Support for non-linear texture as input for the texture combiner.

Make _Metallic attribute 1 if a map is used.

ScriptableRenderPipeline/Core/CoreRP/Editor/TextureCombiner/TextureCombiner.cs

 		return combined;
 	}
 
-	private Texture GetRawTexture (Texture original, bool sRGB = false)
+	private Texture GetRawTexture (Texture original, bool sRGBFallback = false)
 	{
 		if (m_RawTextures == null) m_RawTextures = new Dictionary<Texture, Texture>();
 		if (!m_RawTextures.ContainsKey(original))
 
 				AssetDatabase.ImportAsset(rawPath);
 
-				TextureImporter rawImporter = (TextureImporter) TextureImporter.GetAtPath(rawPath);
+				TextureImporter rawImporter = (TextureImporter) AssetImporter.GetAtPath(rawPath);
 				rawImporter.textureType = TextureImporterType.Default;
 				rawImporter.mipmapEnabled = false;
 				rawImporter.isReadable = true;
-				rawImporter.sRGBTexture = sRGB;
+
+			    Texture2D originalTex2D = original as Texture2D;
+                rawImporter.sRGBTexture = (originalTex2D == null)? sRGBFallback : ( AssetImporter.GetAtPath(AssetDatabase.GetAssetPath(original)) as TextureImporter).sRGBTexture;
+
 				rawImporter.maxTextureSize = 8192;
 
 				rawImporter.textureCompression = TextureImporterCompression.Uncompressed;

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Editor/Material/Lit/StandardsToHDLitMaterialUpgrader.cs

 
             // Metallic
             bool hasMetallic = false;
-            Texture metallicMap;
+            Texture metallicMap = Texture2D.blackTexture;
-                if (hasMetallic) metallicMap = TextureCombiner.GetTextureSafe(srcMaterial, "_MetallicGlossMap", Color.white);
+                if (hasMetallic)
+                {
+                    metallicMap = TextureCombiner.GetTextureSafe(srcMaterial, "_MetallicGlossMap", Color.white);
+                }
+                else
+                {
+                    float metallicValue = Mathf.Pow(srcMaterial.GetFloat("_Metallic"), 2.2f); // Convert _Metallic value from Gamma to Linear
+
+                    dstMaterial.SetFloat("_Metallic", metallicValue);
+                    metallicMap = TextureCombiner.TextureFromColor(Color.white * metallicValue);
+                }
-            else
-                metallicMap = Texture2D.blackTexture;
-            Texture occlusionMap;
+            Texture occlusionMap = Texture2D.whiteTexture;
-            Texture detailMaskMap;
+            Texture detailMaskMap = Texture2D.whiteTexture;
             if (hasDetailMask) detailMaskMap = TextureCombiner.GetTextureSafe(srcMaterial, "_DetailMask", Color.white);
 
             // Smoothness
                 Texture2D maskMap;
 
                 TextureCombiner maskMapCombiner = new TextureCombiner(
-                    TextureCombiner.GetTextureSafe(srcMaterial, "_MetallicGlossMap", Color.white), 4,   // Metallic
-                    TextureCombiner.GetTextureSafe(srcMaterial, "_OcclusionMap", Color.white), 4,       // Occlusion
-                    TextureCombiner.GetTextureSafe(srcMaterial, "_DetailMask", Color.white), 4,         // Detail Mask
-                    smoothnessMap, (srcMaterial.shader.name == Standard_Rough)?-4:3                  // Smoothness Texture
+                    metallicMap, 0,                                                     // R: Metallic from red
+                    occlusionMap, 0,                                                    // G: Occlusion from red
+                    detailMaskMap, 0,                                                   // B: Detail Mask from red
+                    smoothnessMap, (srcMaterial.shader.name == Standard_Rough)?-4:3     // A: Smoothness Texture from inverse greyscale for roughness setup, or alpha
+
+                dstMaterial.SetFloat("_Metallic", 1f);                                  // Force _Metallic value to 1, to use the value stored in the mask map without modification
 
                 string maskMapPath = AssetDatabase.GetAssetPath(srcMaterial);
                 maskMapPath = maskMapPath.Remove(maskMapPath.Length-4) + "_MaskMap.png";