Posted by Ivan Grishchenko, Valentin Bazarevsky, Eduard Gabriel Bazavan, Na Li, Jason Mayes, Google

Pose detection is an important step in understanding more about the human body in videos and images. Our existing models have supported 2D pose estimation for some time, which many of you may have already tried.

Today, we are launching our first 3D model in TF.js pose-detection API. 3D pose estimation opens up new design opportunities for applications such as fitness, medical, motion capture and beyond - in many of these areas we’ve seen a growing interest from the TensorFlow.js community. A great example of this is 3D motion capture to drive a character animation in the browser.

3D motion capture with BlazePose GHUM by Richard Yee (used with permission, live demo available at 3d.kalidoface.com)

This community demo uses multiple models powered by MediaPipe and TensorFlow.js (namely FaceMesh, BlazePose and HandPose). Even better, no app install is needed as you just need to visit a webpage to enjoy the experience. So with that in mind, let’s learn more and see this new model in action!

Try out the live demo!

Installation

The pose-detection API provides two runtimes for BlazePose GHUM, namely MediaPipe runtime and TensorFlow.js runtime.

To install the API and runtime library, you can either use the <script> tag in your html file or use NPM.

Through script tag:

<script src="https://cdn.jsdelivr.net/npm/@tensorflow-models/pose-detection"></script>
<!-- Include below scripts if you want to use TF.js runtime. -->
<script src="https://cdn.jsdelivr.net/npm/@tensorflow/tfjs-core"></script>
<script src="https://cdn.jsdelivr.net/npm/@tensorflow/tfjs-converter"></script>
<script src="https://cdn.jsdelivr.net/npm/@tensorflow/tfjs-backend-webgl"></script>

<!-- Optional: Include below scripts if you want to use MediaPipe runtime. -->
<script src="https://cdn.jsdelivr.net/npm/@mediapipe/pose"></script>

Through NPM:

yarn add @tensorflow-models/pose-detection

# Run below commands if you want to use TF.js runtime.
yarn add @tensorflow/tfjs-core @tensorflow/tfjs-converter
yarn add @tensorflow/tfjs-backend-webgl

# Run below commands if you want to use MediaPipe runtime.
yarn add @mediapipe/pose

To reference the API in your JS code, it depends on how you installed the library.

If installed through script tag, you can reference the library through the global namespace poseDetection.

If installed through NPM, you need to import the libraries first:

import * as poseDetection from '@tensorflow-models/pose-detection';
// Uncomment the line below if you want to use TF.js runtime.
// import '@tensorflow/tfjs-backend-webgl';
// Uncomment the line below if you want to use MediaPipe runtime.
// import '@mediapipe/pose';

Try it yourself!

First, you need to create a detector:

const model = poseDetection.SupportedModels.BlazePose;
const detectorConfig = {
  runtime: 'mediapipe', // or 'tfjs'
  modelType: 'full'
};
detector = await poseDetection.createDetector(model, detectorConfig);

Choose a modelType that fits your application needs, there are three options for you to choose from: lite, full, and heavy. From lite to heavy, the accuracy increases while the inference speed decreases. Please try our live demo to compare different configurations.

Once you have a detector, you can pass in a video stream to detect poses:

const video = document.getElementById('video');
const poses = await detector.estimatePoses(video);

How to use the output? poses represent an array of detected pose predictions in the image frame. For each pose, it contains keypoints and keypoints3D. The keypoints are the same as the 2D model we launched before, it is an array of 33 keypoint objects, each object has x, y in pixel units.

keypoints3D is an additional array with 33 keypoint objects, each object has x, y, z. The x, y, z are in meter units. The person is modeled as if they were in a 2m x 2m x 2m cubic space. The range for each axis goes from -1 to 1 (therefore 2m total delta). The origin of this 3D space is the hip center (0, 0, 0). From the origin, z is positive if moving closer to the camera, and negative if moving away from the camera. See below output snippet for example:

[
  {
    score: 0.8,
    keypoints: [
      {x: 230, y: 220, score: 0.9, name: "nose"},
      {x: 212, y: 190, score: 0.8, name: "left_eye"},
      ...
    ],
    keypoints3D: [
      {x: 0.5, y: 0.9, z: 0.06 score: 0.9, name: "nose"},
      ...
    ]
  }
]

You can refer to our ReadMe for more details about the API.

As you begin to play and develop with BlazePose GHUM, we would appreciate your feedback and contributions. If you make something using this model, tag it with #MadeWithTFJS on social media so we can find your work, as we would love to see what you create.

Model deep dive

The key challenge to build the 3D part of our pose model was obtaining realistic, in-the-wild 3D data. In contrast to 2D, which can be obtained via human annotation, accurate manual 3D annotation becomes a uniquely challenging task. It requires either a lab setup or specialised hardware with depth sensors for 3D scans - which introduce additional challenges to preserve a good level of human and environment diversity in the dataset. Another alternative, which many researchers choose - to build a completely synthetic dataset, which introduces yet another challenge of domain adaptation to real-world pictures.

Our approach is based on a statistical 3D human body model called GHUM, which is built using a large corpus of human shapes and motions. To obtain 3D human body pose ground truth, we fitted the GHUM model to our existing 2D pose dataset and extended it with a real world 3D keypoint coordinates in metric space. During the fitting process the shape and the pose variables of GHUM were optimized such that the reconstructed model aligns with the image evidence. This includes 2D keypoint and silhouette semantic segmentation alignment as well as shape and pose regularization terms. For more details see related work on 3D pose and shape inference (HUND, THUNDR).

Sample GHUM fitting for an input image. From left to right: original image, 3D GHUM reconstruction (different viewpoint) and blended result projected on top of the original image.

Due to the nature of 3D to 2D projection, multiple points in 3D can have the same projection in 2D (i.e. with the same X and Y but different Z). So the fitting can result in several realistic 3D body poses for the given 2D annotation. To minimize this ambiguity, in addition to a 2D body pose, we asked annotators to provide depth order between pose skeleton edges where they are certain (check the figure below). This task proved to be an easy one (compared to a real depth annotation) showing high consistency between annotators (98% on cross-validation) and helped to reduce the depth ordering errors for the fitted GHUM reconstructions from 25% to 3%.

"Depth order" annotation: the wider edge corner denotes the corner closer to the camera (e.g. the person’s right shoulder is closer to camera than left shoulder on both examples)

BlazePose GHUM utilizes a two-step detector-tracker approach where the tracker operates on a cropped human image. Thus the model is trained to predict 3D body pose in relative coordinates of a metric space with origin in the subject's hips center.

MediaPipe vs. TF.js runtime

There are some pros and cons of using each runtime. As shown in the performance table below, the MediaPipe runtime provides faster inference speed on desktop, laptop and android phones. The TF.js runtime provides faster inference speed on iPhones and iPads. The TF.js runtime is also about 1 MB smaller than the MediaPipe runtime.

Acknowledgements

We would like to acknowledge our colleagues, who participated in creating BlazePose GHUM 3D: Andrei Zanfir, Cristian Sminchisescu, Tyler Zhu, the other contributors to MediaPipe: Chuo-Ling Chang, Michael Hays, Ming Guang Yong, Matthias Grundmann, along with those involved with the TensorFlow.js pose-detection API: Ahmed Sabie and Ping Yu, and of course the community who are making amazing work with these models: Richard Yee.