Build a Line Follower with a Rover and a Webcam
Many line-following robots rely on a dedicated array of infrared sensors to follow a dark line on a light background or a light line on a dark background. This tutorial uses a standard webcam in place of these sensors, and allows a robot to follow a line of any color that is at least somewhat different from the background.
Goal: To make a wheeled robot follow a colored line along the floor using a webcam and the Viam Vision Service color detector.
What you will learn:
- How to use the Viam Vision Service including color detectors
- How to use the Viam Python SDK, including:
- How to establish communication between the code you write and your robot
- How to send commands to components of your robot
What you’ll need
- A single board computer running an instance of
viam-server
- This tutorial assumes the use of a Raspberry Pi running a 64-bit Linux distribution, but these instructions could potentially be adapted for other boards.
- A wheeled base component
- We used a SCUTTLE Robot for this project, but any number of other wheeled bases could work, as long as they can carry the compute module and camera, and can turn in place.
- RGB camera
- A common off-the-shelf webcam (such as this) connected to the Pi’s USB port, or something like an ArduCam with a ribbon connector to the Pi’s camera module port.
- You must mount the camera to the front of the rover pointing down towards the floor.
- Colored tape and floor space
- Any color is suitable as long as its color is somewhat different from the floor color. For our tutorial, we used green electrical tape.
- Non-shiny floors tend to work best.

Configuration using Viam
If you haven’t already, please set up the Raspberry Pi per these instructions.
Configuring the hardware components
Configure the board per the Board Component topic.
We named ours local
.
Use type board
and model pi
if you’re using a Raspberry Pi.
Configure the wheeled base per the Base Component documentation.
We named ours scuttlebase
.
Configure the camera as a webcam.
Your webcam configuration will look something like this:
Or if you prefer the raw JSON:
{
"name": "my_camera",
"type": "camera",
"model": "webcam",
"attributes": {
"video_path": "video0"
},
"depends_on": []
}
Configuring a color detector for the color of your tape line
We’ll use the Viam Vision Service color detector to identify the line to follow.
- Use a color picker like colorpicker.me to approximate the color of your line and get the corresponding hexadecimal hash to put in your config.
Put this hash in the
detect_color
parameter. We used #19FFD9 to represent the color of green electrical tape. - We used a segment size of 100 pixels, and a tolerance of 0.06, but you can tweak these later to fine tune your line follower.
Navigate to the robot page on the Viam app. Click on the robot you wish to add the Vision Service to. Select the Config tab, and click on Services.
Scroll to the Create Service section. To create a Vision Service:
- Select
vision
as the Type. - Enter
green_detector
as the Name. - Select
color_detector
as the Model. - Click Create Service.
In your Vision Service’s panel, fill in the Attributes field.
{
"segment_size_px": 100,
"detect_color": "#19FFD9",
"hue_tolerance_pct": 0.06
}
Add the Vision Service object to the services array in your rover’s raw JSON configuration:
"services": [
{
"name": "green_detector",
"type": "vision",
"model": "color_detector",
"attributes": {
"segment_size_px": 100,
"detect_color": "#19FFD9",
"hue_tolerance_pct": 0.06
}
},
... // Other services
]
Configuring the visualizer
This step is optional, but if you’d like to see the bounding boxes that the color detector identifies, you’ll need to configure a transform camera. This isn’t another piece of hardware, but rather a virtual “camera” that takes in the stream from the webcam we just configured and outputs a stream overlaid with bounding boxes representing the color detections.
In the Config tab, make a new component with name show_detections
, type camera
and model transform
.
Set the stream
to "color"
and set the source
to "my_camera"
or whatever you named your webcam.
You’ll need to edit the pipeline
section as well with type
set to "detections"
, and detector_name
set to the name of your color detector ("green_detector"
in our case).
You can paste the following into the Attributes section of the show_detections
config builder:
{
"stream": "color",
"source": "my_camera",
"pipeline": [
{
"type": "detections",
"attributes": {
"detector_name": "green_detector"
}
}
]
}
If you save the config and go to the Control tab, you should now be able to view the camera feed with color detector overlays superimposed on the image.
Full example config
Below is an example JSON file that includes the board, base and camera components, and a Vision Service color detector. You may have different pin numbers and other attributes depending on your hardware setup.
How line following works
You position the rover so that its camera can see the colored line.
If the color of the line is detected in the top center of the camera frame, the rover will drive forward. When it doesn’t see the line color there, it will search for the color in the left side of the camera frame. If it detects the color there, it will turn to the left. If it doesn’t see the color there it’ll repeat the process on the right. Once the line is back in the center front of the camera frame, the rover continues forward.
