LibGDX - How to rotate one point around another

Newbie & Debugging Questions
#1

Hi guys,

I am really close to solving this issue, but my math skills are lacking so I need some input. I have a player, that player has a separate “arm” object that can swing and show the selected item the player is holding. I have the arm swinging motion working fine but now I am trying to display the selected item in his “hand” swinging around the same origin point that his arm is attached. See the video below for the problem.

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I’ve slowed his swing speed quite a bit to illustrate the problem and there is a noticeable gap between the item and his arm at certain points of the rotation. I think the fact that LibGDX offsets the rectangle of the item by originX and originY is what is messing it up, but I don’t know how to fix it. This code determines the position of the arm and the item:


public void update(float dt)
{
	selectedItem = player.getSelectedItem();

        // This set the position of the arm itself
	position.set(player.position.x + 0.3f, player.position.y + 0.4f);

        // This code sets the items position to rotate around the arm position
        // height is the length of the arm which I'm using as the radius of the "circle"
	itemPosition.set(position.x + height * (float)Math.cos((rotation - 90) * MathUtils.degRad), position.y + height * (float)Math.sin((rotation - 90) * MathUtils.degRad));
		
}

This code renders everything which is where I believe I’m going wrong:


public void render(SpriteBatch sb)
{
        // This one draws the arm and works perfectly.  I change the originX to half width
	sb.draw(region, position.x, position.y, width * 0.5f, height, width, height, 1, 1, rotation);

        // This draws the selected item sprite according to the same rotation and where I believe the issue is occuring
	if(selectedItem != null)
	{
		sb.draw(Game.res.getTextureRegion(selectedItem.getID()), 
				itemPosition.x, itemPosition.y, 
				0.2f, 0.6f,  // originX and originY - Been playing with these to get close to desired effect
				0.6f, 0.6f,  // width and height of item texture region
				1,1,
				rotation);
		}
	}

I should mention that I am using a Y-DOWN coordinate system with my batch. Really banging my head with this one! Thanks in advance.

#2

The ‘itemPosition’ uses position.xy as the origin, while the SpriteBatch.draw uses the projection’s origin + origin.xy. These are apparently mismatched. What is the SpriteBatch’s origin?

#3

you can do something like this:


Vector2 baseArmPos = new Vector2(1f, 0f, 0f); // arm is offset 1 unit to the right of player
float armRotation = 45f; // determine this however you need to, we just define it here
Vector2 armPos = new Vector2(baseArmPos).setAngle(armRotation);
Vector2 finalArmPos = new Vector2(playerPos).add(armPos);