Fix noncomformant behavior by not requiring the source field to be present in textures.

gltf-json/src/texture.rs

@@ -179,7 +179,8 @@ pub struct Texture {
     pub sampler: Option<Index<Sampler>>,
 
     /// The index of the image used by this texture.
-    pub source: Index<image::Image>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub source: Option<Index<image::Image>>,
 
     /// Extension specific data.
     #[serde(default, skip_serializing_if = "Option::is_none")]

src/texture.rs

@@ -157,12 +157,12 @@ impl<'a> Texture<'a> {
             .unwrap_or_else(|| Sampler::default(self.document))
     }
 
-    /// Returns the image used by this texture.
-    pub fn source(&self) -> image::Image<'a> {
-        self.document
-            .images()
-            .nth(self.json.source.value())
-            .unwrap()
+    /// Returns the image used by this texture, if any.
+    pub fn source(&self) -> Option<image::Image<'a>> {
+        self.json
+            .source
+            .as_ref()
+            .map(|index| self.document.images().nth(index.value()).unwrap())
     }
 
     /// Returns extension data unknown to this crate version.

tests/blank_texture.gltf

@@ -0,0 +1,8 @@
+{
+  "asset": {
+    "version": "2.0"
+  },
+  "textures": [
+    {}
+  ]
+}

tests/import_sanity_check.rs

@@ -60,7 +60,8 @@ fn run() -> Result<(), Box<dyn StdError>> {
         }
     }
 
-    sparse_accessor_without_buffer_view_test()
+    sparse_accessor_without_buffer_view_test()?;
+    blank_texture_test()
 }
 
 /// Test a file with a sparse accessor with no buffer view
@@ -79,6 +80,16 @@ fn sparse_accessor_without_buffer_view_test() -> Result<(), Box<dyn StdError>> {
     Ok(())
 }
 
+/// Test a file with a texture with neither sampler nor source
+fn blank_texture_test() -> Result<(), Box<dyn StdError>> {
+    let gltf_path = path::Path::new("tests/blank_texture.gltf");
+    print!("{:?}: ", gltf_path);
+    let result = gltf::import(gltf_path)?;
+    sanity_check(&result.0, &result.1, &result.2);
+    println!("ok");
+    Ok(())
+}
+
 #[test]
 fn import_sanity_check() {
     assert!(run().is_ok());