fix: gracefully handle mixed-material OBJ models instead of crashing

Previously, `parseObj()` used `hasColoredVertices === hasColorlessVertices`
to detect inconsistent vertex coloring, but this condition is true in two
very different cases:

1. Both flags `false`  → model has no faces at all (should never crash)
2. Both flags `true`   → model has some faces with material colors and
                         some without (the genuine "mixed" case)

Real-world OBJ exports from Blender, Sketchfab, and other tools commonly
produce files where only some mesh groups have an explicit MTL material
assignment. The previous `throw` caused a hard crash for any such model,
which is a significant barrier for beginners trying to use pre-made 3D
assets in p5.js.

This commit changes the check to `hasColoredVertices && hasColorlessVertices`
(only the genuinely inconsistent case) and replaces the thrown error with a
`console.warn` + `model.vertexColors = []` reset, so the model still loads
and renders using the default fill color.

The existing test that expected a throw is updated to assert that the model
loads successfully with an empty vertexColors array.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Nixxx19

src/webgl/loading.js

@@ -652,9 +652,19 @@ function loading(p5, fn){
     if (model.vertexNormals.length === 0) {
       model.computeNormals();
     }
-    if (hasColoredVertices === hasColorlessVertices) {
-      // If both are true or both are false, throw an error because the model is inconsistent
-      throw new Error('Model coloring is inconsistent. Either all vertices should have colors or none should.');
+    if (hasColoredVertices && hasColorlessVertices) {
+      // Mixed model: some faces have a material diffuse color assigned, others do not.
+      // This is common in real-world OBJ exports (e.g. Blender, Sketchfab) where only
+      // some mesh groups have an explicit MTL material. Rather than crashing the sketch,
+      // we degrade gracefully: strip the partial vertex colors so the model renders
+      // with the default fill color instead of corrupted per-vertex coloring.
+      console.warn(
+        'p5.js: This OBJ model has mixed material coloring — some faces have a ' +
+        'material diffuse color and some do not. Vertex colors will not be applied. ' +
+        'Consider assigning a material to every face group in your 3D software, ' +
+        'or use a model where all faces share the same material.'
+      );
+      model.vertexColors = [];
     }
 
     return model;

test/unit/io/loadModel.js

@@ -79,11 +79,18 @@ suite('loadModel', function() {
     assert.deepEqual(model.vertexColors, expectedColors);
   });
 
-  test('inconsistent vertex coloring throws error', async function() {
-    // Attempt to load the model and catch the error
-    await expect(mockP5Prototype.loadModel(inconsistentColorObjFile))
-      .rejects
-      .toThrow('Model coloring is inconsistent. Either all vertices should have colors or none should.');
+  test('mixed material coloring loads model with empty vertexColors instead of crashing', async function() {
+    // eg1.obj has some faces without a material and some with one.
+    // Real-world exports from Blender/Sketchfab frequently produce this structure.
+    // The loader should degrade gracefully rather than throwing, so beginners'
+    // sketches don't crash when loading common 3D assets.
+    const model = await mockP5Prototype.loadModel(inconsistentColorObjFile);
+    assert.instanceOf(model, Geometry);
+    assert.equal(
+      model.vertexColors.length,
+      0,
+      'Mixed-material model should have no vertex colors (graceful degradation)'
+    );
   });
 
   test('missing MTL file shows OBJ model without vertexColors', async function() {