Android AR / VR Image Processing

Understand and Apply Stereo Rectification for Depth Maps (Part 2)

In part 1 of the article series, we’ve identified the key steps to create a depth map. We have captured a scene from two distinct positions and loaded them with Python and OpenCV. However, the images don’t line up perfectly fine. A process called stereo rectification is crucial to easily compare pixels in both images to triangulate the scene’s depth!

For triangulation, we need to match each pixel from one image with the same pixel in another image. When the camera rotates or moves forward / backward, the pixels don’t just move left or right; they could also be found further up or down in the image. That makes matching difficult.

Wrapping Images for Stereo Rectification

Image rectification wraps both images. The result is that they appear as if they have been taken only with a horizontal displacement. This simplifies calculating the disparities of each pixel!

With smartphone-based AR like in ARCore, the user can freely move the camera in the real world. The depth map algorithm only has the freedom to choose two distinct keyframes from the live camera stream. As such, the stereo rectification needs to be very intelligent in matching & wrapping the images!

Stereo Rectification: reprojecting images to make calculating depth maps easier.
Stereo Rectification: reprojecting images to make calculating depth maps easier.

In more technical terms, this means that after stereo rectification, all epipolar lines are parallel to the horizontal axis of the image.

To perform stereo rectification, we need to perform two important tasks:

  1. Detect keypoints in each image.
  2. We then need the best keypoints where we are sure they are matched in both images to calculate reprojection matrices.
  3. Using these, we can rectify the images to a common image plane. Matching keypoints are on the same horizontal epipolar line in both images. This enables efficient pixel / block comparison to calculate the disparity map (= how much offset the same block has between both images) for all regions of the image (not just the keypoints!).

Google’s research improves upon the research performed by Pollefeys et al. . Google additionally addresses issues that might happen, especially in mobile scenarios.

Android AR / VR Image Processing

Easily Create a Depth Map with Smartphone AR (Part 1)

For a realistic Augmented Reality (AR) scene, a depth map of the environment is crucial: if a real, physical object doesn’t occlude a virtual object, it immediately breaks the immersion.

Of course, some devices already include specialized active hardware to create real-time environmental depth maps – e.g., the Microsoft HoloLens or the current high-end iPhones with a Lidar sensor. However, Google decided to go into a different direction: its aim is to bring depth estimation to the mass market, enabling it even for cheaper smartphones that only have a single RGB camera.

In this article series, we’ll look at how it works by analyzing the related scientific papers published by Google. I’ll also show a Python demo based on commonly used comparable algorithms which are present in OpenCV. In the last step, we’ll create a sample Unity project to see depth maps in action. The full Unity example is available on GitHub.

Quick Overview: ARCore Depth Map API

How do Depth Maps with ARCore work? The smartphone saves previous images from the live camera feed and estimates the phone’s motion between these captures. Then, it selects two images that show the same scene from a different position. Based on the parallax effect (objects nearer to you move faster than these farther away – e.g., trees close to a train track move fast versus the mountain in the background moving only very slowly), the algorithm then calculates the distance of this area in the image.

This has the advantage that a single-color camera is enough to estimate the depth. However, this approach needs structured surfaces to detect the movement of unique features in the image. For example, you couldn’t get many insights from two images of a plain white wall, shot from two positions 20 cm apart. Additionally, it’s problematic if the scene isn’t static and objects move around.

As such, given that you have a well-structured and static scene, the algorithm developed by Google works best in a range between 0.5 and 5 meters.

Artificial Intelligence Image Processing

Hands-On “Deep Learning” Videos: Now on YouTube

Every new product or service claims to use deep learning or neural networks. But: how do they really work? What can machine learning do? How complicated is it to get started?

In the 4-part video series “Deep Learning Hands-On with TensorFlow 2 & Python”, you’ll learn what many of the buzzwords are about and how they relate to the problems you want to solve.

By watching the short videos, your journey will start with the background of neural networks, which are the base of deep learning. Then, two practical examples show two concrete applications on how you can use neural networks to perform classification with TensorFlow:

  • Breast cancer classification: based on numerical / categorical data
  • Hand-written image classification: the classic MNIST dataset based on small images

In the last part, we’ll look at one of the most important specialized variants of neural networks: convolutional neural networks (CNNs), which are especially well-suited for image classification.

Watching all four videos gives you a thorough understanding of how deep learning works and the guidance to get started!

Android AR / VR

Environmental HDR Lighting & Reflections in ARCore: Implementation in Unity 3D (Part 3)

How to make real-time HDR lighting and reflections possible on a smartphone? Based on the unique properties of human perception and the challenges of capturing the world’s state and applying it to virtual objects. Is it still possible?

Google found an interesting approach, which is based on using Artificial Intelligence to fill the missing gaps. In this article, we’ll take a look at how ARCore handles this. The practical implementation of this research is available in the ARCore SDK for Unity. Based on this, a short hands-on guide demonstrates how to create a sphere that reflects the real world – even though the smartphone only captures a fraction of it.

