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D-POINT: Digital Pen with Optical-Inertial Tracking

D-POINT is an open-source digital stylus that uses camera tracking and inertial measurements to achieve 6DoF (six degrees of freedom) inputs, with low latency, pressure sensitivity, and sub-millimetre accuracy. The stylus can be used on any flat surface, and works with consumer-grade webcams.

writing-demo.mp4

This project was part of my undergraduate thesis for electrical engineering. I've open-sourced the code and design files in the hopes that they might be useful to somebody, but it's not intended to be a "plug and play" DIY project. If you want to try building it anyway, follow the setup guide.

Design

This is a very brief overview of how the system works. For all the details, plus literature review and lots of evaluation, read the full thesis (note: I haven't published this online yet).

Block diagram showing how the system works

Hardware

The main body of the stylus was 3D printed as two halves, shown below. The stylus contains a force sensor, a Li-ion battery which charges over USB-C, and an Arduino-based development board for logic and Bluetooth. Eight printed ArUco markers are glued to the back of the stylus, for visual pose estimation.

CAD drawing showing the hardware design of the stylus

Visual pose estimation (VPE)

The VPE process involves the four main steps:

  1. Marker detection: First, we use OpenCV to detect the corners of each visible ArUco marker on the stylus.
  2. Rolling shutter correction: We use a simple 2D motion model to estimate and correct for the effects of rolling shutter on the observed corner locations.
  3. Perspective-n-Point (PnP): From these corner positions, we use a PnP algorithm to estimate the pose of the stylus relative to the camera. When possible, we use the pose from the previous frame as a starting point to refine with virtual visual servoing (VVS), otherwise we fall back to SQPnP.
  4. Coordinate conversion: Using the calibrated pose of the stylus and the drawing surface relative to the camera, we calculate the position and orientation of the stylus tip relative to the drawing surface.

Inertial fusion

We use an Extended Kalman Filter (EKF) to fuse the VPE estimates with the inertial data from the accelerometer and gyroscope, and refine the estimates in real-time using the Rauch-Tung-Striebel (RTS) algorithm. To account for time delay from the camera frames, we use a negative-time measurement update algorithm. The EKF is implemented using NumPy and Numba.

Using inertial measurements allows us to dramatically reduce latency compared to a camera-only implementation, while also improving accuracy and report rate for fast movements.