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What is the difference between OBJ, STL, GLB and USDZ — and which should I choose?

OBJ is read by almost everything, STL is for 3D printing without colour, GLB is one self-contained file for the web and Blender, and USDZ opens directly on iPhone, iPad and Mac.

What a 3D model file of a home actually contains

A 3D model file of a home contains three kinds of information: the geometry (vertices and the triangles stretched between them), the appearance (colours, materials, any images laid onto the surfaces) and the structure (which parts belong together, and where they sit relative to each other).

The difference between the formats is largely how much of those three things they can carry — and whether it all sits in one file or several. That is the whole explanation for why a model that looks right on your end can arrive chalk-white for the recipient: the geometry came along, the appearance did not.

OBJ: the safe choice when you know nothing about the recipient

OBJ (Wavefront OBJ) dates from the 1980s and is read by almost every piece of 3D software that exists. The file is plain text with vertices and faces, and the appearance sits in a separate MTL file alongside it.

This is OBJ's one trap: if you send only the .obj file, the colours do not come along. The MTL file has to travel in the same folder, and if the materials reference image files, those have to come too. Send OBJ as a zipped folder with everything included, or send GLB instead.

Use OBJ when the recipient says "just send a 3D file", when the model needs to go into an older program, or when you want to be able to open it yourself in ten years.

STL: only for 3D printing — and completely without colour

STL is the format of 3D printers. An STL file contains nothing but triangles: no colours, no materials, no names for the parts, no structure. It is a plain description of a surface.

That makes STL excellent for printing a physical model of the house and useless for everything else. Send an STL to an architect or a kitchen company, and they get a lumped-together white house with nothing at all to work with.

If you are going to print the home, it is worth knowing that a printed plan rarely needs to be the whole house: a single floor without a ceiling, printed at 1:100, is easier to read and uses a fraction of the filament. And remember that the printer typically works in millimetres, while the model is often written in metres — a model that arrives 4 mm wide is a unit problem, not a fault in the file.

PLY: the format for points and measurements

PLY (Stanford PLY) can carry both surfaces and loose points, and it can carry a colour per vertex. That makes PLY the standard format when working with scanned data rather than with drawn models.

PLY is what you send to someone working in CloudCompare or MeshLab, and it is the format a point cloud from a LiDAR scan is most often exchanged in. For an ordinary home model that just needs to be looked at, PLY is rarely the right choice — GLB or USDZ are easier for the recipient.

glTF and GLB: the format for the web, Blender and game engines

glTF 2.0 is the modern exchange format for 3D. It is designed to be displayed — fast, consistently and with the materials intact — and it has become the standard on the web and in game engines.

It comes in two versions, and the difference matters in practice:

• glTF (.gltf) is a text file that points to a .bin file with the geometry and possibly to image files. Several files that have to travel together.
• GLB (.glb) is exactly the same thing packed into one binary file. Geometry, materials and textures in the same file, nothing to lose along the way.

If you are sending it, send GLB. If you are editing in a pipeline setup where the textures need to be swappable, .gltf with side files is easier. Blender, Unity, Unreal, three.js and most web viewers read both.

DAE (COLLADA): for SketchUp and older CAD

DAE is the COLLADA format, an older XML-based exchange format. It has largely been superseded by glTF, but it holds its ground in one place: SketchUp and a number of older CAD programs import DAE smoothly, where they can struggle with GLB.

If the recipient specifically asks for DAE, send DAE. Otherwise GLB is the better choice.

USDZ: the one that just opens on an iPhone

USDZ is Apple's format, built on Pixar's USD. The big advantage is that it opens directly in Messages, Mail, Files and Safari on iPhone, iPad and Mac — the recipient taps the file, and the model is right there, with the option to place it in AR on the floor in front of them.

USDZ is one file, and it carries the appearance of the materials with it. That makes it the best choice when you send a home model to a family member, a partner or an estate agent who just needs to see it — not work in it.

The downside is that USDZ is harder outside Apple's world: Windows and Android do not handle it out of the box. If the model needs to be viewable on both an iPhone and a Windows PC, send USDZ and GLB together.

Choose the format for the recipient — not for the quality

There is no single format that is "best". There is only the format the recipient can open. A short rule of thumb:

• Needs to be seen on iPhone or iPad: USDZ.
• Going onto a website or into Blender/Unity: GLB.
• Do not know: OBJ (remember the MTL file) or GLB.
• Needs to be 3D printed: STL.
• Going into SketchUp or older CAD: DAE.
• Is measurements, not a model: PLY.
• Needs further drawing in 2D: not a 3D format at all — send DXF.

HouseSense can write the model in all seven: OBJ, STL, PLY, glTF, GLB, DAE and USDZ. You pick the ones you need, and a cover note sits in the folder telling the recipient what the files are and what unit they are in.

What a home model IS — and what it is not

A 3D model exported from a home takeoff is a building model: walls, doors, windows, floors, ceilings and furniture built from the measured floor plan. It is not a surface scan of your home. The wallpaper pattern, the sofa's upholstery and the crack in the window frame are not included — the model is the house's shape, not its photograph.

It is worth saying out loud to the recipient, because the expectation is often different. In return, the shape is measured, it can be measured against, and it weighs a few megabytes instead of the gigabytes a real surface scan would take up.

And one more thing no model file can answer: whether a wall is load-bearing. Whether the wall sits in a USDZ, a GLB or an STL, it is just geometry. The question of load-bearing is settled by a building surveyor or a consulting engineer — never by a file.