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

[btc]

> ownDirection := self velocity - self position. Taking that purely at face value, it doesn't look right mixing distance(m) & velocity(m.s^-1) in a calculation.  I guess you are integrating velocity over one unit of time... (m.s^-1)*(s)  - (m)  = (m) - (m) = (m) cheers -ben Nicolai Hess wrote: Another observation in Boids>>isInFieldOfVision: aBoid ... ownDirection := self velocity - self position. the own direction should be the same as the velocity. Consider a Boid at position 100@0 with velocity 100@0 would give ownDirection = 0@0. But of course, the steering direction is still 1@0. 2014-03-17 10:07 GMT+01:00 Nicolai Hess <nicolaih...@web.de>: 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 <nicolaih...@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 <w...@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.