On July 20, 2020 we held ShipIt! Presents: AR/VR at Shopify. Daniel talked about what we’re doing with AR/VR at Shopify. The video of the event is available.

Size.link is a free tool built by Shopify’s Augmented Reality (AR) team that lets anyone view the size of a product in the space around them using their smartphone camera.

The tool came out of our April Hack Days focused on helping retailers impacted by Covid-19. Our idea was to create an easy way for merchants to show how big their products are—something that became increasingly important as retail stores were closed. While Shopify does support 3D models of products, it does take time and money to get 3D models made. We wanted to provide a quick stopgap solution.

The ideal flow is for merchants to provide a link on their product page (e.g., https://size.link?l=3&w=4&h=5&units=in) that would open up an AR view when someone clicked on it. For this to be as seamless as possible, it had to be all done on the web (no app required!), fast, accurate, and work on iOS and Android.

Let’s dive into how we pulled it off.

AR on the Web

3D on the web has been around for close to a decade. There are great WebGL libraries like Three.js that make it quick to display 3D content. For Size.link, all we needed to show was a 3D cube which is essentially the “Hello World” of computer graphics.

The problem is that WebAR, the ability to power AR experiences on the web with JavaScript, isn’t supported on iOS. In order to build fully custom AR experiences on iOS, it needs to be in an app.

Luckily, there’s a workaround. iOS has a feature called AR Quick Look, which is a native AR viewer built into the operating system. By opening a link to a 3D model file online, an AR viewer will launch right in your browser to let you view the model. Android has something similar called Scene Viewer. The functionality of both of these viewers is limited to only placing and moving a single 3D object in your space, but for Size.link that was all we needed.

Example of a link to view a 3D model with iOS AR Quick Look:

Example of a link to view a 3D model with Android Scene Viewer:

You might have noticed that the file extensions in the above viewers are different between Android and iOS. Welcome to the wonderful world of 3D formats!

A Quick Introduction to 3D File Formats

Image files store pixels, and 3D files store information about an object’s shape (geometry) and what it’s made of (materials & textures). Let’s take a look at a simple object: a cube. To represent a cube as a 3D model we need to know the positions of each of its corners. These points are called vertices

Left: Each vertex of the cube.
Right: 3d-coordinates of each vertex

These can be represented in array like this:

We also need to know how each side of the cube is connected. These are called the faces.

A face made up of vertices 0, 1, 5, 4

The face array [0, 1, 5, 4] denotes that there’s a face made up by connecting vertices 0, 1, 5, and 4 together. Our full cube would look something like this:

There is also a scale property that lets us resize the whole object instead of moving the vertices manually. A scale of (1,2,3) would scale by 1 along the x-axis, 2 along the y-axis, and 3 along the z-axis.

A scale of (1,2,3) used to resize the whole object

So what file format can we use to store this information? Well, just like how images have different file formats (.jpeg, .png, .tiff, .bmp, etc), there’s more than one type of 3D format. And because the geometry data can get quite large, these formats are often binary instead of ASCII based.

Android’s Scene Viewer uses .glTF, which is quickly becoming a 3D standard in the industry. Its aim is to be the jpeg of 3D. The binary version of a .glTF file is a .glb file. Apple on the other hand is using their own format called USDZ, which is based off of Pixar’s USD file format.

For Size.link to work on both operating systems, we needed a way to create these files dynamically. When a user entered dimensions we’d have to serve them a 3D model of that exact size.

Approach 1: Creating 3D Models Dynamically

There are many libraries out there for creating and manipulating .glTF, and they’re very lightweight. You can use them client side or server side.

USDZ is a whole other story. It’s a real pain to compile all the tools, and an even bigger pain to get running on a server. Apple distributes precompiled executables, but they only work on OSX. We definitely didn’t want to spend half of Hack Days wrestling with tooling, but assuming we could get it working, the idea was:

Generate a cube as a .gltf file
Use the usdzconvert tool to convert the cube.gltf to a .usdz

The problem here is that this process could end up taking a non-trivial amount of time. We’ve seen usdzconvert take up to 3-4 seconds to convert, and if you add the time it takes to create the .gltf, users might be waiting 5 seconds or more before the AR view launches.

There had to be a faster and easier way.

Approach 2: Pre-generate All possible Models

What if we generated every possible combination of box beforehand in both .gltf and .usdz formats? Then when someone entered in their dimensions, we would just serve up the relevant file.

How many models would we need? Let’s say we limited sizes for width, length, and depth to be between 1cm and 1000cm. There’s likely not many products that are over 10 meters. We’d then have to pick how granular we could go. Likely people wouldn’t be able to visually tell the difference between 6.25cm and 6cm. So we’d go in increments of 25mm. That would require 4000 * 4000 * 4000 * 2, or 128,000,000,000 models. We didn’t really feel like generating that many models, nor did we have the time during Hack Days!

