Question

I’m trying to generate a set of points (represented by a Vector struct) that roughly models a spiral galaxy.

The C# code I’ve been playing with is below; but I can only seem to get it to generate a single ‘arm’ of the galaxy.

    public Vector3[] GenerateArm(int numOfStars, int numOfArms, float rotation)
    {
        Vector3[] result = new Vector3[numOfStars];
        Random r = new Random();

        float fArmAngle = (float)((360 / numOfArms) % 360);
        float fAngularSpread = 180 / (numOfArms * 2);

        for (int i = 0; i < numOfStars; i++)
        {

            float fR = (float)r.NextDouble() * 64.0f;
            float fQ = ((float)r.NextDouble() * fAngularSpread) * 1;
            float fK = 1;

            float fA = ((float)r.NextDouble() % numOfArms) * fArmAngle;


            float fX = fR * (float)Math.Cos((MathHelper.DegreesToRadians(fA + fR * fK + fQ)));
            float fY = fR * (float)Math.Sin((MathHelper.DegreesToRadians(fA + fR * fK + fQ)));

            float resultX = (float)(fX * Math.Cos(rotation) - fY * Math.Sin(rotation));
            float resultY = (float)(fY * Math.Cos(rotation) - fX * Math.Sin(rotation));

            result[i] = new Vector3(resultX, resultY, 1.0f);
        }

        return result;
    }

Answer 1

I would abstract that function out into a createArm function.

Then you can store each arm as its own galaxy (temporarily).

So if you want 2 arms, do 2 galaxies of 5000. Then, rotate one of them 0 degrees around the origin (so doesn’t move) and the other 180 degrees around the origin.

With this you can do an arbitrary number of arms by using different rotation amounts. You could even add some “naturalization” to it by making the rotation distance more random, like with a range instead of straight (360 / n). For example, 5 arms would be 0, 72, 144, 216, 288. But with some randomization you could make it 0, 70, 146, 225, 301.

Edit:

Some quick google-fu tells me (source)

q = initial angle, f  = angle of rotation.

x = r cos q
y = r sin q

x' = r cos ( q + f ) = r cos q cos f - r sin q sin f
y' = r sin ( q + w ) = r sin q cos f + r cos q sin f

hence:
x' = x cos f - y sin f
y' = y cos f + x sin f