Google ARCore Approach to Environmental HDR Lighting

To still make environmental HDR lighting possible in real-time on smartphones, Google uses an innovative approach, which they also published as a scientific paper . Here, I’ll give you a short, high-level overview of their approach:

First, Google captured a massive amount of training data. The video feed of the smartphone camera captured both the environment, as well as three different spheres. The setup is shown in the image below.

Android AR / VR

Environmental HDR Lighting & Reflections in ARCore: Virtual Lighting (Part 2)

In part 1, we looked at how humans perceive lighting and reflections – vital basic knowledge to estimate how realistic these cues need to be. The most important goal is that the scene looks natural to human viewers. Therefore, the virtual lighting needs to be closely aligned with real lighting.

But how to measure lighting in the real world, and how to apply it to virtual objects?

Virtual Lighting

How do you need to set up virtual lighting to satisfy the criteria mentioned in part 1? Humans recognize if an object doesn’t fit in:

The left image shows a simple scene setup, where the shadow direction is wrong. The virtual object doesn't fit in.
In the ideal case on the right, the shadow and shading is correct.
Comparing a simple scene setup to environmental HDR lighting. Image adapted from the Google Developer documentation.

The image above from the Google Developer Documentation shows both extremes. Even though you might still recognize that the rocket is a virtual object in the right image, you’ll need to look a lot harder. The image on the left is clearly wrong, especially due to the misplaced shadow.

Android AR / VR

Environmental HDR Lighting & Reflections in ARCore: Human Perception (Part 1)

Realistically merging virtual objects with the real world in Augmented Reality has a few challenges. The most important:

  1. Realistic positioning, scale and rotation
  2. Lighting and shadows that match the real-world illumination
  3. Occlusion with real-world objects

The first is working very well in today’s AR systems. Number 3 for occlusion is working OK on the Microsoft HoloLens; and it’s soon also coming to ARCore (a private preview is currently running through the ARCore Depth API – which is probably based on the research by Flynn et al. ).

But what about the second item? Google put a lot of effort into this recently. So, let’s look behind the scenes. How does ARCore estimate HDR (high dynamic range) lighting and reflections from the camera image?

Remember that ARCore needs to scale to a variety of smartphones; thus, a requirement is that it also works on phones that only have a single RGB camera – like the Google Pixel 2.

App Development Artificial Intelligence Cloud

Computer Vision & Photo Description: Really Simple HTML / JavaScript Example

Image classification & content description is incredibly powerful. Cloud-based computer vision services instantly return a JSON-based description of what they see in photos.

However, most examples are quite complex. As a beginning developer with your main knowledge in HTML + JavaScript, the following code is for you. You don’t need to worry about Node.js or native apps. The code runs directly in your browser from your computer.

AR / VR HoloLens Image Processing

Basics of AR: Anchors, Keypoints & Feature Detection

Creating apps that work well with Augmented Reality requires some background knowledge of the image processing algorithms that work behind the scenes. One of the most fundamental concepts involves anchors. These rely on keypoints and their descriptors, detected in the recording of the real world.

Anchor Virtual Objects to the Real World

AR development APIs hide much of the complexity. As a developer, you simply anchor virtual objects to the world. This ensures that the hologram stays glued to the physical location where you put it.

App Development Artificial Intelligence Digital Healthcare

Using Natural Language Understanding, Part 4: Real-World AI Service & Socket.IO

In this last part, we bring the vital sign check list to life. Artificial Intelligence interprets assessments spoken in natural language. It extracts the relevant information and manages an up-to-date, browser-based checklist. Real-time communication is handled through Web Sockets with Socket.IO.

The example scenario focuses on a vital signs checklist in a hospital. The same concept applies to countless other use cases.

In this article, we’ll query the Microsoft LUIS Language Understanding service from a Node.js backend. The results are communicated to the client through Socket.IO.

Connecting LUIS to Node.JS

In the previous article, we verified that our LUIS service works fine. Now, it’s time to connect all components. The aim is to query LUIS from our Node.js backend.

App Development Artificial Intelligence Digital Healthcare

Using Natural Language Understanding, Part 3: LUIS Language Understanding Service

Training Artificial Intelligence to perform real-life tasks has been painful. The latest AI services now offer more accessible user interfaces. These require little knowledge about machine learning. The Microsoft LUIS service (Language Understanding Intelligent Service) performs an amazing task: interpreting natural language sentences and extracting relevant parts. You only need to provide 5+ sample sentences per scenario.

In this article series, we’re creating a sample app that interprets assessments from vital signs checks in hospitals. It filters out relevant information like the measured temperature or pupillary response. Yet, it’s easy to extend the scenario to any other area.

Language Understanding

After creating the backend service and the client user interface in the first two parts, we now start setting up the actual language understanding service. I’m using the LUIS Language Understanding service from Microsoft, which is based on the Cognitive Services of Microsoft Azure.