How about if we went in increments of 1cm? That would need 1000 * 1000 * 1000 * 2, or 2 billion models. That’s a lot. At 3 KB a model, that’s roughly 6TB of models.

This approach wasn’t going to work.

Approach 3: Modify the Binary Data Directly

All Size.link cubes have the same textures and materials, and the same numbers of vertices and faces. The only difference between them is their scale. It seemed inefficient to regenerate a new file every time just to change this one parameter, and seemed wasteful to pre-generate billions of almost identical files.

But what if we took the binary data of a .usdz file, found which bytes correspond to the scale property, and swapped them out with new values? That way we could bypass all of the usdz tooling.

The first challenge was to find the byte location of the scale values. The scale value (1, 1, 1) would be hard to look for because the value “1” likely comes up many times in the cube’s binary data. But if we scaled a cube with values that were unlikely to be elsewhere in the file, we could narrow down the location. We ended up creating a cube with scale = (1.7,1.7,1.7).

By loading up our file in a hex editor, we’re able to look up and find the value. USDZ stores values as 32-bit floats, so 1.7 is represented as 0x9a99d93f. With a quick search we found at byte offset 1344 the values corresponding to scale along the x, y, and z axes.

Identifying 0x9a99d93f within the USDZ binary

To test our assumption, the next step was to try changing these bytes and seeing that the scale would change.

It worked! With this script we could generate .usdz files on the fly and it was really fast.The best part is that this could also run completely client side with a few modifications. We could modify the .usdz in the browser, encode the data in a URL, and pass that URL to our AR Quick Look link:

Unfortunately, our dreams of running this script entirely on the client were dashed when it came to Android. Turns out you can’t launch Scene Viewer from a local data URL. The file has to be served somewhere, so we’re back to needing to do this on a server.

But before we went about refactoring this script to run on a little server written in Ruby, we wanted to give our lovely cube a makeover. The default material was this grey colour that looked a bit boring.

The new cube is semi-transparent and has a white outline that makes it easier to align with the room

Achieving the transparency effect was as simple as setting the opacity value of the material. The white outline proved to be a bit more challenging because it could easily get distorted with scale.

If one side of the cube is much longer than the other, the outline starts stretching

The outline starts stretching on the box

You can see in the above image how the outlines are no longer consistent in thickness. The ones of the sides are now twice as thick the others.

We needed a way to keep the outline thickness consistent regardless of the overall dimensions, and that meant that we couldn’t rely on scale. We’d have to modify the vertex positions individually.

Vertices are highlighted in red

Above is the structure of the new cube we landed on with extra vertices for the outline. Since we couldn’t rely on the scale property anymore for resizing our cube, we needed to change the byte values of each vertex in order to reposition them. But it seemed tedious to maintain a list of all the vertex byte offsets, so we ended up taking a “find and replace” approach.

Left: Outer vertices. Right: Inner outline vertices

We gave each x, y, z position of a vertex a unique value that we could search for in the byte array and replace. In this case the outer vertices for x, y, z were 51, 52, 53 respectively. We picked these numbers at random just like we picked the number 1.7 before. We wanted something unique.

Vertex values that affected the outline were 41, 42, and 43. Vertex positions with the same value meant that they moved together. Also, since the cube is symmetrical, we gave opposite vertices the same value except negated.

As an example, let’s say we wanted to make a cube 2m long, and 1m wide and tall. We’d first search for all the bytes with a float value of 51 (0x00004c42) and replace it with the value 2 * 0.5 = 1 (0x0000803f). We use 1 instead of 2 because of the vertices being symmetrical. If the left-most corner has a value of -1 and the right-most has a value of 1, then the distance between them is 2

Distance between the vertices is 2

We’d then move the outline vertices by looking for all the bytes with value 41 (0x00002442) and replace them with 1.98 * 0.5 = 0.99 (0xa4707d3f) to keep the outline 2cm thick. We’d repeat this process for the width and the height

The template cube is transformed to the proper dimensions with consistent outline thickness.

Here’s what part of our server side Ruby code ended up looking like to do this:

Et voilà! We now had a way to generate beautiful looking cubes on the fly. And most importantly, it’s blazingly fast.This new way can create usdz files in well under 1 millisecond, something much better than relying on the python USD toolset.

All that remained was to write the .glb version, which we did using the same approach as above.

What’s Next for Size.link?

We’re really happy with how Size.link turned out, and with the simplicity of its implementation. Now anytime you are shopping and want to see how big a product is, you can simply go to Size.link and visualize the dimensions.

But why stop at showing 3D cubes? There are lots of standard-sized products that we could help visualize like rugs, posters, frames, mattresses, etc—Imagine being able to upload a product photo of a 6x9 rug and instantly load it up in front of you in AR. All we need to figure out is how to dynamically insert textures into .usdz and .glb files.

Time to boot up the ol’ hex editor again…