From d07b976d59b053eaf12539c474f4f94243985b84 Mon Sep 17 00:00:00 2001
From: Syoyo Fujita <syoyo@lighttransport.com>
Date: Wed, 28 Apr 2021 19:15:16 +0900
Subject: [PATCH] Do not write negative `skeleton` id(skeleton is not
 assigned). Also ensure `inverseBindMatrices` should not be -2 or other
 negative values.

---
 tiny_gltf.h | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/tiny_gltf.h b/tiny_gltf.h
index 15d738c..0aa9b76 100644
--- a/tiny_gltf.h
+++ b/tiny_gltf.h
@@ -7243,11 +7243,17 @@ static void SerializeGltfScene(Scene &scene, json &o) {
 }
 
 static void SerializeGltfSkin(Skin &skin, json &o) {
-  if (skin.inverseBindMatrices != -1)
-    SerializeNumberProperty("inverseBindMatrices", skin.inverseBindMatrices, o);
-
+  // required
   SerializeNumberArrayProperty<int>("joints", skin.joints, o);
-  SerializeNumberProperty("skeleton", skin.skeleton, o);
+
+  if (skin.inverseBindMatrices >= 0) {
+    SerializeNumberProperty("inverseBindMatrices", skin.inverseBindMatrices, o);
+  }
+
+  if (skin.skeleton >= 0) {
+    SerializeNumberProperty("skeleton", skin.skeleton, o);
+  }
+
   if (skin.name.size()) {
     SerializeStringProperty("name", skin.name, o);
   }