diff --git a/app/Services/ThreeMfGeometryExtractor.php b/app/Services/ThreeMfGeometryExtractor.php
index 96b39a4..db90551 100644
--- a/app/Services/ThreeMfGeometryExtractor.php
+++ b/app/Services/ThreeMfGeometryExtractor.php
@@ -561,7 +561,7 @@ class ThreeMfGeometryExtractor
         $ancestry[] = $objectId;
 
         if ($object['mesh'] !== null) {
-            $defaultColor = $this->resolveColor($baseMaterials, $object['pid'], $object['pindex']);
+            $this->resolveColor($baseMaterials, $object['pid'], $object['pindex']);
             $determinant = $transform[0] * ($transform[4] * $transform[8] - $transform[5] * $transform[7])
                 - $transform[1] * ($transform[3] * $transform[8] - $transform[5] * $transform[6])
                 + $transform[2] * ($transform[3] * $transform[7] - $transform[4] * $transform[6]);
@@ -578,9 +578,11 @@ class ThreeMfGeometryExtractor
                     throw new RuntimeException('3MF triangle references a vertex index that does not exist.');
                 }
 
-                $color = $triangle['pid'] !== null
-                    ? $this->resolveColor($baseMaterials, $triangle['pid'], $this->resolveTriangleVertexIndex($triangle))
-                    : $defaultColor;
+                $color = $this->resolveColor(
+                    $baseMaterials,
+                    $triangle['pid'] ?? $object['pid'],
+                    $this->resolveTriangleVertexIndex($triangle, $object['pindex']),
+                );
 
                 $triangles[] = [
                     'v' => [
@@ -640,13 +642,14 @@ class ThreeMfGeometryExtractor
      * the face. This renderer only paints one flat colour per triangle, so a
      * triangle that actually declares differing per-vertex indices is
      * rejected explicitly rather than silently picking p1 and discarding the
-     * rest of the declared colour data.
+     * rest of the declared colour data. The property group and first index
+     * inherit independently from object pid and pindex; omitted p2/p3 use p1.
      *
      * @param  array{v1:int,v2:int,v3:int,pid:?int,p1:?int,p2:?int,p3:?int}  $triangle
      */
-    private function resolveTriangleVertexIndex(array $triangle): ?int
+    private function resolveTriangleVertexIndex(array $triangle, ?int $objectPindex): ?int
     {
-        $p1 = $triangle['p1'];
+        $p1 = $triangle['p1'] ?? $objectPindex;
         $p2 = $triangle['p2'] ?? $p1;
         $p3 = $triangle['p3'] ?? $p1;
 
diff --git a/tests/Unit/ThreeMfGeometryExtractorTest.php b/tests/Unit/ThreeMfGeometryExtractorTest.php
index 3d18b3f..060355d 100644
--- a/tests/Unit/ThreeMfGeometryExtractorTest.php
+++ b/tests/Unit/ThreeMfGeometryExtractorTest.php
@@ -2,7 +2,11 @@
 
 namespace Tests\Unit;
 
+use App\Services\ManufacturingFileValidator;
+use App\Services\ProductImageRenderer;
 use App\Services\ThreeMfGeometryExtractor;
+use Illuminate\Http\UploadedFile;
+use PHPUnit\Framework\Attributes\DataProvider;
 use RuntimeException;
 use Tests\TestCase;
 use ZipArchive;
@@ -131,6 +135,91 @@ class ThreeMfGeometryExtractorTest extends TestCase
         unlink($path);
     }
 
+    private function validatedTetrahedron(string $triangleProperties): string
+    {
+        $faces = '';
+        foreach ([[0, 2, 1], [0, 1, 3], [0, 3, 2], [1, 2, 3]] as [$v1, $v2, $v3]) {
+            $faces .= '<triangle v1="'.$v1.'" v2="'.$v2.'" v3="'.$v3.'" '.$triangleProperties.'/>';
+        }
+        $resources = '<m:colorgroup xmlns:m="http://schemas.microsoft.com/3dmanufacturing/material/2015/02" id="2">'
+            .'<m:color color="#FF0000"/><m:color color="#0000FF"/><m:color color="#00FF0080"/></m:colorgroup>'
+            .'<object id="1" type="model" pid="2" pindex="0"><mesh><vertices>'
+            .'<vertex x="0" y="0" z="0"/><vertex x="10" y="0" z="0"/>'
+            .'<vertex x="0" y="10" z="0"/><vertex x="0" y="0" z="10"/>'
+            .'</vertices><triangles>'.$faces.'</triangles></mesh></object>';
+        $path = $this->validArchive($this->model($resources, '<item objectid="1"/>'));
+        try {
+            $validated = (new ManufacturingFileValidator)->validate(new UploadedFile($path, 'tetrahedron.3mf', test: true));
+            $this->assertSame('3mf', $validated['extension']);
+        } catch (\Throwable $exception) {
+            unlink($path);
+            throw $exception;
+        }
+
+        return $path;
+    }
+
+    /** @return array<string, array{string}> */
+    public static function flatInheritedProperties(): array
+    {
+        return [
+            'p2 and p3 default to p1' => ['p1="1"'],
+            'all vertex properties specified' => ['p1="1" p2="1" p3="1"'],
+        ];
+    }
+
+    #[DataProvider('flatInheritedProperties')]
+    public function test_inherited_triangle_property_group_applies_flat_override_through_png_rendering(string $properties): void
+    {
+        $path = $this->validatedTetrahedron($properties);
+        try {
+            $scene = $this->extractor->extract($path);
+            $this->assertSame(4, $scene['triangle_count']);
+            foreach ($scene['triangles'] as $triangle) {
+                $this->assertSame([0, 0, 255, 255], $triangle['color']);
+            }
+            $rendered = (new ProductImageRenderer)->render($scene['triangles']);
+            $this->assertSame('image/png', $rendered['mime']);
+            $image = imagecreatefromstring($rendered['bytes']);
+            $this->assertNotFalse($image);
+            try {
+                $pixel = imagecolorsforindex($image, imagecolorat($image, 256, 256));
+                $this->assertSame(0, $pixel['red']);
+                $this->assertSame(0, $pixel['green']);
+                $this->assertGreaterThan(0, $pixel['blue']);
+                $this->assertSame(0, $pixel['alpha']);
+            } finally {
+                imagedestroy($image);
+            }
+        } finally {
+            unlink($path);
+        }
+    }
+
+    public function test_inherited_triangle_property_group_rejects_interpolated_override(): void
+    {
+        $path = $this->validatedTetrahedron('p1="0" p2="1" p3="0"');
+        try {
+            $this->expectException(RuntimeException::class);
+            $this->expectExceptionMessageMatches('/interpolat/i');
+            (new ProductImageRenderer)->render($this->extractor->extract($path)['triangles']);
+        } finally {
+            unlink($path);
+        }
+    }
+
+    public function test_inherited_triangle_property_group_rejects_transparent_override(): void
+    {
+        $path = $this->validatedTetrahedron('p1="2"');
+        try {
+            $this->expectException(RuntimeException::class);
+            $this->expectExceptionMessageMatches('/transparen/i');
+            (new ProductImageRenderer)->render($this->extractor->extract($path)['triangles']);
+        } finally {
+            unlink($path);
+        }
+    }
+
     public function test_resolves_object_level_color_from_a_colorgroup(): void
     {
         $resources = '<colorgroup id="5"><color color="#00CC33"/></colorgroup>'