Optical Flow walk through Aidean Sharghi Spring 14

Optical Flow The objective in this assignment is to setup and use Optical Flow This implementation is fast and so you will see it can be used for real-time purposes Required steps to setup and run are provided in this presentation

Contents OpenCV installation process Setting Environment Variables How to run Optical Flow

OpenCV OpenCV stands for Open-source Computer Vision It is a library of useful functions used in Computer Vision To run Optical Flow: – Install OpenCV 2.2 if you have VS 2010 installed on your machine – Install OpenCV 2.4 if you have VS 2012

Install OpenCV2.2 for VS 2010 Download OpenCV2.2 from Extract the files in C:\OpenCV2.2 Set Environment Variables. – If variable ‘path’ exists, just add the C:\OpenCV2.2\bin\ to the end of its value – If ‘path’ does not exist, just add a new variable and name it ‘path’ and set its value to what mentioned above

Install OpenCV2.4 for VS 2012 Download OpenCV2.4 from Extract the files in C:\opencv Set Environment Variables. – Add the new variable ‘path’ and set its value to “C:\opencv\build\(your VS build x86/x64)\vc11\bin\ “ – E.g. if you have VS x64 version installed on your machine, the value will be: C:\opencv\build\x64\vc11\bin

Set the Environment Variable in Windows 7/8/8.1

Building the Project Just download and build the project using your VS Open a command prompt and change its directory to your project’s debug folder Give the input

Command Prompt Open Command Prompt – To do so, you can hold Ctrl key and press R. Run window will show up. Then write cmd in the box and press enter. Change Directory to the Debug Folder – cd is the command you need to use. For example if you enter: – cd C:\Users\’your username’\Desktop This will take you to your desktop

Two ways to use When you are in the Debug folder, then you have two options Compute Optical Flow frames imageFile1 imageFile2 change directory to the debug folder then enter these: > OpticalFlow.exe frames image0.jpg image1.jpg Find the Optical Flow by webcam camera or NULL > OpticalFlow.exe camera

Four keys to control w : increment (by 10) number of corners to track (max = 400) s : decrement (by 10) number of corners to track (min=10) d : double pyramid window size (max=64) a : half pyramid window size (min=2)

Code Analysis Load Image 1 Interest Points KLT Tracker using 2 images

Harris Corner Detector

KLT Tracker Detect Harris corners in the first frame Build the Optical Flow Pyramid For each Harris corner compute motion between consecutive frames Link motion vectors in successive frames to get a track for each Harris point Introduce new Harris points by applying Harris detector at every m frames

void cvCalcOpticalFlowPyrLK(const CvArr* prev, const CvArr* curr, CvArr* prevPyr, CvArr* currPyr, const CvPoint2D32f* prevFeatures, CvPoint2D32f* currFeatures, int count, CvSize winSize, int level, char* status, float* track_error, CvTermCriteria criteria, int flags) Two useful functions

int buildOpticalFlowPyramid(InputArray img, OutputArrayOfArrays pyramid, Size winSize, int maxLevel, bool withDerivatives=true, int pyrBorder=BORDER_REFLECT_101, intderivBorder=BORDER_CONSTANT, bool tryReuseInputImage=true)