What the mockup tool does with your screenshot
Nothing leaves your browser. Here is exactly what happens between dropping a file and downloading the render, for people who want to check.
"Nothing is uploaded" is easy to write on a page. Here is what it means in practice, step by step, so you can verify it yourself with your browser's network tab.
What the page loads
When you open the tool, your browser fetches the page, one stylesheet, one script, a small cookieless analytics script that counts the visit, and the scene you selected: a photograph, a mask and a reflection image. That is all the traffic. The scene files are the same for everyone, and once loaded they stay in the tab.
What happens with your file
When you drop a screenshot, the browser decodes it into memory. The tool's script hands it to your graphics card as a texture, together with the scene's three images and the screen's corner points. A small program on the graphics card, a shader, then computes the finished image: for every pixel of the photograph it works out which pixel of your screenshot lands there, applies the mask and the reflection, and writes the result to a canvas on the page.
No request is made. There is no server that sees the image, no queue, no processing job. If you switch off your connection after the scene has loaded, the tool keeps working.
What happens when you download
The same computation runs again at full size into an invisible canvas, which the browser turns into a PNG and saves to your downloads folder. Again, no request.
What does go to the server
Two things, both optional. If you enter a tool key, the key alone is sent to our server, which asks Polar whether it is valid. If you use the newsletter form, your email address goes to Kit. Neither involves your images.
Why we built it this way
Partly principle, partly practice. A screenshot is often an unreleased design, a client's product, or a page from something you sell. Handing that to a third party for a mockup is a strange trade. And a tool that does the work locally is faster than any upload could be, which is the other reason it feels the way it does.
If you want to check, open the developer tools, watch the network tab, and drop a file. Nothing appears.
Try the mockup tool, free, in your browser.