . "LINE"))))) (setq pn (q:geo:normal.to.3p p1 p2 p3)) (foreach x c (setq pa (cdr (assoc 10 (entget x))) pb (cdr (assoc 11 (entget x)))) (setq int (q:geo:line-intersect-plane-1 pa pb pn p1 T)) (if (and int (not (q:geo:is-samepoint int pa))(not (q:geo:is-samepoint int pb))) (progn (q:mulentmod (entget (entmakex (entget x))) (list 62 11) (list 1 int)) (q:mulentmod (entget (entmakex (entget x))) (list 62 10) (list 2 int)) (entdel x) (q:entmake:point int "0") ) ) ) ) (princ "\n These three points can not form a plane.") ) ) (princ) (princ "\n Break 3d lines by a 3d Plane,by qjchen,the command is :test") (princ)

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Excellent – thank you!

Nice one.