Entity interpolation for high speed projectiles is wildly inaccurate

Whilst testing arrows launched at high speeds I’ve noticed that the motion of the arrows seems really strange. After investigating further I have found there are wild inaccuracies in the way projectiles are rendered to the client each frame. This appears to only affect projectile entities and not other types of entities like TnT.

I have included a video clearly demonstrating the issue. In the video the green hitbox represents the actual position of the projectile entity, whilst the white hitbox follows the entity as it is rendered for the client. For very low speeds of around 1 block per tick its not very noticable, but once you start getting to higher speeds the discrepancy becomes very pronounced.

To reproduce the setup you can use the following commands:

summon minecraft:arrow ~ ~1 ~ {Motion:[0.0,1.0,0.0]}

summon minecraft:arrow ~ ~1 ~ {Motion:[0.1,1.0,0.0]}

summon minecraft:arrow ~ ~1 ~ {Motion:[0.0,10.0,0.0]}

summon minecraft:arrow ~ ~1 ~ {Motion:[1.0,10.0,0.0]}

Thanks to Velizar we now know exactly what part of the code is causing this issue. When the game goes to compile entity data to transmitto the client. It clamps the motion values to a range of [-3.9,3.9] and multiplies by 8000 to ensure the data is transmitted in 32 signed integer format. This seems like a completely unnecessary networking optimization that just detracts from the visual accuracy of gameplay features. Included is the code that causes this issue.

package net.minecraft.network.protocol.game;

import net.minecraft.network.FriendlyByteBuf;
import net.minecraft.network.codec.StreamCodec;
import net.minecraft.network.protocol.Packet;
import net.minecraft.network.protocol.PacketType;
import net.minecraft.util.Mth;
import net.minecraft.world.entity.Entity;
import net.minecraft.world.phys.Vec3;

public class ClientboundSetEntityMotionPacket implements Packet<ClientGamePacketListener> {
	public static final StreamCodec<FriendlyByteBuf, ClientboundSetEntityMotionPacket> STREAM_CODEC = Packet.codec(
		ClientboundSetEntityMotionPacket::write, ClientboundSetEntityMotionPacket::new
	);
	private final int id;
	private final int xa;
	private final int ya;
	private final int za;

	public ClientboundSetEntityMotionPacket(Entity entity) {
		this(entity.getId(), entity.getDeltaMovement());
	}

	public ClientboundSetEntityMotionPacket(int i, Vec3 vec3) {
		this.id = i;
		double d = 3.9;
		double e = Mth.clamp(vec3.x, -3.9, 3.9);
		double f = Mth.clamp(vec3.y, -3.9, 3.9);
		double g = Mth.clamp(vec3.z, -3.9, 3.9);
		this.xa = (int)(e * 8000.0);
		this.ya = (int)(f * 8000.0);
		this.za = (int)(g * 8000.0);
	}

	private ClientboundSetEntityMotionPacket(FriendlyByteBuf friendlyByteBuf) {
		this.id = friendlyByteBuf.readVarInt();
		this.xa = friendlyByteBuf.readShort();
		this.ya = friendlyByteBuf.readShort();
		this.za = friendlyByteBuf.readShort();
	}

	private void write(FriendlyByteBuf friendlyByteBuf) {
		friendlyByteBuf.writeVarInt(this.id);
		friendlyByteBuf.writeShort(this.xa);
		friendlyByteBuf.writeShort(this.ya);
		friendlyByteBuf.writeShort(this.za);
	}

	@Override
	public PacketType<ClientboundSetEntityMotionPacket> type() {
		return GamePacketTypes.CLIENTBOUND_SET_ENTITY_MOTION;
	}

	public void handle(ClientGamePacketListener clientGamePacketListener) {
		clientGamePacketListener.handleSetEntityMotion(this);
	}

	public int getId() {
		return this.id;
	}

	public double getXa() {
		return this.xa / 8000.0;
	}

	public double getYa() {
		return this.ya / 8000.0;
	}

	public double getZa() {
		return this.za / 8000.0;
	}
}