GLB and USDZ explained, the two files AR needs on iPhone and Android

Two files for one product

A 3D model is a file that describes a product’s shape and surface so a screen can draw it from any angle. Two formats matter for shopping.

GLB is the single-file version of glTF, an open format published by the Khronos Group, a standards body. The glTF specification calls it a « transmission » format that is « compact, and efficient to process and render at runtime ». In plain words, it is made for delivery to a screen, not for editing. A GLB packs the shape, the materials and the images into one file.

USDZ comes from Pixar’s Universal Scene Description, and Apple uses it for AR. The USDZ specification describes it as an uncompressed zip archive that holds the scene and its images, PNG or JPEG, in one file.

Which phone reads which

On an iPhone or iPad, the AR viewer is AR Quick Look. Apple’s documentation says a web page enables it by linking a USDZ file, and iOS then shows the product in AR « on your behalf ». The shopper can move it, resize it with touch gestures, and share it. Quick Look also opens a .reality file, a format from Apple’s own tools.

On Android, the AR viewer is Google’s Scene Viewer. It reads glTF and GLB files. Google’s page says it needs a phone that supports ARCore, Google’s AR system, running Android 7.0 or later, with up-to-date Google Play Services for AR and the Google app.

Most shops never call these viewers directly. They use model-viewer, a free web component from Google that shows the 3D model on the page and adds an AR button. Its documentation lists three AR modes. WebXR runs AR inside Chrome on Android. Scene Viewer and Quick Look open the phone’s own viewer. If you give model-viewer only a GLB, it generates a USDZ on the fly when an iPhone user taps the AR button. You can also supply your own USDZ, which gives you control over how the iPhone version looks. Shopify themes use the same component, and Shopify converts each uploaded model so both versions exist.

Size and texture limits from the official pages

A phone has far less memory than a computer, so the viewers set limits. Google’s Scene Viewer page gives the clearest numbers.

• File size of 10 MB, since « bigger models may result in poor user experience ». Its validator warns above 10 MB and reports an error above 15 MB.
• Up to 100,000 triangles, the small flat faces a 3D shape is made of. Google calls 30,000 to 50,000 the ideal range.
• Textures, the images that give a model its colour and grain, of 2048 by 2048 pixels at most.
• One set of texture coordinates per mesh, and no more than 10 materials.
• JPG textures are preferred when nothing is transparent, because they are smaller.

Shopify’s checklist for model makers is stricter. It asks for a total file of about 4 MB and textures of no more than 2048 by 2048 pixels on mobile.

Apple publishes no file size or texture limit for Quick Look on the pages we could read. Its documentation lists the formats and how to link them, not a size budget. Treat the Google numbers as a safe ceiling for the iPhone file too, and test it on an iPhone.

Google Merchant Center has its own rule for 3D models sent to Google Shopping. The recommended file size is 10 MB, and it shouldn’t exceed 15 MB.

Why real-world scale matters

AR shows the product at the size written in the file. If the file is wrong, the shopper sees a wrong product in their room, and that is worse than no AR at all.

The glTF specification says « the units for all linear distances are meters ». Google’s Scene Viewer page repeats it, one unit equals one metre, « to ensure the model is placed in AR in true scale ». USD files work differently. Pixar’s documentation says a USD file that does not state its unit is read in centimetres. A converter that gets this wrong makes a sofa a hundred times too big or too small, so check the size in AR after every conversion.

Both viewers let shoppers pinch to resize by default. Apple’s allowsContentScaling setting « defaults to YES », and Scene Viewer’s resizable setting is true by default. model-viewer offers ar-scale="fixed", which stops shoppers from scaling the object in all three AR modes. For furniture and art, where the point is to judge size, a fixed scale shows shoppers the true size every time.

What to check on a phone

A model that looks right on a computer can still fail on a phone. Run these checks on a real iPhone and a real Android phone before a product goes live.

1. Open the product page on mobile data, not only on Wi-Fi, and see how long the 3D view takes to appear.
2. Tap the AR button. On iPhone, Quick Look should open. On Android, Scene Viewer or the in-browser AR view should open.
3. Place the product on the floor and measure it with a tape measure. Its width, depth and height should match your product sheet.
4. Walk around it. Look for missing parts, stretched textures, shiny spots that should be matte, and dark patches.
5. Check the colour in daylight against the real product or your best photo.
6. For wall art, check that it hangs on the wall, not on the floor.

Google also offers a free online previewer that validates a GLB file against Scene Viewer’s rules, linked from its Scene Viewer page.

In short

• GLB serves Android phones and web browsers, and USDZ serves iPhones and iPads. A shop needs both.
• model-viewer shows the model on the page and opens the right AR viewer on each phone.
• Google recommends files of 10 MB or less, textures of 2048 pixels at most, and up to 100,000 triangles. Shopify asks for about 4 MB.
• glTF measures in metres, and AR places the product at the size in the file, so the size must be exact.
• Always measure the product in AR on a real iPhone and a real Android phone.

Roomhero builds both files for each product at its real size, installs them on your product pages and measures each one against your real dimensions. If you want it done for you, get your first model free.