Optical flow method for a brief introduction to __ video

The concept of optical flow was first proposed by Gibson in 1950. It is the instantaneous velocity of the pixel motion of the space moving object in observing the imaging plane, it is a method to find out the corresponding relationship between the previous frame and the current frame by using the change of the pixel in the time domain and the correlation between the adjacent frames in order to compute the motion information of the object between the adjacent frames. In general, the optical flow is caused by the movement of the foreground object itself in the scene, the motion of the camera, or the joint motion of both. Its calculation methods can be divided into three categories:

(1) Matching method based on region or feature;

(2) The method based on frequency domain;

(3) The method based on gradient;

In short, the optical flow is the "instantaneous velocity" of the pixel motion of the space moving object on the observation plane. The study of optical flow is to determine the "motion" of pixel position by using the time domain variation and correlation of pixel intensity data in the image sequence. The purpose of studying the optical flow field is to approximate the field which can not be obtained directly from the image sequence.

The assumption of the optical flow method:

(1) The brightness between adjacent frames is constant;

(2) The frame time of adjacent video frames is continuous, or the movement of objects between adjacent frames is relatively "small";

(3) Maintain space consistency; that is, the pixel points of the same child image have the same motion

Here are two concepts to explain:

Sports field, in fact, is the movement of objects in the three-dimensional real world;

The Optical flow field is the projection of the sports field on the two-dimensional image plane.

As shown in the figure above, the pixel (x,y) in H is moved to (X+U,Y+V) in the position of (U,V).

The principle of optical flow method for target detection: a velocity vector is given to each pixel point in the image, thus a motion vector field is formed. At a particular moment, the point on the image corresponds to the point one by one on the three-dimensional object, which can be computed by projection. According to the velocity vector characteristics of each pixel, the image can be dynamically analyzed. If there is no moving target in the image, the amount of light vectoring is continuously changing in the whole image region. When there is a moving object in the image, there is a relative movement between the target and the background. The velocity vector formed by the moving object must be different from the velocity vector of the background, so the position of the moving object can be calculated. It is necessary to remind that the method of optical flow is used to detect moving objects, which can not guarantee real-time and practicability.

The principle of optical flow method for target tracking:

(1) The processing of a continuous video frame sequence;

(2) For each video sequence, a certain target detection method is used to detect potential foreground targets;

(3) If a frame appears the foreground target, find its representative key characteristic point (can be randomly produced, also can use corner point to do feature point);

(4) for any two adjacent video frames, the best position of the key feature points appearing in the previous frame is found in the current frame, thus the position coordinates of the foreground target in the current frame are obtained.

(5) This iterative, then can achieve the target tracking;