Boat glitches when going off two block tall step on ice

[Tomasan7]

When you ride a baot on ice and go down over a ledge two or more blocks tall, you lose your forward momentum.

Illustration video in attachments. (there isn't directly two blocks tall ledge, but the boat goes two blocks down in total when it happens)

Recording.2023-06-14.094124.mp4

[TheMeinerLP]

Can you create a code to replecate that more easy?

[Tomasan7]

Yes, but unfortunately no sooner than 7/15, I am currently without pc for some time.

[TheMeinerLP]

Yes, but unfortunately no sooner than 7/15, I am currently without pc for some time.

Thats fine

[Tomasan7]

Here I have the example code. I hope that it's okay that it is not a repository and is written in Kotlin 🙂
Just run it, place a boat on the eleveted area, and ride it down the step, you'll see, that the boat loses it's forward momentum.

import net.minestom.server.MinecraftServer
import net.minestom.server.entity.*
import net.minestom.server.event.player.PlayerBlockInteractEvent
import net.minestom.server.event.player.PlayerLoginEvent
import net.minestom.server.event.player.PlayerPacketEvent
import net.minestom.server.instance.block.Block
import net.minestom.server.item.ItemStack
import net.minestom.server.item.Material
import net.minestom.server.network.packet.client.play.ClientSteerVehiclePacket
import kotlin.experimental.and
import kotlin.random.Random

fun main()
{
    val minecraftServer = MinecraftServer.init()
    val instanceManager = MinecraftServer.getInstanceManager()
    val instance = instanceManager.createInstanceContainer()

    instance.setGenerator { unit ->
        val bottom = unit.absoluteStart().y().toInt()
        if (Random.nextBoolean())
            unit.modifier().fillHeight(bottom, bottom + 3, Block.PACKED_ICE)
        else
            unit.modifier().fillHeight(bottom, bottom + 1, Block.PACKED_ICE)
    }

    val globalEventHandler = MinecraftServer.getGlobalEventHandler()

    globalEventHandler.addListener(PlayerLoginEvent::class.java) { event ->
        event.setSpawningInstance(instance)
        event.player.gameMode = GameMode.CREATIVE
        event.player.inventory.setItemInHand(Player.Hand.MAIN, ItemStack.of(Material.OAK_BOAT))
    }

    globalEventHandler.addListener(PlayerBlockInteractEvent::class.java) { event ->
        if (event.player.inventory.itemInMainHand.material() != Material.OAK_BOAT)
            return@addListener

        val boat = Entity(EntityType.BOAT)
        boat.setInstance(instance, event.blockPosition.add(0.0, 1.0, 0.0))
        boat.addPassenger(event.player)
    }

    globalEventHandler.addListener(PlayerPacketEvent::class.java) { event ->
        val packet = event.packet

        if (packet !is ClientSteerVehiclePacket)
            return@addListener

        if (packet.flags and 0x2 == 0.toByte())
            return@addListener

        val player = event.player
        val vehicle = player.vehicle
        vehicle?.removePassenger(player)
        vehicle?.remove()
        player.teleport(player.position.add(0.0, 1.0, 0.0))
    }

    minecraftServer.start("localhost", 25565)
}

[TheMeinerLP]

Kotlin its fine. I work also all day with it. Thx

[Tomasan7]

Any update?

[TheMeinerLP]

Not yet i need still testing. Sorry for no update. I am on it 😅

[TheMeinerLP]

@Tomasan7 I tested it on our fork Microtus. I can't replicated.
Can you test maybe our fork and confirm it if it on your server works ?

[Tomasan7]

Sure, I'll let you know tomorrow

[TheMeinerLP]

Thanks

[Tomasan7]

Still the same issue.
Repo: https://github.com/Tomasan7/MinestomBoatGlitch

javaw_NwRSjnE2UH.mp4

[Tomasan7]

Vanilla behaviour:

javaw_ZwwQ5MHe94.mp4

[TheMeinerLP]

I have the vanilla behavior on my server. I tested yesterday

[Tomasan7]

Have you cloned the repo I provided?
Do you use vanilla client?
Otherwise I don't know what could be wrong. It was happening to me on 2 different computers across 2 different versions.

[TheMeinerLP]

Do you use vanilla client?

Only no modifications.

Have you cloned the repo I provided?

Yes

[Tomasan7]

Is anyone else here able to reproduce this bug?

[FluxCapacitor2]

From looking at decompiled Fabric sources, it seems like boat physics are entirely handled by client prediction. When a boat is moving around horizontally on ice, the server thinks its velocity is 0 (at least on my Minestom server).

Because of this, when it tries to apply gravity, it sends a velocity packet with the current horizontal velocity (which the server thinks is 0) and a modified vertical component based on the acceleration of gravity. This makes the boat stop moving horizontally on the client.

Here's a simple fix: BlueDragonMC@04f0919

It's definitely not the best way to fix the issue, but it has worked for me so far.

You could also make a custom Boat entity and prevent all velocity packets from being sent:

// in a custom Entity subclass
    override fun sendPacketToViewers(packet: SendablePacket) {
        if (packet is EntityVelocityPacket) return
        super.sendPacketToViewers(packet)
    }

However, this solution isn't localized to the entity's automatic velocity handling, so it could prevent you from sending velocity packets in scenarios where you'd want to.