What is a formula to get a vector perpendicular to another vector?

Question

What is a formula to get a three dimensional vector B lying on the plane perpendicular to a vector A?

That is, given a vector A, what is a formula f(angle,modulus) w

Answer 1

One way would be to find a rotation transform from the positive z-axis (or any other axis) to your given vector. Then transform using this transform.

def getPerpendicular(v1,modulus,angle):
    v2 = vector(0,0,1)
    v1_len = v2.length()

    axis = v1.cross_product(v2)
    sinAngle = axis.length() / v1_len       # |u x v| = |u| * |v| * sin(angle)
    cosAngle = v1.dot_product(v2) / v1_len  # u . v = |u| * |v| * cos(angle)
    axis = axis.normalize()
    # atan2(sin(a), cos(a)) = a, -pi < a < pi
    angle = math.atan2(sinAngle, cosAngle)

    rotationMatrix = fromAxisAngle(axis, angle)

    # perpendicular to v2
    v3 = vector(modulus*cos(angle),modulus*sin(angle),0)

    return rotationMatrix.multiply(v3);

To calculate the rotation matrix, see this article: WP: Rotation matrix from axis and angle

Another method would be to use quaternion rotation. It's a little more to wrap your head around, but it's less numbers to keep track of.