# When a sprite is rotating to follow an angle, how to prevent it from reversing direction after a point?

Below is a section of my code which handles a player's sprite rotating towards an angle after a user touches the screen:

touchState = TouchPanel.GetState();
Vector2 touchPosition;

if (touchState.Count > 0)
{
touchPosition = new Vector2(touchState[0].Position.X, touchState[0].Position.Y);
targetPosition = Math.Atan2(player.Position.X - touchPosition.X, player.Position.Y - touchPosition.Y);

{
}

{
}

}


The problem I'm having is that when the angle goes past a certain point (I believe 3.15 radians?), the logic no longer functions correctly and the sprite reverses direction in a 360 circle until it meets the target position again.

I know there is something I'm missing from this logic, and I can see the problem, but I'm not sure how to approach handling it.

How would I prevent the sprite from reversing direction?

Thanks

EDIT: I tried the solution below but I only receive a positive number from 0-6 for angle_distance, what have I done wrong?

            if (touchState.Count > 0)
{
touchPosition = new Vector2(touchState[0].Position.X, touchState[0].Position.Y);
targetPosition = Math.Atan2(player.Position.X - touchPosition.X, player.Position.Y - touchPosition.Y);
angle_distance = targetPosition - angle_radians + Math.PI;
angle_distance = angle_distance - Math.Floor(angle_distance / 2 / Math.PI) * 2 * Math.PI;

if (angle_distance < 0)
{
}

if (angle_distance > 0)
{
}

}


EDIT: I now realise my mistake, correct solution is below

if (touchState.Count > 0)
{
touchPosition = new Vector2(touchState[0].Position.X, touchState[0].Position.Y);
targetPosition = Math.Atan2(player.Position.X - touchPosition.X, player.Position.Y - touchPosition.Y);

angle_distance = targetPosition - angle_radians + Math.PI;
angle_distance = angle_distance - Math.Floor(angle_distance / 2 / Math.PI) * 2 * Math.PI;

if (angle_distance < Math.PI)
{
}

if (angle_distance > Math.PI)
{
}

}

• It should reverse at exactly 0 or 2 * pi radians – Bálint Nov 7 '17 at 21:41

First, find the signed distance between the two angles:

dist = targetAngle - currentAngle + pi
dist = dist - floor(dist / 2 / pi) * 2 * pi - pi


If it's negative, then subtract from the current angle to reach the other angle in the shortest time, if it's positive, then add to it.

• Thanks very much for your reply, please could you see my edit above for my response? – pytheron Nov 7 '17 at 22:57
• @pytheron of course you get something between 0 and 6.28, it's in radians, it goes from 0 to 2pi – Bálint Nov 8 '17 at 6:49
• Myself, I like the formula (toAngle - fromAngle + 900) % 360 - 180 if both angles are >= -360 (That's (toAngle - fromAngle + 5*pi) % (2*pi) - pi in radians). Or to insure against angles in any range, ((toAngle - fromAngle + 180) % 360 + 360) % 360 - 180 – DMGregory Nov 8 '17 at 11:56
• @DMGregory Yeah, (x % 360 + 360) % 360 is the same as dist - floor(dist / 360) * 360 – Bálint Nov 8 '17 at 12:57
• @Bálint thanks very much again for your reply - I now realise that you meant the angle distance should be > or < than pi, and the sprite rotates correctly – pytheron Nov 9 '17 at 1:11