XNA Tutorial: Smooth Sprite Rotation

Its been a while since my last post. I've bee pretty busy since. You may not have heard but me and my friends have started a games company(Unlucky For Some Studios). So that's been keeping me busy. Any ways a while ago I wrote a tutorial about sprite rotation. Recently I have been experimenting with lerping rotation values so the sprite appears to be rotating smoothly. I have been using the MathHelper.Lerp method to do this.

rotation = MathHelper.Lerp(rotation, nextRotation, 0.06f);

view raw Game1.cs hosted with ❤ by GitHub

Initially I thought this would work perfectly however I was getting a weird behaviour shown in the following video.

I have a pretty good idea as to what is causing this but explaining it would be kinda hard. I have said may times that I am just crap at Maths and was unable to figure out this problem myself. After a very long google search I managed to find the solution. I thought I would write a blog post about it and hopefully it will make it easier for other people to find and fix.

I used the following code that I found on app hub.

	private float CurveAngle(float from, float to, float step)
	{
	if (step == 0) return from;
	if (from == to || step == 1) return to;
	Vector2 fromVector = new Vector2((float)Math.Cos(from), (float)Math.Sin(from));
	Vector2 toVector = new Vector2((float)Math.Cos(to), (float)Math.Sin(to));
	Vector2 currentVector = Slerp(fromVector, toVector, step);
	return (float)Math.Atan2(currentVector.Y, currentVector.X);
	}
	private Vector2 Slerp(Vector2 from, Vector2 to, float step)
	{
	if (step == 0) return from;
	if (from == to || step == 1) return to;
	double theta = Math.Acos(Vector2.Dot(from, to));
	if (theta == 0) return to;
	double sinTheta = Math.Sin(theta);
	return (float)(Math.Sin((1 - step) * theta) / sinTheta) * from + (float)(Math.Sin(step * theta) / sinTheta) * to;
	}

view raw Game1.cs hosted with ❤ by GitHub

After I used these methods I got the following result which was exactly what I originally set out to do. Instead of calling MathHelper.Lerp you now call CurveAngle and pass in the same parameters.

I hope this helps someone as it helped me. Also give our game a go.