How do I rotate a vector?

Question

If I have [1, 0] and I rotate it 90 degrees = 1/2PI radians , I should get [0, -1] . How do I achieve this?

I looked at this page and implemented this:

var rotateVector = function(vec, ang)
{
    // not right!
    return new PVector(
        vec.x * cos(ang) - vec.y * sin(ang),
        vec.x * sin(ang) + vec.y * cos(ang)
    );
};

But that doesn't seem to work.

It doesn't matter if it's degrees or radians, because the geometry remains the same. I use radians right now.

Answer 1

Your formulas are correct, but you need to use the angle in radians instead of degrees:

radians = degrees * (Math.PI/180)

And if you want to rotate it clockwise, notice that your angle is -90 degrees instead of 90. Or, consider it 90 but change the formula to:

radians = -degrees * (Math.PI/180)

Edit:

According to your last edit, probably you get the following result:

[6.123233995736766e-17, -1]

This is because the computer cannot store "infinite" numbers. But, as you can see that number is very small, so it can be considered 0 . You can round the result.

Eg:

var rotateVector = function(vec, ang)
{
    ang = -ang * (Math.PI/180);
    var cos = Math.cos(ang);
    var sin = Math.sin(ang);
    return new Array(Math.round(10000*(vec[0] * cos - vec[1] * sin))/10000, Math.round(10000*(vec[0] * sin + vec[1] * cos))/10000);
};

And test it using:

rotateVector([1,0], 90)

Result:

[0, -1]

Answer 2

后来我发现PVector有一个方法： rotate(angle)