[1.7.10] How to detect if an entity is moving?

[TheRealMcrafter]

Hey guys, I want to check if an entity is moving from inside my Tile Entity updateEntity method. I have already checked if entity.prevPosX != entity.posX, I've tried using entity.motionX, and I've tried storing the old coordinates in a variable and checking against them. Entity.motion* only works if the player is sprinting, and all of the other methods I've tried don't work. Does anyone know how to do this?

[coolAlias]

Entity.motionX/Y/Z should work fine. Are you checking on the server, the client, or both? How are you getting the player?

 

Show some code.

[TheRealMcrafter]

I'm checking on the server, and it is not just for the player, it is for any living entity. I am making a motion detector, so if any entity in the list moves, isDetected turns to true.

 

 

 if (!worldObj.isRemote){
		List list = this.worldObj.getEntitiesWithinAABB(EntityLivingBase.class, AxisAlignedBB.getBoundingBox(this.xCoord - 10, this.yCoord - 10, this.zCoord - 10, this.xCoord + 10, this.yCoord + 10, this.zCoord + 10));
		if (list.isEmpty()){
			this.isDetecting = false;
		} else {
			for (int i = 0; i < list.size(); i ++){
				EntityLivingBase entity = (EntityLivingBase) list.get(i);

				if (entity.motionX != 0){
					this.isDetecting = true;
				} else {
					this.isDetecting = false;
				}					
			}
		}

 

Right now I'm focusing only on motionX, but it doesn't work for Y and Z also. A System.out.println() always prints out 0.0

[TheRealMcrafter]

Does anyone have a workaround for this? It appears coolAlias had troubles with this before, did you ever find a way to do it?

 

http://www.minecraftforge.net/forum/index.php?topic=13721.0

[jabelar]

Does anyone have a workaround for this? It appears coolAlias had troubles with this before, did you ever find a way to do it?

 

http://www.minecraftforge.net/forum/index.php?topic=13721.0

 

In that thread, TheGreyGhost gives the solution. Create your own field to remember the previous tick's position. Then each tick you compare current position to that previous position, do what you need to do, and then always update the previous position field with the current position.

[TheRealMcrafter]

How could I do this when I have more than one entity to track, and I dont know how many entities that are being tracked? An ArrayList?

[jabelar]

How could I do this when I have more than one entity to track, and I dont know how many entities that are being tracked? An ArrayList?

 

Yes, that would work.

 

I think I would do it like this in your tile entity:

1) create a hashmap field for mapping entities to their previous position

2) scan for entities in a bounding box around the tile entity with a getEntititesWithinAABB() type function and store that in a List or ArrayList.

3) update the hashmap keys by removing any entities that are no longer within range, and adding any new ones in range.

4) process the hashmap values (representing the previous position) as I mentioned above -- i.e. compare with current value, take action as needed.

5) update the hashmap values by setting the previous position to value of current position.

 

Note that you can store all three dimensions (x, y, z) of a position in a Vec3 so your hashmap could map entity to Vec3.

 

Definitely a bit of coding needed with this approach, but it should work.

[TheRealMcrafter]

Or just use IExtendedEntityProperties. It takes care of all the loading and unloading for you.

 

How would I go about doing this?

[SanAndreaP]

By the way, this is bad:

entity.motionX != 0

since motion fields are all floats, they suffer from approximation errors. See here, it is a really good article on that topic: http://www.theregister.co.uk/2006/08/12/floating_point_approximation/

So I'd suggest you check for "approximately" 0, like

entity.motionX > 0.0001 || entity.motionX < -0.0001

[TheRealMcrafter]

By the way, this is bad:

entity.motionX != 0

since motion fields are all floats, they suffer from approximation errors. See here, it is a really good article on that topic: http://www.theregister.co.uk/2006/08/12/floating_point_approximation/

So I'd suggest you check for "approximately" 0, like

entity.motionX > 0.0001 || entity.motionX < -0.0001

 

Yeah, I just quickly did that to show my point. I figured that out when I was testing for motionY, which is about 1.17. Thanks for reminding me though.