[Pharo-users] Calculate angle between two vectors (probably @Igor :)

MartinW wm at fastmail.fm
Sun Mar 16 12:01:46 EDT 2014


Hello,
i probably made some embarassing mistake, but as i cannot find it, i ask you
to have a look at this method.
It should calculate the angle between two vectors. And vectors are instances
of Point (perhaps here is already a misconception?).
It is used in my flocking simulation PharoBoids
(http://smalltalkhub.com/#!/~MartinWalk/Boids)

Here is my version of the method:

angleBetween: vector1 and: vector2 onError: aBlock
	| cosinusOfAngle innerProductOfVectors productOfVectorsLengths|
	innerProductOfVectors := (vector1 dotProduct: vector2).
	productOfVectorsLengths := (vector1 r) * (vector2 r).
	productOfVectorsLengths = 0 ifTrue: [ ^ aBlock value ].
	cosinusOfAngle := innerProductOfVectors / productOfVectorsLengths.
	^ cosinusOfAngle arcCos
	
But my Boids behave wrong when i use it. I found another implementation of
the same problem by Igor Stasenko which works and looks like this:

angleBetween: p1 and: p2 ifDegenerate: aBlock
" Calculate an angle (in radians) between two vectors. 
Evaluate a block, in case if calculation not possible because one of the
vectors has zero length "

	| x1 y1 x2 y2 dot2 n2 |
	x1 := p1 x.
	y1 := p1 y.
	x2 := p2 x.
	y2 := p2 y.
	
	dot2 := x1 * x2 + (y1 * y2).
	dot2 := dot2 * dot2.
	
	n2 := (x1*x1 + (y1*y1)) * (x2*x2 + (y2*y2)).
	
	n2 = 0 ifTrue: [ ^ aBlock value ].
	
	^ (dot2 / n2) arcCos
	
Can anybody explain the problem of my method? M.



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