Picture this. You're holding a chicken in your hands. You turn around, but the chicken stays in the same spot.

So, now the chicken is literally floating behind you.

 

That's the problem I'm having right now. I'm heavily assuming trigonometry is involved with a solution to this. I've looked at the Item.java class to see how an item follows the player's hand.

 

It's been hours, and I've been staring at this code, but I can't seem to get it to work.

Taken from the Item.java Class

   protected MovingObjectPosition getMovingObjectPositionFromPlayer(World worldIn, EntityPlayer playerIn, boolean useLiquids)
    {
        float f = playerIn.rotationPitch;
        float f1 = playerIn.rotationYaw;
        double d0 = playerIn.posX;
        double d1 = playerIn.posY + (double)playerIn.getEyeHeight();
        double d2 = playerIn.posZ;
        Vec3 vec3 = new Vec3(d0, d1, d2);
        float f2 = MathHelper.cos(-f1 * 0.017453292F - (float)Math.PI);
        float f3 = MathHelper.sin(-f1 * 0.017453292F - (float)Math.PI);
        float f4 = -MathHelper.cos(-f * 0.017453292F);
        float f5 = MathHelper.sin(-f * 0.017453292F);
        float f6 = f3 * f4;
        float f7 = f2 * f4;
        double d3 = 5.0D;
        if (playerIn instanceof net.minecraft.entity.player.EntityPlayerMP)
        {
            d3 = ((net.minecraft.entity.player.EntityPlayerMP)playerIn).theItemInWorldManager.getBlockReachDistance();
        }
        Vec3 vec31 = vec3.addVector((double)f6 * d3, (double)f5 * d3, (double)f7 * d3);
        return worldIn.rayTraceBlocks(vec3, vec31, useLiquids, !useLiquids, false);
    }

 

My code:

					        float f1 = thePlayer.rotationYaw;
					        double d1 = thePlayer.posY + 6;
					        float f = thePlayer.rotationPitch;
					        double d0 = thePlayer.posX;
					        double d2 = thePlayer.posZ;
					        Vec3 vec3 = new Vec3(d0, d1, d2);
					        float f2 = MathHelper.cos(-f1 * 0.017453292F - (float)Math.PI);
					        float f3 = MathHelper.sin(-f1 * 0.017453292F - (float)Math.PI);
					        float f4 = -MathHelper.cos(-f * 0.017453292F);
					        float f5 = MathHelper.sin(-f * 0.017453292F);
					        float f6 = f3 * f4;
					        float f7 = f2 * f4;
					        double d3 = 5.0D;
					        if (thePlayer instanceof net.minecraft.entity.player.EntityPlayerMP)
					        {
					            d3 = ((net.minecraft.entity.player.EntityPlayerMP)thePlayer).theItemInWorldManager.getBlockReachDistance();
					        }
					        Vec3 vec31 = vec3.addVector((double)f6 * d3, (double)f5 * d3, (double)f7 * d3);
						theEntity.setPosition(d0 * (f2 * Math.PI), d1 , d2 * (f3 * -Math.PI));

 

This code did some improvement, but I feel like I'm doing the Math wrong.

 

What am I doing wrong? Please help. Thank you.

Items in a player's hand don't do anything special to stay in their hand as the player rotates.  That's all handled by the GL calls.  Namely, GL rotates the context, then draws the player and their equipment.  All of the trig is hidden away from you.

 

*Digs out some code from a project he's currently working on.*

 

You'll have to convert some stuff, but the math is here.

 

//SpecialBulletLogic is an XML based parameterized system that drive various aspects, should be apparent what things do
public void MeleeShot_UpdatePosition() {
	double distance = this.SpecialBulletLogicOffsetFromFiringEntity; //how far from parent's center;
	double angle = 0;
	if(this.SpecialBulletLogic.RotatesAroundFiringEntity) { //you want this
		angle = MathHelper.SafeAngleAddRadians(angle, this.SpecialBulletLogic.ParentBullet.CurrentAngleRadians); //add parent's angle
	}
	Vector2 effectiveOffset = Vector2.zero;
	if(distance != 0) {
		//calculate rotated offsets
		effectiveOffset.x = (float)(distance * Math.Cos(angle));
		effectiveOffset.y = (float)(distance * Math.Sin(angle));
	}

	Vector2 newLocation = this.SpecialBulletLogic.ParentBullet.WorldLocation;

	newLocation.x += effectiveOffset.x;
	newLocation.y += effectiveOffset.y;

	this.SetWorldLocation(newLocation);
}

Didn't seem to work. It rotates in a circular motion when I move, but the circle is so small and the chicken is like 5 blocks adjacent from me, now. Although, that could be that I'm adding the cosine and sin of the rotationYaw of the player with the posX and posZ of the player. 

You should be taking the player's rotation, getting the sin and cos, and adding that to the player's position.  I don't know why the circle would be small, unless the offset isn't being multiplied by a large enough value (the

double distance = this.SpecialBulletLogicOffsetFromFiringEntity;

line dictates how far away from the player the chicken will be).

						double distance = -3;
						float f1 = thePlayer.rotationYaw;
					        double d1 = thePlayer.posY + 6;
					        double d0 = thePlayer.posX;
					        double d2 = thePlayer.posZ;
					        double f2 = distance * MathHelper.cos(f1);
					        double f3 = distance * MathHelper.sin(f1);
					        double newPosX = d0 + f3;
					        double newPosZ = d2 + f2;
						theEntity.setPosition(newPosX + - 4, d1, newPosZ + 4);

 

 

I'm think I'm doing something wrong.

(newPosX + - 4, d1, newPosZ + 4);

 

Wha?

Oh I was trying to fix the chicken's position with that. Forgot to remove it when the distance value fixed it.

 

But the circle still is small, for some reason. Am I doing it wrong?

Alright, so I found a solution to the circle being too small and stuff by multiplying the rotationYaw with pi/180 and subtracting that by pi, and it creates a proper circle.

 

The thing is, the rotationYaw moves even when my head moves. I want it to only calculate based on my body, not my head, so that if I simply turn my head, the chicken doesnt move out of place.

 

 

Alright, so I found a solution to the circle being too small and stuff by multiplying the rotationYaw with pi/180

 

Mm, degrees and radains. Don't know why you had to subtract pi though.

Multiplying it by pi/180 alone would put the entity across the angle behind me. Like if you would look on a unit circle, if I wanted the entity to be at pi/2, they'd be at 3pi/2.

 

Subtracting pi from it fixed this.

 

 

EDIT:

 

Added to the rotationYaw and it gave me the desired result. The chicken now stays in my hands.

 

Thank you, Draco18s. Not only for helping me get to this result, but for also being so patient with me. You rock, man.

 

Gotcha.  I figured that was why, but didn't see how the angle of rotation put it behind the player rather than in front.  Huh.