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Query Player getDeltaMovement() from Serverside


Syric

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I'm trying to make a block that is slippery in one direction: its friction depends on the angle of your motion. I've got it to work perfectly for nonplayer entities, but (once again), the player poses a different issue because of siding. My testing indicates that calling getDeltaMovement is giving (0, -1, 0) on serverside when it shouldn't be, and this screws up the calculation. How can I get accurate information here?

    @Override
    public float getFriction(BlockState state, LevelReader level, BlockPos pos, @org.jetbrains.annotations.Nullable Entity entity) {
        if (entity == null) {
            return 0.7F;
        }

        //Get a unit vector in the appropriate direction
        Vec3 axisUnitVector = Vec3.ZERO;
        if (state.getValue(AXIS) == Direction.Axis.X) {
            axisUnitVector = new Vec3(1, 0, 0);
        } else if (state.getValue(AXIS) == Direction.Axis.Z) {
            axisUnitVector = new Vec3(0, 0, 1);
        }
        //The block can't be placed vertically

        //Take the dot product of that vector with a unit vector in the direction of the entity's movement
        //This effectively just returns the cosine of the angle between the entity's vector and the chosen axis
        //i.e. 0 when perpendicular, 1 when parallel, etc.
        Vec3 normalizedMovement = entity.getDeltaMovement().normalize();
        double dotProduct = Math.abs(normalizedMovement.dot(axisUnitVector));

        //Produce a string for reporting
        String vectorString = "(" + StringUtils.truncate(String.valueOf(normalizedMovement.x), 4) + "," + StringUtils.truncate(String.valueOf(normalizedMovement.y), 4) + "," + StringUtils.truncate(String.valueOf(normalizedMovement.z), 4) + ")";


        //Friction ranges from 0.6 to 1 depending on that cosine
        double finalMultiplier = Mth.lerp(dotProduct, 0.6, 1);
        chatPrint("Friction: " + StringUtils.truncate(String.valueOf(finalMultiplier), 5) + ", dot:" + StringUtils.truncate(String.valueOf(dotProduct), 5) + ", vector:" + vectorString + (level.isClientSide() ? "clientside" : "serverside"), (Level) level);
        return (float) finalMultiplier;
    }

Walking on the block in the slippery direction produces this output:

[CHAT] Friction: 0.6, 	dot:0.0, 	vector:(0.0,-1.0,0.0)		serverside
[CHAT] Friction: 0.981, dot:0.953, 	vector:(-0.0,-0.3,-0.9)		clientside
[CHAT] Friction: 0.6, 	dot:0.0, 	vector:(0.0,-1.0,0.0)		serverside
[CHAT] Friction: 0.981, dot:0.952, 	vector:(-0.0,-0.3,-0.9)		clientside
[CHAT] Friction: 0.6, 	dot:0.0, 	vector:(0.0,-1.0,0.0)		serverside
[CHAT] Friction: 0.981, dot:0.952, 	vector:(-0.0,-0.3,-0.9)		clientside
[CHAT] Friction: 0.6, 	dot:0.0, 	vector:(0.0,-1.0,0.0)		serverside
[CHAT] Friction: 0.981, dot:0.952, 	vector:(-0.0,-0.3,-0.9)		clientside
[CHAT] Friction: 0.6, 	dot:0.0, 	vector:(0.0,-1.0,0.0)		serverside
[CHAT] Friction: 0.980, dot:0.952, 	vector:(-0.0,-0.3,-0.9)		clientside
[CHAT] Friction: 0.6, 	dot:0.0, 	vector:(0.0,-1.0,0.0)		serverside
[CHAT] Friction: 0.980, dot:0.952, 	vector:(-0.0,-0.3,-0.9)		clientside
[CHAT] Friction: 0.6, 	dot:0.0, 	vector:(0.0,-1.0,0.0)		serverside
[CHAT] Friction: 0.980, dot:0.952, 	vector:(-0.0,-0.3,-0.9)		clientside

 

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I don't know if you can really answer your question on this one. I can't say I have ever properly understood how this works. 🙂

 

This is kind of an exception to the sides rule (every rule has an exception, including this one!)

Mostly the client calculates the movement for a player and then sends positions to the server, while for everything else it is the server.

But there exceptions to this, e.g. if an entity is being riding by a player, but not for a pig without a carrot on a stick.

There are couple of methods Entity/Mob.isControlledByLocalInstance() and LivingEntity.isEffectiveAi() that get used for some of this logic.

 

The takeaway for you is, I don't think Entity.getDeltaMovement() is very reliable on the server for a player, I don't think the client ever sends its value to the server either.

But it shouldn't really matter to you either? Because the place where it is doing the real calculation you should have a good value. But maybe I am wrong about that?

Edited by warjort

Boilerplate:

If you don't post your logs/debug.log we can't help you. For curseforge you need to enable the forge debug.log in its minecraft settings. You should also post your crash report if you have one.

If there is no error in the log file and you don't have a crash report then post the launcher_log.txt from the minecraft folder. Again for curseforge this will be in your curseforge/minecraft/Install

Large files should be posted to a file sharing site like https://gist.github.com  You should also read the support forum sticky post.

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7 hours ago, warjort said:

But it shouldn't really matter to you either? Because the place where it is doing the real calculation you should have a good value. But maybe I am wrong about that?

Unfortunately, the block is definitely not as slippery as it should be for the player (because the server side is always returning more friction than it should, presumably). The server side is therefore relevant to actual movement, so I do need its calculation to be correct. The client side calculations seem to be relevant as well, and working correctly, but insufficient on their own.

I set server side to always be high friction and client side always low, and the friction the player gets is somewhere in between.

Edit: Switching server side to low friction and client side to high produces the strange effect of a block which has very high friction whenever you aren't touching the keyboard but which flings you at absurd speeds as soon as you attempt to walk on it.

Edited by Syric
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