Ignore ambient texture if it is the same as the diffuse texture

lib/loadMtl.js

@@ -165,6 +165,11 @@ function loadMtl(mtlPath, options) {
     }
 
     function loadMaterialTextures(material) {
+        if (material.diffuseTexture === material.ambientTexture) {
+            // OBJ models are often exported with the same texture in the diffuse and ambient slots but this is typically not desirable, particularly
+            // when saving with PBR materials where the ambient texture is treated as the occlusion texture.
+            material.ambientTexture = undefined;
+        }
         loadMaterialTexture(material, 'diffuseTexture', diffuseTextureOptions, mtlDirectory, texturePromiseMap, texturePromises, options);
         loadMaterialTexture(material, 'ambientTexture', ambientTextureOptions, mtlDirectory, texturePromiseMap, texturePromises, options);
         loadMaterialTexture(material, 'emissiveTexture', emissiveTextureOptions, mtlDirectory, texturePromiseMap, texturePromises, options);

specs/data/box-diffuse-ambient-same/box-diffuse-ambient-same.mtl

@@ -0,0 +1,14 @@
+# Blender MTL File: 'box.blend'
+# Material Count: 1
+
+newmtl Material
+Ns 96.078431
+Ka 0.000000 0.000000 0.000000
+Kd 0.640000 0.640000 0.640000
+Ks 0.500000 0.500000 0.500000
+Ke 0.000000 0.000000 0.000000
+Ni 1.000000
+d 1.000000
+illum 2
+map_Kd cesium.png
+map_Ka cesium.png

specs/data/box-diffuse-ambient-same/box-diffuse-ambient-same.obj

@@ -0,0 +1,46 @@
+# Blender v2.78 (sub 0) OBJ File: 'box.blend'
+# www.blender.org
+mtllib box-diffuse-ambient-same.mtl
+o Cube
+v -1.000000 -1.000000 1.000000
+v -1.000000 1.000000 1.000000
+v -1.000000 -1.000000 -1.000000
+v -1.000000 1.000000 -1.000000
+v 1.000000 -1.000000 1.000000
+v 1.000000 1.000000 1.000000
+v 1.000000 -1.000000 -1.000000
+v 1.000000 1.000000 -1.000000
+vt 0.0000 0.0000
+vt 1.0000 0.0000
+vt 1.0000 1.0000
+vt 0.0000 1.0000
+vt 0.0000 0.0000
+vt 1.0000 0.0000
+vt 1.0000 1.0000
+vt 0.0000 1.0000
+vt 0.0000 0.0000
+vt 1.0000 0.0000
+vt 1.0000 1.0000
+vt 0.0000 1.0000
+vt 0.0000 0.0000
+vt 1.0000 0.0000
+vt 1.0000 1.0000
+vt 0.0000 1.0000
+vt 1.0000 0.0000
+vt 1.0000 1.0000
+vt 0.0000 0.0000
+vt 0.0000 1.0000
+vn -1.0000 0.0000 0.0000
+vn 0.0000 0.0000 -1.0000
+vn 1.0000 0.0000 0.0000
+vn 0.0000 0.0000 1.0000
+vn 0.0000 -1.0000 0.0000
+vn 0.0000 1.0000 0.0000
+usemtl Material
+s off
+f 1/1/1 2/2/1 4/3/1 3/4/1
+f 3/5/2 4/6/2 8/7/2 7/8/2
+f 7/9/3 8/10/3 6/11/3 5/12/3
+f 5/13/4 6/14/4 2/15/4 1/16/4
+f 3/5/5 7/17/5 5/18/5 1/16/5
+f 8/19/6 4/6/6 2/15/6 6/20/6

specs/lib/loadMtlSpec.js

@@ -17,6 +17,7 @@ var externalMaterialPath = 'specs/data/box-external-resources/box-external-resou
 var resourcesInRootMaterialPath = 'specs/data/box-resources-in-root/box-resources-in-root.mtl';
 var externalInRootMaterialPath = 'specs/data/box-external-resources-in-root/box-external-resources-in-root.mtl';
 var transparentMaterialPath = 'specs/data/box-transparent/box-transparent.mtl';
+var diffuseAmbientSameMaterialPath = 'specs/data/box-diffuse-ambient-same/box-diffuse-ambient-same.mtl';
 
 var diffuseTexturePath = 'specs/data/box-textured/cesium.png';
 var transparentDiffuseTexturePath = 'specs/data/box-complex-material/diffuse.png';
@@ -251,6 +252,17 @@ describe('loadMtl', function() {
             }), done).toResolve();
     });
 
+    it('ambient texture is ignored if it is the same as the diffuse texture', function(done) {
+        expect(loadMtl(diffuseAmbientSameMaterialPath, options)
+            .then(function(materials) {
+                expect(materials.length).toBe(1);
+                var material = materials[0];
+                var pbr = material.pbrMetallicRoughness;
+                expect(pbr.baseColorTexture).toBeDefined();
+                expect(pbr.occlusionTexture).toBeUndefined();
+            }), done).toResolve();
+    });
+
     describe('metallicRoughness', function() {
         it('creates default material', function() {
             var material = loadMtl._createMaterial(undefined, options);