What is a Charuco board and why you should use it?

What is a ChArUco?

The word ChArUco is derived from the combination of chessboard + aruco. You can comprehend it clearly with the below image.

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An ArUco marker is a synthetic square marker composed of a wide black border and an inner binary matrix which determines its identifier (ID). The black border facilitates its fast detection in the image and the binary codification allows its identification and the application of error detection and correction techniques. The marker size determines the size of the internal matrix. For instance, a marker size of 4x4 is composed of 16 bits.

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In simple words, charuco is a combination of chessboard and ArUco that facilitates fast corner detection (as chessboard) and also allows you to uniquely identify the corners independent of the other corners. Contrary to a chessboard where for detection you need to have a complete chessboard in the field of view, a visible charuco board can be identified even with a partial board visible in the image. The ArUco is defined by a dictionary which is the number of bits present and each pattern represents a unique number/ID.

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Corner Detection can be used to get points in the real world whose measurements you know and whose correlation can be used for calibration of different equipment that will be used by different machines for different purposes like ADAS systems in automobiles, for the surrounding identification by robots, etc.
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You can easily generate a charuco board from this link: https://calibrate.deepen.ai/target-generator

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Why choose a digital target (like a monitor or TV) over a physical target?

1. The targets are flat (unless you use a curved monitor) which is a very important feature in calibration. You don’t have to worry about small air bubbles which might have been formed while sticking the target to a board.
2. Perfect measurement of square length: You can use any flat display and generate a charuco that fits end to end and calculate the length of the squares mathematically using the number of squares and the dimensions of the display. Even a tiny difference in the square size used can give significant error in the results. If you use physical targets instead, you have to approach third-party vendors with precision printing or manufacturing tools to get high accuracy numbers.
3. No special types of equipment are required: we don’t require any special equipment to have high-accuracy measurements which saves a lot of time.
4. Dependence on external lighting is reduced significantly.
5. The display can be reused for other purposes once calibration is done.
6. Fast and Easy change in the number of squares and dictionary type which saves a lot of time, effort, and other resources.

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Summary:

Charuco board gives a lot of advantages over the checkerboard / chessboard especially when you are looking to reduce the number of images captured and get high coverage. It gives us flexibility in a lot of ways. Having a digital target and generating a charuco that fits the screen can help you get very high accuracy in the measurement of squares without special equipment which increases the accuracy of your calibration. Use this link to generate a charuco board for your digital target: https://calibrate.deepen.ai/target-generator

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Visit www.deepen.ai/calibrate to learn more about our calibration offerings.