GSOC 26: apply mtl material state to single-material models

src/webgl/loading.js

@@ -190,10 +190,21 @@ async function loadMaterialTextures(materials, modelPath, instance) {
 // as the aggregate; each part gets its own localised verts with faces re-indexed
 // against them, plus its material's state.
 function buildMaterialParts(model, faceMaterials, materials) {
-  // only split when there are genuinely multiple materials. a single material
-  // (or none) stays as the geometry's own part and renders as before. one group
-  // per material, plus a null group for faces before any usemtl so none drop.
+  // one group per material, plus a null group for faces before any usemtl so
+  // none drop.
   const names = [...new Set(faceMaterials)];
+
+  // one material covering every face. the geometry is already its own part, so
+  // hand it the state directly: splitting would duplicate every vertex to say
+  // the same thing, and would stop parts[0] being the geometry itself. without
+  // this a single material model never receives its maps at all.
+  if (names.length === 1 && names[0] != null) {
+    Object.assign(model.partState, mtlToPartState(materials[names[0]]));
+    return;
+  }
+
+  // nothing to split on: no materials, or one material alongside faces that
+  // were declared before any usemtl and so have none.
   if (names.filter(name => name != null).length < 2) return;
 
   const hasUvs = model.uvs.length > 0;

test/unit/assets/single_material_textured.mtl

@@ -0,0 +1,5 @@
+newmtl only
+Kd 1.000 1.000 1.000
+Ks 0.500 0.500 0.500
+Ns 60
+map_Kd cat.jpg

test/unit/assets/single_material_textured.obj

@@ -0,0 +1,12 @@
+mtllib single_material_textured.mtl
+v 0 0 0
+v 1 0 0
+v 1 1 0
+v 0 1 0
+vt 0 0
+vt 1 0
+vt 1 1
+vt 0 1
+usemtl only
+f 1/1 2/2 3/3
+f 1/1 3/3 4/4

test/unit/io/loadModel.js

@@ -159,6 +159,25 @@ suite('loadModel', function () {
     assert.equal(model.parts[0], model, 'the geometry is its own single part');
   });
 
+  test('a single-material OBJ still receives its maps', async function () {
+    const fakeImage = { width: 1, height: 1 };
+    mockP5Prototype.loadImage = async () => fakeImage;
+    try {
+      const model = await mockP5Prototype.loadModel(
+        '/test/unit/assets/single_material_textured.obj'
+      );
+      // one material, so no split: the geometry stays its own only part
+      assert.equal(model.parts.length, 1);
+      assert.equal(model.parts[0], model);
+      // and it carries the material's state rather than dropping it
+      assert.equal(model.partState.texture, fakeImage);
+      assert.equal(model.partState.shininess, 60);
+      assert.deepEqual(model.partState.specularColor, [0.5, 0.5, 0.5]);
+    } finally {
+      delete mockP5Prototype.loadImage;
+    }
+  });
+
   test('a 12-material OBJ splits into 12 parts', async function () {
     const model = await mockP5Prototype.loadModel(
       '/test/unit/assets/multi_material_12.obj'