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How to Ground Truth Label Your Datasets

December 1, 2021

Ground truth labeling is the process of checking machine learning results for accuracy in the real world. For example, autonomous driving systems can move without a human driver or human intervention.

Instead, these types of cars perceive the environment using sensors. For example, the ground truth labeler app labels videos and images for automotive applications.

We all know that labeling is a tedious and time-consuming process. But, it is necessary to create test data to evaluate our perception algorithms accurately. A ground truth labeler is a tool that we have created to reduce some of these pain points.

This tutorial will look at creating ground truth data for object detection and semantic segmentation. We will use algorithms to automate the labeling process and integrate ground truth information from other sensors.

Prerequisites

To follow along with this tutorial, you’ll need to have:

How to locate the app

To find this app:

  • At the top of the window, click on APPS and click on the dropdown arrow as shown below:

image

  • Locate the ground truth labeler in the automation section.

image 2

  • Click on the ground truth labeler to open it.

Ground truth labeler allows us to label videos and images for automotive applications. As we all know, labeling is a tedious and time-consuming process, but it is necessary.

It gives us test data to evaluate the perception algorithm.

Our workflow for this tutorial will be:

  • Creating ground truth data for the detection and semantic segmentation.
  • Using an algorithm to automate the labeling process.
  • Integrating ground truth information from other sensors.

Creating ground truth data for object detection and semantic segmentation

Once we open the app, we need to load data into it. To do this, click on load at the top-left corner.

If you click on it, the app will ask about the data type you are inputting. As you can see, we have three options: video, image sequence, and custom reader.

The most common file format for data in this app is video and image sequences. But, we use the custom reader if you have a different file format from those mentioned.

For this tutorial, we will load a 25-second video into the app. After doing this, we start with the ROI (region of interest) label by clicking on the label button.

labeling button

When you click on it, the following window opens up:

new window

Now, let us begin with vehicle then click ok. Note that you can label your data before defining the ROI.

Also, this app has a workspace shown below where all your labels are stored, and you can see them.

workspace

After completing this, label the vehicle with the bounding box. To label the data with a bounding box, click on the label at the top of the workspace.

vehicles labelled

Let us label the sub-labels. Sub-labels are parts that are associated with a parent label. For example, we can have vehicle number plates or even wheels as the sub-labels.

Let’s, for example, name our vehicles tarlight. To label, the sub-labels, click on the sub-labels right next to the label button and label them on your input using bounding boxes.

You can have as many bounding boxes as you want. If you click on the sub-label button, a new window like that of labeling prompts the user to input the name of the sub-labels.

To make the labels on the input image, you have to select the bounding box of the parent label, and then you make your label inside the bounding box of this main bounding box. If you do not select the parent label first, you will not make your sub-labels.

From the definition, sub-labels are parts of the parent data or ROI.

vehicles and the sublabels

We can assign attributes for our labels and sub-labels. Attribute information makes up the metadata associated with the label.

In our example, we have the vehicle as the parent label. So now the attribute for vehicles can be vehicles type. This attribute window allows the user to input various types as a list. You can have as many as you want.

list of the vehicle types

After making your attributes, you click ok, which appears on the left side of the window. For example, you select the vehicle, move to the left, and choose the vehicle type below to label your vehicle types.

giving vehicle type

We are happy with the labels. You may notice that the labels are not present when we switch frames, as shown below. To go to the next frame, click on the forward button circled in the image below:

next frame

We’ll notice that we can go ahead and repeat the entire process of labeling, but this will be redundant. Therefore, it is ideal to introduce automation in such scenarios.

Use an algorithm to automate the labeling process.

There are so many in-built algorithms in Matlab that help with this. You can use an in-built algorithm or write your algorithm. To do this Matlab allows all these options.

To use an algorithm, click on the select algorithm and select or import algorithm to use.

algorithms

For this tutorial, we will use the Point Tracker algorithm. After selecting the algorithm, you select the vehicles you want to track.

Also, you input how long you want to track it. It is specified beside the play section where you input the start time and end time.

For example, we are going to track the cars for seven seconds.

tracking

Once done, click on automate. Once clicked, the window will display the instruction to use the specified algorithm. Read through the instructions and click on the play button at the top of the window.

automation

Once completed, click on accept. You can look at the label summary to see how the labeling proceeded.

label summary

Other capabilities

Let’s say that we want to label the lanes. In this case, we cannot use bounding boxes; we will use the line and label the lanes. It means that you draw lines on the lanes and label them.

labeling the lane

You can also label individual pixels in the image using the pixel label. For example, let’s say we want to label the road. We will make a label named road and use the pixel label. There is a future flood fill at the top of the window in the image below. It enables you to label a wide region at once.

using pixel label

Note that you can erase sections that you never intended to label using erase. Because flood fill does the labeling randomly over a wide region, you should erase the mislabeling. Also, you can include those that you may have missed to label using the brush.

Another feature called smart polygon allows us to make boundaries of what you want to label and makes its segmentation. After this you can save your work.

Integrate ground truth information from other sensors

This feature allows you to integrate ground truth information from other signals. For example, let’s open a video that has been time-synchronized with lidar data.

To open this, execute the command below:

groundTruthLabeler('01_city_c2s_fcw_10s.mp4', 'ConnectorTargetHandle', @LidarDisplay);

Note that the video 01_city_c2s_fcw_10s.mp4 is a Matlab video and it is readily available. So all you need to do is to execute the command in the command window.

This command opens up two windows.

lidar windows

The first is the ground truth labeler with the video data and the point cloud player, which contains synchronized lidar data. You’ll notice that it is synchronized because if you scrub through the lidar player, we are also scrubbing through the video player.

It is important because lidar data can provide information that vision sensors alone cannot determine. For example, the lidar data provides the vehicle’s distance from the surrounding objects.

Conclusion

Ground truth labeler is very effective in the automation of the labeling process. Despite the labeling process being time-consuming and tedious, this app can do that with a button click.

Also, as we have seen, using the app is very simple. This app opens the door for all the possible algorithm.

It is now up to the user to decide the algorithm to use depending on what she/he does. However, as we know that the accuracy of algorithms varies, not restricting anything to specific algorithms is a good idea.

Happy coding!


Peer Review Contributions by: Dawe Daniel