Here is a routine I wrote some time back as an exercize. It may give you some ideas on how to proceed.
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;; elb.lsp Draw an elbow from user input
;;
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(defun c:elb (/ p90 p45 p30 p60 p180 p270
rtd dtr fij ing draw choose om
calc_points pt1 pt2 pt3 pt4 pt5 pt6 pt7 pt8 ptc
get_direction midpoint direction dir alpha ductdia
)
;;; -=<*>=- -=<*>=- -=<*>=- -=<*>=- -=<*>=- -=<*>=- -=<*>=- -=<*>=-
;;; Local Functions
;;; -=<*>=- -=<*>=- -=<*>=- -=<*>=- -=<*>=- -=<*>=- -=<*>=- -=<*>=-
;; RADIAN TO DEGREE function
(defun rtd (r) (/ (* r 180.0) pi))
;;define DEGREE to RADIAN function
(defun dtr (d) (/ (* d pi) 180.0))
;; find mid point between two points
(defun midpoint (p1 p2)(mapcar '(lambda (a b) (* 0.5 (+ a b))) p1 p2))
;; ================================================== =======
;; (vector-side <origin> <direction> <point> )
;;
;; Returns an integer code indicating position of <point>
;; in relation to the directed vector whose endpoints are
;; <origin> and <direction>.
;;
;; Result Meaning
;; ---------------------------------------------------------
;; -1 Point is to the right of vector.
;; 0 Point is on (colinear with) vector
;; 1 Point is to the left of vector.
(defun vector-side (v1 v2 p / r *fuzz*)
(setq *fuzz* 1e-10)
(setq r (- (* (-(car v2)(car v1))(-(cadr p)(cadr v1)))
(* (-(cadr v2)(cadr v1))(-(car p)(car v1)))
)
)
(cond ((equal r 0.0 *fuzz*) 0)
(t (fix (/ (abs r) r)))
)
) ; end defun
;; ================================================== =======
;; determine the elbow direction
(defun calc_direction (/ dircode ang midpt vpt dir)
(if (= (setq dircode (vector-side pt1 pt2 ptside)) 0)
(prompt "\nError, Point was on elbow base line.")
(progn ; got which side of the elbow base line
;; get direction for elbow
(cond
((= dircode 1) ;ccw
(setq dir '+)
)
((= dircode -1) ; cw
(setq dir '-)
)
) ; end cond stmt
(setq ang (apply dir (list (angle pt1 pt2) p90))
midpt (midPoint pt1 pt2)
vpt (polar midpt ang 20) ; vector point
)
(if (= (setq dircode (vector-side midpt vpt ptside)) 0)
(prompt "\nError, Point was in the middle.")
(progn ; got which side of duct
;; get direction for elbow
(cond
((= dircode 1) ;ccw
(setq direction "CCW")
(setq dir '+)
)
((= dircode -1) ; cw
(setq direction "CW")
(setq dir '-)
)
) ; end cond stmt
) ; end progn
) ;end if
) ; end progn
) ;end if
)
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;;DATA INPUT PER USER
(defun Get_Location ()
(initget 7) ; NO 0, NULL, NIETHER NEGATIVE VALUE ALLOWED
(setq om (getvar "osmode"))
(setvar "osmode" 1) ; osmode to End
(if (setq pt1 (getpoint "\nSelect insertion point of elbow."))
(if (setq pt2
(getpoint pt1 "\nSelect the end point of the other duct side.")
)
(progn
(setvar "osmode" 0) ; osmode to None
(command "._Line" pt1 pt2 "")
(if (setq ptside
(getpoint "\nSelect point near the other end of the elbow.")
)
(progn
(entdel (entlast))
(if (> (distance pt1 ptside)(distance pt2 ptside))
(setq tmp pt1 ; keep pt1 as inside point of elbow
pt1 pt2
pt2 tmp)
)
(setq alfa (angle pt1 pt2) ; angle between sides
ductdia (distance pt1 pt2)) ; get the duct size
t ; return True
) ; end progn
(entdel (entlast))
) ; endif
);end progn
) ; endif
) ; endif
) ;_end ing func
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;;Calculate Points
(defun calc_points (/ ang elbrad elbstr)
(setq elbrad (* ductdia 1.5) ;Duct Radious = 1.5 OF A
elbstr (* ductdia 0.25) ; THE ELLBOW'S STRAIGHT PART
ang (apply dir (list alfa p90))
pt3 (polar pt2 ang elbstr)
pt4 (polar pt1 ang elbstr)
ptc (polar pt4 (+ alfa pi) elbrad)
pt5 (polar ptc ang elbrad)
pt6 (polar pt5 ang ductdia)
pt7 (polar pt6 (+ alfa pi) elbstr)
pt8 (polar pt5 (+ alfa pi) elbstr)
) ; end setq
) ;_END CALC FOR CW
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(defun draw ()
(command ".undo" "begin")
(setvar "osmode" 0) ; osmode to None
;; draw the straigth part of the elbow beside the duct
(command ".pline" pt1 pt2 pt3 pt4 "c")
;; idem for the elbows's end
(command ".pline" pt5 pt6 pt7 pt8 "c")
(cond
((= direction "CCW")
(command ".arc" "c" ptc pt3 pt6) ; THE ARC CCW
(command ".arc" "c" ptc pt4 pt5) ; THE ARC CCW
) ;_END cond CCW
((= direction "CW")
(command ".arc" "c" ptc pt5 pt4) ;THE ARC CW
(command ".arc" "c" ptc pt6 pt3) ;THE ARC CW
) ;_END cond CW
) ; end cond stmt
(command ".undo" "end")
) ;_END DRAW
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;;;=============================================== =================
;;; Start of Routine
;;;=============================================== =================
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;;constant definition
(setq p90 (/ pi 2.0)) ; 90 DEGREE
(setq p45 (/ pi 4.0)) ;45 DEGREE
(setq p30 (/ pi 6.0)) ; 30 DEGREE
(setq p60 (/ pi 3.0)) ;60 DEGREE
(setq p180 pi) ;180 degre
(setq p270 (* pi 1.5)) ;270 degree
;;************************************************ ******
(setvar "angdir" 0)
;;(setvar "lunits" 4); the user unit
(setvar "lunits" 2) ; my units system
;;-------------------------------------------------------
(if (Get_Location) ; get points
(if (setq dir (calc_direction))
(progn
(calc_points)
(draw)
) ; end progn
(prompt "\nCalculate direction failed...")
) ; endif
(prompt "\nGet location failed...")
) ; endif
(princ)
(setvar "osmode" om)
) ;_end defun c:elb
(prompt "\nElbow DRAWER ... type ELB as a command")
(princ)
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