I am working on an entity that functions in a similar way to the arrow. its a projectile that flies in a straight line and gets stuck in blocks / entities. It renders as an item that is oriented in the way the projectile is going.
This all works as expected, but sometimes when I summon my entity  it glitches around: The rotation yaw and pitch just randomly change to other values for like one render tick and then change back. I'm pretty sure Ive got all the partialTicks interpolation right, and the values for rotationyaw/pitch are correct at the end of the entity tick on both client and server. I have debugged that by logging the values. However, in my entityrenderer sometimes the rotation fields values have weird spikes for a single tick.

This means that somethign happens in between the ticks that changes the value somehow and I have no clue what it is. Any ideas?

//ENTITY RENDERER
matrixStack.push();
matrixStack.rotate(Vector3f.YP.rotationDegrees(90 - MathHelper.lerp(partialTicks, entity.prevRotationYaw, entity.rotationYaw)));
matrixStack.rotate(Vector3f.ZP.rotationDegrees(-45 - MathHelper.lerp(partialTicks, entity.prevRotationPitch, entity.rotationPitch)));
ItemRenderer renderer = Minecraft.getInstance().getItemRenderer();
renderer.renderItem(new ItemStack(ModItems.MANA_SHARD.get()), TransformType.GROUND, lightlevel, OverlayTexture.NO_OVERLAY, matrixStack, buffer);
matrixStack.pop();
super.render(entity, entityYaw, partialTicks, matrixStack, buffer, lightlevel);

 

//ENTITY TICK

super.tick();
this.prevPosX = this.getPosX();
this.prevPosY = this.getPosY();
this.prevPosZ = this.getPosZ();
this.prevRotationPitch = this.rotationPitch;
this.prevRotationYaw = this.rotationYaw;

this.ticksAlive++;

if (!inGround) {
	this.ticksInAir++;
	Vec3d motion = this.getMotion();
			
	float f = this.isInWater() ? this.getMotionFactor() : 0.7F;
			
	double x = this.getPosX() + motion.x * f;
	double y = this.getPosY() + motion.y * f;
	double z = this.getPosZ() + motion.z * f;
			
	ProjectileHelper.rotateTowardsMovement(this, 0.2F);

	if (this.isInWater()) {
		for (int i = 0; i < 4; ++i) {
			this.world.addParticle(ParticleTypes.BUBBLE, x - motion.x * 0.25D, y - motion.y * 0.25D, z - motion.z * 0.25D, motion.x, motion.y, motion.z);
		}
	}
			
	Vec3d newMotion = motion.add(this.accelerationX, this.accelerationY, this.accelerationZ);
	this.setMotion(newMotion);
	this.setPosition(x, y, z);

 

Howdy

Perhaps related to these two methods:

  /**
   * Sets a target position for the client to interpolate towards over the next few ticks
   * For normal projectiles, this is important in order to make sure that the client stays in sync with the server
   */
  @OnlyIn(Dist.CLIENT)
  @Override
  public void setPositionAndRotationDirect(double x, double y, double z, float yaw, float pitch, int posRotationIncrements, boolean teleport) {
    boolean isInFlight = this.dataManager.get(IN_FLIGHT_DMP);
    if (isInFlight) return;   // if we are in flight, the client tick will force the position, yaw and pitch every tick so we
    // don't need to set it here; i.e. just ignore it

    // otherwise, update position and rotation to match the server
    this.setPosition(x, y, z);
    this.setRotation(yaw, pitch);
  }

  /**
   * Updates the entity motion clientside, called by packets from the server
   */
  @OnlyIn(Dist.CLIENT)
  @Override
  public void setVelocity(double x, double y, double z) {
    this.setMotion(x, y, z);
  }

For more context, see here:

https://github.com/TheGreyGhost/MinecraftByExample/blob/1-15-2-final/src/main/java/minecraftbyexample/mbe81_entity_projectile/BoomerangEntity.java

 

-TGG