Does anyone know how I would rotate an object to look at a point (xyz entity location) in my TESR? I've gotten my 3 axis rotating perfectly when I only do one of them, but when I try to do all 3 of them, it doesn't work at all. (I'm using GlStateManager.rotate(angle, x, y, z) rather that Quaternions because I just cannot understand them). I'm relatively new to rendering, and haven't rendered anything more complex than ItemBlocks & Models in my TESRs before. Any help would be greatly appreciated.

Link to the file in my GitHub: https://github.com/Cadiboo/WIPTech/blob/master/src/main/java/cadiboo/wiptech/client/render/tileentity/TESRWire.java

Same code but with most comments removed

	@Override
	public void render(TileEntityWire tileEntity, double x, double y, double z, float partialTicks, int destroyStage, float alpha) {
		
      	if (tileEntity.electrocutionTime > 0) {
      		
			double dX; //entity xy&z - set later
			double dY;
			double dZ;
			final int NumberOfBranches = 1; //for rendering the electricity
			final int NumberOfPossibleSubBranches = 2;
		
			List<Entity> entities = tileEntity.getAllEntitiesWithinRangeAt(tileEntity.getPos().getX(), tileEntity.getPos().getY(), tileEntity.getPos().getZ(), 113);
			for (int i = 0; i < entities.size(); i++) {
              
            	GlStateManager.pushMatrix();
				GlStateManager.translate(x + 0.5, y + 0.5, z + 0.5);
              
              	dX = entities.get(i).posX - tileEntity.getPos().getX() - 0.5;
				dY = entities.get(i).posY - tileEntity.getPos().getY() - 0.5;
				dZ = entities.get(i).posZ - tileEntity.getPos().getZ() - 0.5;
              
              
              	//Y rotation
              	double aa = dX;
				double bb = dZ;
				double cc = Math.sqrt(aa * aa + bb * bb);

				double angle = 180F / Math.PI * Math.acos((bb * bb - (-cc * cc) - aa * aa) / (2 * bb * cc));
              
             	GlStateManager.rotate(90F + (float) angle * (aa < 0 ? -1 : 1), 0, 1, 0);
              
              	//X rotation
              	aa = dY;
				bb = dX;
				cc = Math.sqrt(aa * aa + bb * bb);

				angle = 180F / Math.PI * Math.acos((bb * bb - (-cc * cc) - aa * aa) / (2 * bb * cc));

				//GlStateManager.rotate(70F + (float) angle * (aa < 0 ? -1 : 1), 0, 0, 1);
              
              	//Z rotation
              
              	aa = dY;
				bb = dZ;
				cc = Math.sqrt(aa * aa + bb * bb);

				angle = 180F / Math.PI * Math.acos((bb * bb - (-cc * cc) - aa * aa) / (2 * bb * cc));

				GlStateManager.rotate(270F + (float) angle * (aa < 0 ? 1 : -1), 1, 0, 0);
              
              	//... render electricity
              
              GlStateManager.popMatrix();
              
            }
		}
      //... render wires
    }

 

Edited October 16, 20187 yr by Cadiboo

I think your problem here is that rotations are fundamentally a 4-axis object and you're trying to piecemeal it with three. Yes, Euler Angles are a Thing, but they suffer from gimbal lock.

 

This is the closest thing I can find that has the right math:

https://stackoverflow.com/questions/14720264/create-a-rotation-matrix-from-a-position-and-a-look-at-position

17 hours ago, Draco18s said:

I think your problem here is that rotations are fundamentally a 4-axis object and you're trying to piecemeal it with three. Yes, Euler Angles are a Thing, but they suffer from gimbal lock.

 

This is the closest thing I can find that has the right math:

https://stackoverflow.com/questions/14720264/create-a-rotation-matrix-from-a-position-and-a-look-at-position

What is the 4th axis??? (time aha?)

Its not gimbal lock so much as that when I rotate on 1 axis the other 2 axis rotate with it. This is as everything should be, but for my code I NEED them not to rotate (or better maths). I also don't understand Matrixes (Matricies??) or Quaternions at all and I don't really understand vectors very well either. I've set myself to fail pretty badly

4 hours ago, Cadiboo said:

What is the 4th axis???

Feel free to read up: https://en.wikipedia.org/wiki/Quaternion

4 hours ago, Cadiboo said:

when I rotate on 1 axis the other 2 axis rotate with it.

This is expected for one of two reasons:

1) GL deals with rotation as a quaternion and when you make an adjustment in Euler, the end result is not neccessarily the same Euler (to avoid gimbal lock)

2) Rotation around a rotated axis changes all of the Euler values from a non-rotated reference frame.

