add webp fallback support at the gltf json level

gltf-json/src/texture.rs

@@ -1,3 +1,4 @@
+use crate::extensions::texture;
 use crate::validation::{Checked, Validate};
 use crate::{extensions, image, Extras, Index};
 use gltf_derive::Validate;
@@ -179,10 +180,7 @@ where
     P: Fn() -> crate::Path,
     R: FnMut(&dyn Fn() -> crate::Path, crate::validation::Error),
 {
-    if cfg!(any(
-        feature = "allow_empty_texture",
-        feature = "EXT_texture_webp"
-    )) {
+    if cfg!(any(feature = "allow_empty_texture",)) {
         if !source_is_empty(source) {
             source.validate(root, path, report);
         }
@@ -207,7 +205,7 @@ pub struct Texture {
 
     /// The index of the image used by this texture.
     #[serde(default = "source_default", skip_serializing_if = "source_is_empty")]
-    pub source: Index<image::Image>,
+    source: Index<image::Image>,
 
     /// Extension specific data.
     #[serde(default, skip_serializing_if = "Option::is_none")]
@@ -220,6 +218,27 @@ pub struct Texture {
     pub extras: Extras,
 }
 
+impl Texture {
+    /// The index of the image used by this texture.
+    pub fn source(&self) -> Index<image::Image> {
+        #[allow(unused_mut)]
+        let mut source = self.source;
+        #[cfg(feature = "EXT_texture_webp")]
+        {
+            if let Some(texture_webp) = &self.extensions {
+                if let Some(texture_webp) = &texture_webp.texture_webp {
+                    // Only use the webp source if the source is not empty
+                    // Otherwise, fallback to whatever was there originally
+                    if !source_is_empty(&texture_webp.source) {
+                        source = texture_webp.source;
+                    }
+                }
+            }
+        }
+        source
+    }
+}
+
 impl Validate for Texture {
     fn validate<P, R>(&self, root: &crate::Root, path: P, report: &mut R)
     where
@@ -230,7 +249,8 @@ impl Validate for Texture {
             .validate(root, || path().field("sampler"), report);
         self.extensions
             .validate(root, || path().field("extensions"), report);
-        source_validate(&self.source, root, || path().field("source"), report);
+
+        source_validate(&self.source(), root, || path().field("source"), report);
     }
 }
 

src/texture.rs

@@ -1,5 +1,3 @@
-use std::error::Error;
-
 use crate::{image, Document};
 
 pub use json::texture::{MagFilter, MinFilter, WrappingMode};
@@ -159,29 +157,10 @@ impl<'a> Texture<'a> {
             .unwrap_or_else(|| Sampler::default(self.document))
     }
 
-    fn source_index(&self) -> usize {
-        let default = self.json.source.value();
-
-        #[cfg(feature = "EXT_texture_webp")]
-        {
-            let opt_texture_webp = self
-                .json
-                .extensions
-                .as_ref()
-                .and_then(|ext| ext.texture_webp.as_ref());
-
-            if let Some(texture_webp) = opt_texture_webp {
-                return texture_webp.source.value();
-            }
-        }
-
-        return default;
-    }
-
     /// Returns the image used by this texture.
     #[cfg(feature = "allow_empty_texture")]
     pub fn source(&self) -> Option<image::Image<'a>> {
-        let index = self.source_index();
+        let index = self.json.source().value();
         if index == u32::MAX as usize {
             None
         } else {
@@ -192,7 +171,10 @@ impl<'a> Texture<'a> {
     /// Returns the image used by this texture.
     #[cfg(not(feature = "allow_empty_texture"))]
     pub fn source(&self) -> image::Image<'a> {
-        self.document.images().nth(self.source_index()).unwrap()
+        self.document
+            .images()
+            .nth(self.json.source().value())
+            .unwrap()
     }
 
     /// Returns extension data unknown to this crate version.