CV_SS08_Lab_2

Computer Vision

SS 2008

Tutorial/Lab II

MSc in CS Moritz Vieth

SS 2008 Tutorial/Lab I 2

Overview

Reading

Programming in CV

− Basic steps− OpenCV− Starter kit

The CV Lab

− Equipment− Accounts− Source control (SVN)


ReadingsWhat is the main underlying problem of CV?What is a model?What are the four representations presented in

the reading?In which way can concepts of AI/Machine

Learning aid CV?Does CV have to exactly reproduce the human

perception system?Name some CV applications (and their means)


Programming in CVBasic tasks:

− Data acquisitionSingle imagesImage sequences (video file/camera)

− Image processingManipulationAnalysis

− OutputDisplayFilesetc.


OpenCVOpen Source Library

− Originally developed by Intel− Written in C/C++ (but mostly C)− http://opencvlibrary.sourceforge.net/

Basic functions and structures

Advanced algorithms

“Head start into CV programming”

Nomenclature:

− Methods start with cv (lowercase)− Structures start with Cv (uppercase)


OpenCV: Basic FunctionsStructures & methods for

− Image handling− Image acquisition

Single image fileVideo fileCamera

− Image manipulation− Output

DisplaySave to file


OpenCV: Advanced AlgorithmsAlgorithms and functions commonly used in CV

applications

− Edge/Corner detection− Pattern matching− Structural analysis− Motion detection− Tracking− Calibration (not that good, use MATLAB instead)− Face Detection− ...


Using OpenCVImage acquisition (single file)

#include "cv.h"#include "highgui.h"

int main( int argc, char** argv ){ IplImage* img; if(img = cvLoadImage( “image.jpg”,

CV_LOAD_IMAGE_COLOR)) != 0 ){

doSomething(img); // your processing method cvWaitKey(5); // very important, contains event // processing loop inside

return 0; } return 1;}


Using OpenCVImage acquisition (video file)


int main( int argc, char** argv ){ IplImage* img; CvCapture* capture = cvCaptureFromFile(“video.mpg”); while(img = cvQueryFrame(capture) != 0 ))

{ doSomething(img); // your processing method cvWaitKey(5); // very important, contains event // processing loop inside

return 0; }

cvDestroyCapture(&capture); return 1;}


Using OpenCVImage acquisition (camera)


int main( int argc, char** argv ){ IplImage* img; CvCapture* capture = cvCaptureFromCAM(0); while(img = cvQueryFrame(capture) != 0 ))

{ doSomething(img); // your processing method cvWaitKey(5); // very important, contains event // processing loop inside

return 0; }

cvDestroyCapture(&capture); return 1;}


Using OpenCVImage handling: Accessing a pixel

Easy, isn't it?− We'll come back to that

//Set pixel value://IplImage* image, int channel, int value

(((uchar*)(image>imageData + image>widthStep*(y))))[x *image>nChannels + channel] = (uchar)value;

//Get pixel value://IplImage* image, int channel, int value

value = (((uchar*)(image>imageData + image>widthStep*(y)))[(x * image>nChannels) + channel] ;


Using OpenCVImage manipulation: Drawing

Same goes for cvCircle, cvLine, cvEllipse, ...

IplImage* img;if(img = cvLoadImage( “image.jpg”, CV_LOAD_IMAGE_COLOR)) != 0){ //draw a red rectangle from (20,20) to (100,100)

cvRectangle(img, cvPoint(20,20), cvPoint(100,100), CV_RGB(255,0,0));

cvWaitKey(5); // very important, contains event // processing loop inside

}


Using OpenCVImage manipulation: Color conversion

IplImage* img;IplImage* hsv_img;

if(img = cvLoadImage( “image.jpg”, CV_LOAD_IMAGE_COLOR)) != 0){ hsv_img = cvCreateImg(cvGetSize(img),8,3); cvCvtColor(img, hsv_img, CV_BGR2HSV);


}


Using OpenCVImage output: Windows

Windows are created and referenced by nameYou can use as many NamedWindows as you

like

CvCapture* capture = cvCaptureFromCAM(0);IplImage* frame = cvQueryFrame(capture)cvNamedWindow( "myWindow", CV_WINDOW_AUTOSIZE );

while(frame != 0){cvShowImage(“myWindow”, frame);cvWaitKey(5);frame = cvQueryFrame(capture);

}cvReleaseCapture(&capture);cvDestroyWindow(“myWindow”);