9 hours ago, Draco18s said:

Feel free to read up: https://en.wikipedia.org/wiki/Quaternion

This is expected for one of two reasons:

1) GL deals with rotation as a quaternion and when you make an adjustment in Euler, the end result is not neccessarily the same Euler (to avoid gimbal lock)

2) Rotation around a rotated axis changes all of the Euler values from a non-rotated reference frame.

Thanks! The maths for quaternions is over my head for the moment (I’m trying to fix this). You specified that because I was using Euler angles the axis were also rotated, do you know (off the top of your head) if there is a way use quaternions to avoid this?

*I* don't understand quaternions. I just know what they're good for.

2 minutes ago, Draco18s said:

*I* don't understand quaternions. I just know what they're good for.

All good thanks for the help!

After ≈ 6 days of solid work I have made it work, I hope this helps someone.

you find your y rotation through Math.atan2(originX - pointX, originZ - pointZ);

and your up down (z/x) rotation through Math.atan2(originY - pointY, Math.sqrt(originX * originX + originZ * originZ) - Math.sqrt(pointX * pointX + pointZ * pointZ);

 

then you convert both from radians to degrees.

then you rotate around your y axis by your y rotation

GlStateManager.rotate((float) yAngle + 90, 0, 1, 0);

 

then you rotate around your x or z angle (I chose z because a) your normal rotation order is Y, Z, X and b) I was already using x for rotating my object 180 degrees from straight down to straight up)

GlStateManager.rotate(90 - (float) _Angle, 0, 0, 1);

 

I really hope this helps someone.

Here is the full java code that I used

 

Entity Position code (removes render shake)

Spoiler

double flyingMultiplier = 1.825;

double yFlying = 1.02;

double yAdd = 0.0784000015258789;

 

if (entities.get(i) instanceof EntityPlayer) {

if (((EntityPlayer) entities.get(i)).capabilities.isFlying) {

flyingMultiplier = 1.1;

yFlying = 1.67;

yAdd = 0;

}

}

 

double yGround = entities.get(i).motionY + yAdd == 0 && entities.get(i).prevPosY > entities.get(i).posY ? entities.get(i).posY - entities.get(i).prevPosY : 0;

double xFall = 1;

if (flyingMultiplier == 1.825) {

if (entities.get(i).motionX != 0) {

if (entities.get(i).motionY + yAdd != 0) {

xFall = 0.6;

} else if (yGround != 0) {

xFall = 0.6;

}

} else {

xFall = 0.6;

}

}

 

double zFall = 1;

if (flyingMultiplier == 1.825) {

if (entities.get(i).motionZ != 0) {

if (entities.get(i).motionY + yAdd != 0) {

zFall = 0.6;

} else if (yGround != 0) {

zFall = 0.6;

}

} else {

zFall = 0.6;

}

}

 

double dX = entities.get(i).posX - (entities.get(i).prevPosX - entities.get(i).posX) * partialTicks - (entities.get(i).motionX * xFall) * flyingMultiplier - tileEntity.getPos().getX() - 0.5;

double dY = entities.get(i).posY - yGround - (entities.get(i).prevPosY - entities.get(i).posY) * partialTicks - (entities.get(i).motionY + yAdd) * yFlying - tileEntity.getPos().getY() - 0.5;

double dZ = entities.get(i).posZ - (entities.get(i).prevPosZ - entities.get(i).posZ) * partialTicks - (entities.get(i).motionZ * zFall) * flyingMultiplier - tileEntity.getPos().getZ() - 0.5;

dY += entities.get(i).getEyeHeight();

 

Rotation code

//get Y angle
double yAngle = Math.atan2(0 - dX, 0 - dZ);
//convert angle to degrees
yAngle = yAngle * (180 / Math.PI);
yAngle = yAngle < 0 ? 360 - (-yAngle) : yAngle;
//rotate by angle
GlStateManager.rotate((float) yAngle + 90, 0, 1, 0);
//rotate object (It starts facing down, I want it to start facing up)
GlStateManager.rotate(180, 1, 0, 0);

//get Up/Down (X/Z) angle
double _Angle = Math.atan2(dY, Math.sqrt(dX * dX + dZ * dZ));
//convert angle to degrees
_Angle = _Angle * (180 / Math.PI);
_Angle = _Angle < 0 ? 360 - (-_Angle) : _Angle;
//rotate by angle
GlStateManager.rotate(90 - (float) _Angle, 0, 0, 1);

//...draw object

 

GitHub perma-link (High-accuracy entity positioning and object rotation)

https://github.com/Cadiboo/WIPTech/blob/1bc15df55251b6631d31b6d101ae5a729ed2a4ca/src/main/java/cadiboo/wiptech/client/render/tileentity/TESRWire.java#L77-L126

Edited May 7, 20187 yr by Cadiboo
added GitHub perma-link