Instead of using AdvancedPosition objects with a value for spectatedEntityID, use SpectatorData objects for Spectator Keyframes. This allows us to add more options to Spectator Keyframes.

src/main/java/eu/crushedpixel/replaymod/holders/SpectatorData.java

@@ -0,0 +1,47 @@
+package eu.crushedpixel.replaymod.holders;
+
+import eu.crushedpixel.replaymod.interpolation.AdvancedPositionLinearInterpolation;
+import eu.crushedpixel.replaymod.interpolation.AdvancedPositionSplineInterpolation;
+import eu.crushedpixel.replaymod.interpolation.Interpolation;
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+import lombok.NoArgsConstructor;
+import net.minecraft.client.Minecraft;
+import net.minecraft.entity.Entity;
+
+@Data
+@NoArgsConstructor
+@EqualsAndHashCode(callSuper=true)
+public class SpectatorData extends AdvancedPosition {
+
+    private int spectatedEntityID;
+
+    public SpectatorData(double x, double y, double z, double pitch, double yaw, double roll, int entityID) {
+        super(x, y, z, pitch, yaw, roll);
+        this.spectatedEntityID = entityID;
+    }
+
+    public SpectatorData(int entityID) {
+        this(Minecraft.getMinecraft().theWorld.getEntityByID(entityID));
+    }
+
+    public SpectatorData(Entity e) {
+        super(e.posX, e.posY, e.posZ, e.rotationPitch, e.rotationYaw);
+        this.spectatedEntityID = e.getEntityId();
+    }
+
+    @Override
+    public SpectatorData newInstance() {
+        return new SpectatorData();
+    }
+
+    @Override
+    public Interpolation getCubicInterpolator() {
+        return new AdvancedPositionSplineInterpolation();
+    }
+
+    @Override
+    public Interpolation getLinearInterpolator() {
+        return new AdvancedPositionLinearInterpolation();
+    }
+}