# Collinear Angles ## Recommended Posts  It's intuitive to know that 90 degrees and 270 degrees are collinear assuming the same reference point and 0 degrees.  It gets harder to test for other angles though. This gets even more frustrating to test if your angle is negative, or more than 2*pi in radians.

```;********************************************************;
;; MA:str8_ang - Check if two angles are straight (collinear)
;; Arguments:
;; - ang1 (float): First angle in radians
;; - ang2 (float): Second angle in radians
;; Returns:
;; - test (bool): True if the angles are collinear, False otherwise
;; Description:
;; This function checks if two angles are collinear (straight) by calculating the cross product of their corresponding unit vectors.
;; The provided angles are translated to positive equivalents and normalized to the range of 0 to 2π.
;; The function then calculates the unit vectors u and v from the angles and calculates their cross product.
;; If the cross product is nearly zero (within a tolerance), the angles are considered collinear.
;; The function returns True if the angles are collinear, and False otherwise.
;; Usage: (MA:str8_ang ang1 ang2)
(defun MA:str8_ang (ang1 ang2 / u1 u2 u3 v1 v2 v3 cross_prod test)
(if (and ang1 ang2)
(progn
;; Translate negative angles to positive equivalents
(if (< ang1 0)
(setq ang1 (+ (* 2 pi) (rem ang1 (- (* 2 pi)))))
)
(if (< ang2 0)
(setq ang2 (+ (* 2 pi) (rem ang2 (- (* 2 pi)))))
)

;; Normalize angles to the range of 0 to 2π
(setq ang1 (rem ang1 (* 2 pi)))
(setq ang2 (rem ang2 (* 2 pi)))

(setq u1 (cos ang1)
u2 (sin ang1)
u3 0.0
v1 (cos ang2)
v2 (sin ang2)
v3 0.0
)
(setq cross_prod (list (- (* u2 v3) (* u3 v2))
(- (* u3 v1) (* u1 v3))
(- (* u1 v2) (* u2 v1))
)
)
(if
(and (= (car cross_prod) 0.0)
)
(setq test T)
(setq test nil)
)
test ; return the test result
) ; end progn
) ; end if
)```

This basically outputs T if the two angles have the same angle or reflection of each other.

Do you guys have an easier way of checking this? A refactor would be nice.

##### Share on other sites  And from polyface.de ?

Exemple:

```;;functions VectorProduct & collinear
;;Armin Antkowiak, Berlin
;;http://www.polyface.de/general.html
;;mailto:info@polyface.de
(defun vectorProduct (v1 v2 / )
(list
)
)
(defun collinear (p1 p2 p3 p4 / tol)
(setq tol 1E-12)
(equal
'(0.0 0.0 0.0)
(vectorProduct (mapcar '- p2 p1) (mapcar '- p3 p4))
tol
)
)
;*********************************
(defun c:colineaire ( / e1 e2 pt1 pt2 pt3 pt4)
(setq e1 (entget (car (entsel "\nSelect first line: "))))
(setq e2 (entget (car (entsel "\nSelect second line: "))))
(setq
pt1 (cdr (assoc 10 e1))
pt2 (cdr (assoc 11 e1))
pt3 (cdr (assoc 10 e2))
pt4 (cdr (assoc 11 e2))
)
(if (collinear pt1 pt2 pt3 pt4)
(princ "\nLine are collinear")
(princ "\nLine aren't collinear")
)
(princ)
)```

##### Share on other sites  Posted (edited)
```(defun collinear-p ( p1 p p2 )
(equal (distance p1 p2) (+ (distance p1 p) (distance p p2)) 1e-6)
)```

```;; Collinear-p  -  Lee Mac
;; Returns T if p1,p2,p3 are collinear

(defun LM:Collinear-p ( p1 p2 p3 )
(
(lambda ( a b c )
(or
(equal (+ a b) c 1e-8)
(equal (+ b c) a 1e-8)
(equal (+ c a) b 1e-8)
)
)
(distance p1 p2) (distance p2 p3) (distance p1 p3)
)
)```

Edited by marko_ribar
##### Share on other sites

• 3 weeks later...  On 7/16/2023 at 6:31 AM, Tsuky said:

And from polyface.de ?

I don't even know that website. Nobody owns Linear Algebra afaik. Maybe I should get some good code from there as well. Seems neat and nice. Thanks for sharing.

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