Lecture-14 Motion detection

1、12022-3-6Lecture 1: Introduction 14.1 Introduction 14.2 Motion detection methods 14.3 Video processingLecture 14: Motion D22022-3-614.1 Introduction First, the world is dynamic, and motion information is one of the keys to many practical applications such as video mining, Robert navigation, intellig

2、ent traffic system, etc. Motion Detection is also a basic step to machine learning, pattern recognition in dynamic context.Why Motion Detection? 32022-3-614.1 IntroductionExtract the moving foreground from image sequence, which is to separate the moving foreground from background.将运动前景从图像序列中提取出来，也就是

3、说将背景与运动前景分离开。42022-3-6 Static Camera Static Objects Static Camera Moving Objects Moving Camera Static Objects Moving CameraMoving Objects14.1 I52022-3-614.2 Motion detectionTwo approaches : 1. Motion detection based on foreground information directly.Foreground information直接利用前景信息检测前景。62022-3-614.2

4、Motion detectionTwo approaches : 2. The input image subtract the background image and get a foreground image.先得到背景图象，然后将输入图象减去背景图像从而得到前景图象。Input imageBackground 72022-3-6一些场景的背景相对固定，而前景变化较大。Two approaches : 1. Motion detection based on foreground information directly.2. The input image subtract the

5、background image and get a foreground image.According to the specific application decide which approach is adopted. 两种思路，视具体环境选择.一些场景的背景有较大变化，而前景的一些特征变化不大。14.2 Motion 82022-3-6帧间差分法(Frame differencing)背景差法(Background subtraction)光流法(Optical flow)前景建模方法14.2 Motion 92022-3-614.2 Motion detection A sim

6、ple way to detect the variation between frames is to do subtraction on gray-scale values of corresponding pixels. Double frame difference（双帧差分）1( , ) |( , )( , )|nnnD i jI i jIi jDifference Image：1,nnIIwhere are two images of adjacent frame.0( , )1nMi j( , )( , )nnD i jTD i jTMoving foregroundT10202

7、2-3-614.2 Motion detection Three-frame difference （三帧差分）11( , ) |( , )( , )|( , )( , )|nnnnnD i jIi jI i jI i jIi jDifference Image：11,nnnIIIwhere are three images of adjacent frame.0( , )1nMi j( , )( , )nnD i jTD i jTMoving foregroundT112022-3-6 优点：适用于动态变化的背景环境。 不足：较难准确检测运动速度过快的物体；较难准确检测运动速度过慢的物体；较

8、难准确检测场景中同时存在的多个运动物体。14.2 Motion 122022-3-614.2 Motion detectionPrinciple：Construction the background image of scene , Calculate the difference on gray-scale value between current image and background image pixel-by-pixel.构建背景图像，计算当前图像与背景图像的逐象素的灰度差。( , ) |( , )( , )|nnD i jI i jB i jCurrent imageBack

9、ground image0( , )1nMi j( , )( , )nnD i jTD i jTMoving foregroundT132022-3-6Input imageBackground image14.2 Motion detection?Moving foreground How to construct background image？- -求其均值图像；求其均值图像；背景未知：背景已知：- -图像训练集的中值/均值图像；142022-3-6 1 2 3 4 5 6a.均值图像：若干背景图像求逐点的灰度均值。14.2 Motion detection简单，易于实现；不适用于背景变化的情况152022-3-614.3 Video processingCommands for Image ProcessingRead a video file:avi