The additive flag is only automatically applied for Premultiply Alpha shaders such as Spine/Skeleton
.
If you are using other shaders and settings, you need to use a custom material just for the slots that are additive.
You can use a component like this to apply it with your own additive material of your choice. If you don't have one, you need to make a new Material asset using the same texture but an additive shader.
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using Spine;
using Spine.Unity;
public class SpineAdditiveMaterialSetter : MonoBehaviour {
public Material additiveMaterial;
void Start () {
var skeletonAnimation = GetComponent<SkeletonAnimation>();
var slots = skeletonAnimation.Skeleton.Slots;
foreach (var slot in slots) {
if (slot.Data.BlendMode == BlendMode.Additive)
skeletonAnimation.CustomSlotMaterials.Add(slot, additiveMaterial);
}
}
}
This will cause it to use multiple materials to render your skeleton.