This was of course wrong, I don't know what the
other method computes, but not the angle between two vectors.

Now the question is, why do you think your method is wrong / the boids� behave wrong?

2014-03-16 18:41 GMT+01:00 Nicolai Hess <nicolaihess@web.de>:

Rounding error?

This may be the reason, igor's version is working with

cos = (a dot b)^2 / (|ab|^2)

while you are using

cos = (a dot b) / |ab|

2014-03-16 17:01 GMT+01:00 MartinW <wm@fastmail.fm>:

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.



--
View this message in context: http://forum.world.st/Calculate-angle-between-two-vectors-probably-Igor-tp4749351.html
Sent from the Pharo Smalltalk Users mailing list archive at Nabble.com.