When the rover no longer sees any of the line color anywhere in the front portion of the camera frame, it stops and the program ends.
Let’s write some code
Make sure you have Python installed. You can double-check this by running:
python --version
or if you have multiple versions of Python installed, try
python3.9 --version
or
python3.8 --version
We at Viam are running Python 3.9.2 (Python 3.8 is also supported) for this tutorial.
Make sure you have the Viam Python SDK installed.
Open a file in your favorite IDE and paste in the code from the earlier referenced repo.
Adjust the component names to match the component names you created in your config file. In this case, the component names that you may need to change are scuttlebase, my_camera, and green_detector.
From your robot’s page on the Viam app, go to the Code Sample tab. Find the Python SDK field and copy the robot address (which will likely have the form
robotName-main.1234abcd.local.viam.cloud:8080 ) and payload (a nonsensical string of numbers and letters) from it into the corresponding fields towards the top of your command file. This allows your code to connect to your robot.Caution
Do not share your robot secret or robot address publicly. Sharing this information compromises your system security by allowing unauthorized access to your computer.
Save the code in a directory of your choice.
To get the code onto the Pi you have a few options. If you intend to make lots of tweaks to the code over time it may be most convenient for you to set up a Mutagen Sync session from a directory on your computer to a directory on your Pi. If you’re just trying to get this running as quickly as possible, do the following:
- In your Pi terminal, navigate to the directory where you’d like to save your code.
Run,
nano rgb_follower.py (or replacergb_follower with the your desired filename). - Paste all your code into this file. Type CTRL + X to close the file. Type Y to confirm file modification, then press enter to finish.
- In your Pi terminal, navigate to the directory where you’d like to save your code.
Run,
Controlling your rover with Viam
Go to your robot’s page on the Viam app. Verify that it’s connected by refreshing the page and ensuring that Last Online (in the top banner) says, “Live.”
Go to the Control tab and try viewing the camera and also pressing buttons in the Base section to move your robot around. Ensure that the base moves as expected. If one or both drive motors are going backwards, you can power down the Pi by running
sudo poweroff
, unplug the battery, and switch the wires to the motor before powering it back on.Now for the creative part: Use your colored tape to make a path for your robot to follow. Perhaps a circle or other shape, or perhaps a path from one point of interest to another. Sharp corners will be more challenging for the robot to follow so consider creating more gentle curves.
Set your robot on the line such that the line appears in the front of the camera’s view. Verify that the camera sees the line by viewing the camera feed on the Control tab of the robot page.
In a terminal window, SSH to your Pi by running:
ssh <your_username>@<your_pi’s_name>.local
Replace the angle brackets and the example text with your actual Pi username and the name of your Pi. Remember to delete the angle brackets!
In this Pi terminal go ahead and run the code:
python ~/myCode/rgb_follower.py
Be sure to replace ~/myCode with the path to the directory where you saved your Python script, and
rgb_follower.py with whatever you named your Python script file. You may need to callpython3.9
orpython3.8
instead ofpython
, depending on how you configured your Pi.
The robot should continue moving along the line until it no longer sees the color of your detector except at the back of the frame, at which point it should stop moving and the code will terminate.
Summary
By now you have learned how to configure a wheeled base and camera with Viam. You have access to the Control tab from which you can drive your rover around with WASD keys. You have learned to use the Viam Vision Service color detector, which can be useful in many other projects. You have a rover following a path of your choice, anywhere you want it to go!
Troubleshooting
Issue: The rover moves too fast to track the line
If your rover keeps driving off the line so fast that the color detector can’t keep up, you can try two things:
- Slow down the move straight and turning speeds of the rover by decreasing the values of
linear_power
andangular_power
.- Conversely, if your rover is moving too slowly or stalling, increase the numbers (closer to 1.0 which represents full power).
Issue: The robot is not detecting the color accurately
Things to try:
- Add a
saturation_cutoff_pct
and/or avalue_cutoff_percent
(documented here) to your Vision Service parameters. - Try to achieve more consistent lighting on and around the line.
- Try a different color of line, or a different background.
Be sure to update your
detect_color
parameter accordingly.
Additional Troubleshooting
You can find additional assistance in the Troubleshooting section.
You can also ask questions in the Community Discord and we will be happy to help.
Bonus Challenges
- Automatically detect what color line the robot is on and follow that.
- Use two differently colored lines that intersect and make the robot switch from one line to the other.
- Put two rovers on intersecting lines and write code to keep them from crashing into each other.
Have questions, or want to meet other people working on robots? Join our Community Discord.
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