Pebble, the smartwatch for your smartphone, released an early preview of its user interface on Vimeo today … including a look behind the scenes at how the massively successful crowdfunding project is building the software.

The challenge is not small: How to fit displays and controls for music, email, apps, timers, rangefinder, clockfaces, and any other apps developers dream up … all into one tiny 144-x-168 pixel black-and-white display.

Pebble’s user interface engineer, Martijn Thé, had a clear goal:

Above: Prototyping designs in real time.

Image Credit: Pebble

“I figured Pebble should be so easy that our grandmothers should be able to pick it up and use it right away.”

The difficult part, however, was how to make it simple enough for granny and also support the needs and desires of power users.

To design the Pebble’s interface efficiently, the team created a tool that allows them to prototype designs while viewing them in real time on a mock up (see image). Pebble will be making the software available for developers to use as they create third-party apps for the device.

In addition, the company created an iPhone app that acts as a Pebble, so that designers can interact with it on their wrists and ensure that the feel of the interface is natural and simple.

See it in action in the video below:

Image credit: Pebble