Follow ROCK's workflow to calibrate your camera.
Understanding the camera calibration process is crucial for users of ROCK LiDAR units. Your ROCK LiDAR system provides the flexibility of removing the camera for handheld SLAM dock or Mobile Mount use, before reattaching it for drone-based data collection. However, this necessitates the need for precise camera calibration to achieve optimal alignment between the RGB and the LiDAR data in Intensity view. This article will provide an in-depth, step-by-step guide for camera calibration using ROCK Desktop.
When to calibrate the camera on your ROCK LiDAR unit:
- If the camera has been removed and reattached to your LiDAR unit. The most common example of this is when the your ROCK LiDAR unit has the camera removed to collect a SLAM or Mobile Mount Dataset.
- The mounting screws for the camera are loosened or tightened by someone. Even if it feels like the mounting screw was minimally tightened, ROCK would recommend completing a calibration.
- If you are seeing offsets in distinct features within your LiDAR point cloud.
Camera Calibration Flight Specifics:
- Your camera calibration flight needs to be collected with a flight height of 40 meters, a flight speed of 4-5 meters per second, and a 50% overlap.
- You camera calibration flight should take place over an empty parking lot or field, where there are distinct features that can be used for calibration. A parking lot with dark asphalt and crisp parking lines is ideal. You can also use a large sports field which has clearly painted lines.
- A minimum of three flight lines need to be collected over the camera calibration area.
- Your camera calibration flight needs to include all necessary calibration flights.
Note: When locating a site for your camera calibration flight, please follow any and all rules related to flying in that area.
1. Preparing for Calibration
Start your calibration process by opening ROCK Desktop, and follow the steps to produce a LiDAR point cloud. These steps include processing the trajectory, selecting the trajectory, adjusting the angle gate, and adjusting the range gate. Once completed, you will be on the "Process Point Cloud" step. Click on the "START PROCESSING" drop down, and select "Calibrate Camera & Process.
2. Choosing the Ideal Photos for Calibration
A key element to a successful camera calibration is choosing good photos. These photos should feature distinct characteristics in all corners and the center. Ideal images should contain varied features across their entire surface area.
ROCK recommends that you select 5-7 photos to use for the calibration process. Once you've selected your photos, click "START PROCESSING".
3. Adding Key Pair Point Matches
ROCK Desktop will now display a single view of the Point Cloud data for the photo(s) you have selected. Your task is to add key pair point matches in both the Intensity view and the RGB view. Take the time to make sure the Intensity Range is adjusted so that the distinct features you will need to locate are identifiable. It is also recommended that you switch back and forth between the Intensity view and RGB view, and try to locate the spots that you will use for the pair points.
The above images show the RGB (left) view and Intensity (right) view for the same photo. The right angle on the painted yellow line would be a good area to create a pair point.
- Once you have located the pair points you intend to use for the photo, click on "ADD CAMERA POINT", then use your mouse to select the unique point in the RGB view for the pair point.
- Click on "ADD LiDAR POINT", and add the an Intensity point corresponding to the RGB point's location.
NOTE: After selecting your LiDAR or RGB point, you can click to "VIEW" the point, and edit it's location if needed. You can also delete a pair point by clicking on the trash icon to the right of the pair point. - Repeat the pair point process throughout the image so that you ideally have 5 pair points, distributed through the photo in the top left, top right, bottom left, bottom right, and center.
- After completing the process for a single photo, click "NEXT" so that you can move to the next photo and repeat the process of generating pair point.
- You will need to generate a minimum of 10 pair points to calibrate your camera, however ROCK recommends using more than that.
- Once you have completed the process of generating the pair points in your photos, you will see a "CALIBRATE CAMERA" button in the lower right of ROCK Desktop, click there.
4. Calibrating the Camera
After creating all the pair point matches, click "CALIBRATE CAMERA". You'll be presented with four options:
- Auto Extrinsic
- Extrinsic
- Intrinsic
- Both (Intrinsic and Extrinsic)
Important: You should only calibrate the Extrinsic on the R3Pro V2 or R3Pro LiDAR units. The Intrinsic calibration is fixed to the camera and change minimally over time. The camera's orientation with respect to the LiDAR body, however, changes whenever you attach or detach the camera. For R2A and R360 LiDAR units you'll need to do Both (Intrinsic and Extrinsic).
Once the calibration process is complete, you should inspect your results, and save the calibration if you are satisfied with the results. Following this comprehensive guide will ensure you obtain the best results from your ROCK LiDAR system. Precise camera calibration is essential for achieving optimal alignment of RGB and LiDAR data, which ultimately leads to higher quality, more accurate outputs.