Need some help understanding Vector2 behaviour

Ataxia

Hi there, I've just picked up the engine and have been tooling about with it for a bit. I'm a bit confused by the following code snipped though (c#):

        var normLinVel = v.LinearVelocity.Normalized() * 100f;
        var arcLimit = Mathf.Deg2Rad(20.0f);
        var a =  pos + normLinVel.Rotated(arcLimit);
        var b =  pos + normLinVel.Rotated(-arcLimit);
        var ngA = a.AngleTo(normLinVel);
        var ngB = b.AngleTo(normLinVel);
        var c =  pos + normLinVel.Rotated(ngA);
        var d =  pos + normLinVel.Rotated(ngB);
        
        DrawLine(pos, a, _colors["BLACK"], 1.5f);
        DrawLine(pos, b, _colors["BLACK"], 1.5f);
        DrawLine(pos, c, _colors["GREEN"], 1.5f);
        DrawLine(pos, d, _colors["GREEN"], 1.5f);

The expectation is that I take the linear velocity of a Body and draw two lines (a and b) at angles -20 degrees and + 20 degrees, scaled to 100px relative to the velocity.

This works as expected, which is great.

The next thing i want to do (this is just to prove my understanding of the engine) is to find the angles between the velocity and the vectors which I just made. I then want to draw two more lines over the lines which I've already drawn (b and c).

The expectation is that the angles returned from AngleTo should be identical to the variable arcLimit, however this isn't the case. It appears that I don't quite understand what the function is doing and I believe it's quite an important one to grock.

I've attached an image of the output of this. a and b are in black, linear velocity is pink and c and d are in green.

If someone can show me where i'm going wrong i'd be really grateful :D

Many thanks!

TwistedTwigleg

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If you need any help, please let us know! Thanks! :smile:

Ataxia

Oops.. Apologies for that.. It looks like I also managed to set this to be a discussion rather than a question. Is it possible to alter the type?

Thanks for your help! :D

TwistedTwigleg

No problem. :smile:

I changed this discussion to a question. Only forum staff can change a topic from a discussion to a question, so if anyone needs a discussion changed just let one of us know we'll help as soon as we can.

Ataxia

Awesome, cheers! :D

wombatstampede

@Ataxia said: ... The expectation is that the angles returned from AngleTo should be identical to the variable arcLimit, however this isn't the case. It appears that I don't quite understand what the function is doing and I believe it's quite an important one to grock.

angleTo() returns the angle between the two vectors. But your a and b are position vectors (you added the position before). Therefor they are now vectors from the local coordinate origin (0,0) to the end of your drawn vectors. So you draw a line from position to a but the a vector is actually going from (0,0) to a.

Probably you wanted to calculate something like this?

var aRel =  normLinVel.Rotated(arcLimit);
var ngA = aRel.AngleTo(normLinVel);

Ataxia

I'm such an idiot! :S

Thanks for the explanation, it seems pretty obvious now!