Install on Windows, step by step

This page walks through the whole install on Windows, one step at a time, and says what you should see at each step. It takes about ten minutes, most of it the one-time setup on the first start. There is nothing to run as administrator and nothing to uninstall later: the app lives in one folder.

Before you start

• Windows 10 or 11. The launcher can install Python for you through winget, which comes with current versions of Windows, and falls back to a direct download if winget is missing.
• About 2 GB of free disk space on the drive where you put the folder.
• An internet connection for the first start. After that the app runs offline.
• Chrome, Edge or Brave, if you want the app in a window of its own. Without one of them it opens in a tab of your usual browser, and works the same.

Python 3.10, 3.11 or 3.12 is needed as well. You do not have to check for it first: the launcher looks, and offers to install it if it is missing.

Step 1: download and unpack

1. Open the Download page of this site and press the download button. Your browser saves a zip file, usually into your Downloads folder.
2. Right-click the zip and choose Extract All…, then Extract. Do not double-click your way into the zip and run the launcher from there: Windows lets you look inside a zip, but the launcher needs real files next to it.
3. Move the unpacked folder (it is named something like MeshOptimiser-main) to a place you are allowed to write to, for example your Documents folder. The app creates two folders inside it, .venv and inbox.
4. Open the folder. You should see start.bat, start_hidden.vbs, serve.py and index.html, among others.

Step 2: Python (usually nothing to do)

There are three cases, and the launcher handles all of them.

You already have Python 3.10, 3.11 or 3.12

Nothing to do. The launcher finds it, even when you have several versions installed, and picks the newest supported one. A newer Python, such as 3.13, can stay installed next to it: it is simply not used.

You have no Python

The launcher says “Python was not found on this PC” and asks “Install Python 3.12 automatically now”. Press Y. It tries winget first, and if that fails it downloads the official installer from python.org and runs it quietly, for your user only, with Python added to the PATH. When it is done, the launcher carries on by itself.

You only have an unsupported Python

The launcher names the version it found, for example “Found Python 3.13”, and offers to install 3.12 next to it. Your own Python is left as it is.

If you prefer to install Python yourself, go to https://www.python.org/downloads/, download Python 3.12 and run the installer. On its first screen, tick “Add Python to PATH”. Then close every open terminal window before you start the launcher.

Step 3: the first start

Double-click start.bat. A black window with white text opens. This is the launcher, and it is also the server that the app runs on, so leave it open while you work. This is what it prints on a first run:

No file dropped - starting viewer empty.

You started it by double-clicking, so it opens the app without a file. You can also drag a file onto start.bat; see Start it the first time.

[1/4] Checking Python...

It looks for a supported Python and prints its version, for example Python 3.12.7.

[2/4] Checking local Python environment...

Then First-time setup - creating .venv (takes ~30 seconds).... This builds the private Python environment inside the app folder.

Installing dependencies (takes 1-3 minutes the first time)...

It downloads and installs the three add-ons the converter needs: cadquery-ocp (the CAD reader), trimesh and numpy. This is the step that needs the internet. The text scrolls quickly. It is working, even if it looks quiet for a while.

Setup complete.

The environment is ready. On every later start this part is replaced by Found .venv - reusing it.

[3/4] Starting the local server...

The server starts and prints the address it is using, normally http://localhost:4242. The app opens in a window of its own and shows the start screen.

[4/4] Server stopped.

You will only see this when the app has been quit. A clean quit closes the launcher window by itself.

Step 4: check that it works

1. On the start screen, click one of the four shapes (Cube, Sphere, Cylinder or Torus). It opens in the 3D view. This needs no file, so it is a quick way to know that the app itself is fine.
2. Press F1 to open the help window, and Esc to close it again.
3. Optional: drag a small STEP file onto the window, to test the converter as well. See Open your first file.

If something stopped on the way, find the message in Install problems, message by message.

What the install left on your computer

• The app folder, with a .venv folder (up to about 2 GB) and an inbox folder (converted files) inside it.
• Python 3.12, only if the launcher installed it for you. It shows up in Settings, Apps as a Python 3.12 entry, and you can remove it there like any other program.
• Nothing else from the app itself: no service, no startup entry and no registry entries. (Python’s own installer adds its usual entries, if it was installed.) Your settings are kept by your browser.