Using OpenCVImage output: Saving

IplImage* img;if(img = cvLoadImage( “image.jpg”, CV_LOAD_IMAGE_COLOR)) != 0){ //draw a red rectangle from (20,20) to (100,100)

cvRectangle(img, cvPoint(20,20), cvPoint(100,100), CV_RGB(255,0,0));


cvSaveImage(“image.bmp”, img);

return 0;}


Using OpenCVInteractivity

CvCapture* capture = cvCaptureFromCAM(0);IplImage* frame = cvQueryFrame(capture);

cvNamedWindow( "myWindow", CV_WINDOW_AUTOSIZE );int key = 0;

while(frame != 0 && key != 'c'){cvShowImage(“mywindow”, frame);frame = cvQueryFrame(capture);key = cvWaitKey(250);

}cvReleaseCapture(&capture);cvDestroyWindow(“myWindow”);

[spot the error]


OpenCV: CompilingIf you are compiling OpenCV from scratch,

don't forget to add ffmpeg support!To compile programs using OpenCV:

− Specify the include path:-I/usr/local/include/opencv

− Specify the library path:-L/usr/local/lib/

− (or wherever you installed it)− Tell the linker to use the shared libraries:-lcv -lcvaux -lhighgui


OpenCV: CompilingCommon errors:

− “Undefined reference to...” when compiling/linking-> Make sure you have linked to the libs (-l...), and set

the library path correctly (-L...)

− “Undefined reference to...” when starting the programTell the shell how to find the shared libraries(Linux:) export LD_LIBRARY_PATH=/usr/local/lib

− (or wherever the missing libs are)


OpenCV: ExamplesFind Circles in an Image:

− cvHoughCircles(...)

[Images by Thomas Hofhammer]


OpenCV: ExamplesDetect Faces:

− cascade = (CvHaarClassifierCascade*) cvLoad( cascade_name, 0, 0, 0 );

− CvSeq* faces = cvHaarDetectObjects( small_img, cascade, storage,1.1, 2, 0, cvSize(30, 30) );

www.lenna.org


See it liveA short demo

− Read and display a video stream− Paint circles− React to user input


CvUtilsOpenCV is not complete

− Sometimes a little complicatedRemember pixel access?

− Some (utility) functions are just missing

“CvUtils” library (by me)− ImageUtils -> Mostly access and drawing− MathUtils -> Frequently used shortcuts− Pixel -> used to store coordinates along with color− Blob -> a collection of pixels− ParFile (by Stefan Hahne) -> Easy config file

access


Starter KitShould accelerate the “getting started” partA bit more sophisticated than the demoMain file

− reads config file and command line

ControllerBasic input (video file or camera)Basic output (one main window)Error codesUses CvUtils


Output LibraryShared library for outputUsed to format outputControls output level (no more searching for

those DEBUG messages)Automated loggingAutomatic display of function names (if desired)Thread-safeBacktrace support (for Linux/Unix)Silly name (niftout)


EquipmentEquipment available for projects:

− Various IEEE1394 industrial cams Color or b/w, mostly 640x480Various lenses (wide angle and tele)

− Sony IEEE1394 zoom cams− Several USB webcams− 2 (3) PTZ cams− Tripods

Some other stuff (e.g. Stereo Cam) in the RoboCup lab

− Just ask them!


CV-LabYou can work on your projects in the CV lab

− Get accounts on the lab server− Work on fast machines (some dual or quad core)− Multiple FireWire support

Use source control− There is a SVN repository on the lab server− Access via lab account− ->Ask!


Project ProposalsContents Name(s) of Student(s)Student ID(s) (Matr. Nr.)Title of the CV project

(after approval of the instructor)Description of the CV project (plan of attack)

(max. 2 pages, incl. graphics)(kind of working contract between you and me)

References (min. 2 peer reviewed)of a project proposal


Project ProposalsDescription of the CV project should contain:

− IntroductionWhat is the project about? What is the context of the

project? What is the Motivation?

− MaterialsWhat is the Equipment needed?What setup do you have in mind?

− MethodsHow are you going to achieve your goals? (CV methods,

not a “storyline”)


Project ProposalsDescription of the CV project should contain

(cont'd):− Assumptions

What are the prerequisites for your project (e.g. lighting conditions, background, etc.)?

What are the restrictions on you project?What may be changed in the environment?

− Expected results− Distinction: who does what?

Documents

CV_SS08_Lab_2