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[RCIX]

I've written some code for a player controlling a space ship:

Code:

                Vector2 currentPositionNormal = this.Position;
                currentPositionNormal.Normalize();

                Vector2 demandedDirectionNormal = _input.LeftStickPosition;
                demandedDirectionNormal.Normalize();

                Vector2 velocity = Body.LinearVelocity;
                float currentRotationRadians = (float)Math.Atan2(velocity.Y, velocity.X);

                float demandedRotationRadians = (float)Math.Atan2(demandedDirectionNormal.Y, demandedDirectionNormal.X);
                float rotationDelta = demandedRotationRadians - currentRotationRadians;
                Vector2 targetVelocity = Vector2.Zero;
                if (rotationDelta < (Info.TurnRate * deltaTime) &&
                    rotationDelta > -(Info.TurnRate * deltaTime))
                {
                    targetVelocity = new Vector2(
                        (float)Math.Cos(demandedRotationRadians),
                        (float)Math.Sin(demandedRotationRadians));
                }
                else if (rotationDelta > Math.PI)
                {
                    targetVelocity = TurnLeft(currentRotationRadians, deltaTime);
                }
                else if (rotationDelta < -(float)Math.PI)
                {
                    targetVelocity = TurnRight(currentRotationRadians, deltaTime);
                }
                else if (rotationDelta > 0)
                {
                    targetVelocity = TurnRight(currentRotationRadians, deltaTime);
                }
                else if (rotationDelta < 0)
                {
                    targetVelocity = TurnLeft(currentRotationRadians, deltaTime);
                }
                targetVelocity *= _input.LeftStickPosition.Length();
                Body.LinearVelocity = targetVelocity;
                Body.Rotation = (float)Math.Atan2(targetVelocity.Y, targetVelocity.X) + (float)(Math.PI / 2); //done because we treat 0 radians as straight up but the engine treats 0 radians as straight right
                Body.AngularVelocity = 0;

However, in the case that the player points the control stick straight left, the ship "vibrates". I know it has something to do with the fact that the transition between negative and positive radians happens at the left, but I'm stumped as to what to patch to fix it. Anyone mind helping?

[Sam]

I don't follow everything that's going on in your code, but I think you can get your rotationDelta value to be neater.  If your demandedRotation is 359 degrees (I know, it's in radians really) and currentRotation is 2 degrees then rotationDelta will come out at 357.  But of course you want the ship to turn -3 degrees rather than an almost full circle.

Code:

function shortestDifferenceBetweenRadianAngles(start:Number, end:Number):Number
{
	var diff:Number = end - start;
	while (diff > onePi)
	{
		diff -= twoPi;
	}
	while (diff < -onePi)
	{
		diff += twoPi;
	}
	return diff;
}

While loops are used instead of just If statements in case it's given crazy angles that need multiple corrections to get into the right range.  If you're regularly getting such angles, it'd probably be better to do that in a smart arithmetic way rather than looping.

[RCIX]

I probably should have been doing that off the bat, thanks!

[dcarrigg]

I don't follow everything that's going on in your code, but I think you can get your rotationDelta value to be neater.  If your demandedRotation is 359 degrees (I know, it's in radians really) and currentRotation is 2 degrees then rotationDelta will come out at 357.  But of course you want the ship to turn -3 degrees rather than an almost full circle.

Code:

function shortestDifferenceBetweenRadianAngles(start:Number, end:Number):Number
{
	var diff:Number = end - start;
	while (diff > onePi)
	{
		diff -= twoPi;
	}
	while (diff < -onePi)
	{
		diff += twoPi;
	}
	return diff;
}

While loops are used instead of just If statements in case it's given crazy angles that need multiple corrections to get into the right range.  If you're regularly getting such angles, it'd probably be better to do that in a smart arithmetic way rather than looping.

Out of curiosity, is there a better/faster way to compute that? One that avoids the two while loops?

[___]

You could avoid crazy angles by just doing angle % 360, couldn't you?

[Glaiel-Gamer]

shortest distance between radian angles would be much better suited treating them as vectors.


Angle between 2 vectors = acos(v1•v2/(|v1||v2|)) (don't need to divide by length if the vectors are normalized, • = dotproduct. v1•v2 = v1.x*v2.x+v1.y*v2.y)

this will get you the angle between 2 vectors.

Now to find the DIRECTIONAL angle between 2 vectors, you multiply that by the sign of the cross product, which is kinda reverse of the dot product.

sign(v1.x*v2.y-v1.y*v2.x)

acos((v1.x*v2.x+v1.y*v2.y)/(v1.length*v2.length)) * sign(v1..x*v2.y-v1.y*v2.x)

^^ that should do the trick

[RCIX]

(aside from the fact that the libraries i use use radians as well) ...aaaand that's why i don't do it that way