Expose relative parameters to setRotation

paper-api/src/main/java/org/bukkit/entity/Entity.java

@@ -135,6 +135,16 @@ public interface Entity extends Metadatable, CommandSender, Nameable, Persistent
      */
     public void setRotation(float yaw, float pitch);
 
+    /**
+     * Sets the entity's rotation.
+     *
+     * @param yaw the yaw
+     * @param relativeYaw whether the yaw is relative to the current yaw
+     * @param pitch the pitch
+     * @param relativePitch whether the pitch is relative to the current pitch
+     */
+    void setRotation(float yaw, boolean relativeYaw, float pitch, boolean relativePitch);
+
     // Paper start - Teleport API
     /**
      * Teleports this entity to the given location.

paper-api/src/main/java/org/bukkit/entity/Player.java

@@ -3795,6 +3795,20 @@ public int getPing() {
      */
     void setRotation(float yaw, float pitch);
 
+    /**
+     * Set the player's rotation.
+     * <p>
+     * Note: When relative parameters are set, client will do interpolations to
+     * make the rotation smooth, it's different from just send a new rotation with
+     * yaw or pitch addition.
+     *
+     * @param yaw the yaw
+     * @param relativeYaw whether the yaw is relative to the current yaw
+     * @param pitch the pitch
+     * @param relativePitch whether the pitch is relative to the current pitch
+     */
+    void setRotation(float yaw, boolean relativeYaw, float pitch, boolean relativePitch);
+
     /**
      * Causes the player to look towards the given entity.
      *

paper-server/src/main/java/org/bukkit/craftbukkit/entity/CraftEntity.java

@@ -277,6 +277,14 @@ public void setRotation(float yaw, float pitch) {
         this.getHandle().forceSetRotation(yaw, false, pitch, false);
     }
 
+    @Override
+    public void setRotation(float yaw, final boolean relativeYaw, float pitch, final boolean relativePitch) {
+        NumberConversions.checkFinite(pitch, "pitch not finite");
+        NumberConversions.checkFinite(yaw, "yaw not finite");
+
+        this.getHandle().forceSetRotation(yaw, relativeYaw, pitch, relativePitch);
+    }
+
     @Override
     public boolean teleport(Location location) {
         return this.teleport(location, TeleportCause.PLUGIN);

paper-server/src/main/java/org/bukkit/craftbukkit/entity/CraftPlayer.java

@@ -1299,6 +1299,13 @@ public void setRotation(float yaw, float pitch) {
         super.setRotation(yaw, pitch);
     }
 
+    @Override
+    public void setRotation(final float yaw, final boolean relativeYaw, final float pitch, final boolean relativePitch) {
+        if (this.getHandle().connection == null) return;
+
+        super.setRotation(yaw, relativeYaw, pitch, relativePitch);
+    }
+
     @Override
     public void lookAt(org.bukkit.entity.@NonNull Entity entity, @NonNull LookAnchor playerAnchor, @NonNull LookAnchor entityAnchor) {
         this.getHandle().lookAt(toNmsAnchor(playerAnchor), ((CraftEntity) entity).getHandle(), toNmsAnchor(entityAnchor));