As we all know, the world we live in is three-dimensional, while photos are two-dimensional, so we can think of the camera as a function, the input is a scene, and the output is a grayscale image. The function of this process from three-dimensional to two-dimensional is irreversible. The goal of camera calibration is to find a suitable mathematical model and calculate the parameters of the model, so that we can approximate the three-dimensional to two-dimensional process and find the inverse function of the three-dimensional to two-dimensional process function. This approximation process is called "camera calibration". We use a simple mathematical model to express the complex imaging process and find out the inverse process of imaging. After calibration, the camera can reconstruct the 3D scene, that is, depth perception, which is a major branch of computer vision. Therefore, in this process, the calibration board is often needed. Do you know what role the calibration plate can play in camera calibration? Here's a brief introduction:
The calibration plate can play the following roles in camera calibration:
1. The calibration plate can determine the correspondence between object points and image points.
The principle is mainly "perspective invariance". For example, if you look closely at a person and look far away at a person, although the size of his nose has changed, and the perspective of your nose has also changed, the topology must be unchanged, and you can't think of the nose as a mouth.
Therefore, in the calibration board, the topology is printed, and the checkerboard and dot grid are widely used. The reason why these two become mainstream is not only because their topology is clear and uniform, but also because the algorithm to detect their topology is simple and effective. The checkerboard detects corners, which can be obtained by calculating gradients in both horizontal and vertical directions of the captured checkerboard image; The detection of dot grid only needs to calculate the centroid of the captured dot grid pattern. If you develop a perfect algorithm to detect all the features of a face, you can use your photos as a calibration plate.
2. The corner points in the calibration plate are transformed to the coordinates under the camera coordinate system [Formula].
For beginners of calibration, it is easy to ignore that the calibration plate has a calibration plate coordinate system. In other words, the position of each corner point in the calibration plate coordinate system is determined and known.
The elements of the transformation matrix from the calibration plate coordinate system to the camera coordinate system are called external parameters, so in our eyes, the external parameters for camera calibration are basically the by-products of the calibration internal parameters, which will vary with the placement of the calibration plate. The transformation matrix from the calibration plate coordinate system to the camera coordinate system changes.
To sum up, the calibration plate can play two roles in camera calibration. At present, the calibration plate is mainly used to correct lens distortion in machine vision, image measurement, photogrammetry, 3D reconstruction and other applications. The geometric model of the camera can be obtained through the camera shooting a flat plate with a fixed spacing pattern and the calculation of the calibration algorithm, so as to obtain high-precision measurement and reconstruction results.
