Introduction to OpenCV

Computer Vision
OpenCV
Luigi De Russis
Politecnico di Torino
Dipartimento di Automatica e Informatica (DAUIN)
Torino - Italy
luigi.derussis@polito.it

This work is licensed under the Creative Commons (CC BY-SA)
License. To view a copy of this license, visit
http://creativecommons.org/licenses/by-sa/3.0/

What is OpenCV?
 Open source Computer Vision library
 http://opencv.org/
 Originally developed by Intel
 Has more than 2500 optimized algorithms
 C/C++/Python API
 it is written natively in C++
 Cross-platform
includes a
 also available for Android and iOS
Java API
 Released under a BSD license (it’s free)
 Current release version: 2.4.3RC (October 2012)
10/26/2012 Luigi De Russis - Computer Vision - OpenCV 2

Various applications…
 Human-Computer Interaction (HCI)
 Object Identification
 Object Recognition
 Face Recognition
 Gesture Recognition
 Motion Tracking
 Image Processing
 Mobile Robotics
 … and so on.


Why OpenCV? (I)
 Best competitor here: Matlab
 Pros
 Specific
 OpenCV was made for image processing
 Matlab is quite generic
 Speed
 around 30 frames processed per seconds in real time image
processing (OpenCV)
 around 4-5 frames processed per seconds in real time image
processing (Matlab)
 Efficient
 Matlab needs more system resources than OpenCV

Why OpenCV? (II)
 Cons
 Easy of use
 Integrated Development Environment
 you can use Eclipse, Netbeans, Visual Studio, Qt, XCode, … a
simple text editor for OpenCV
 Memory management
 Two more “pros”…
 Price (!)
 OpenCV Wrappers
 SimpleCV, JavaCV, Emgu CV, JavacvPro, …


Modules (I)
 OpenCV has a modular structure, i.e., the package
includes several shared or static libraries:
 core
 basic structures and algorithms
 imgproc
 Image Processing algorithms (such as image filtering,
geometrical image transformations, histograms, etc.)
 video
 video analysis (such as motion estimation and object tracking)
 highgui
 built-in simple UI
 in addition, we will use Qt

Modules (II)
 calib3d
 camera calibration and 3D reconstruction
 features2d
 2D features framework (feature detectors, descriptors, and
descriptor matchers)
 objdetect
 detection of objects and other items (e.g., faces, eyes, mugs,
people, …)
 ml
 machine learning classes used for statistical classification,
regression and clustering of data
 gpu
 GPU-accelerated algorithms

Data structures (I)
 We speak about C++ API
 All the OpenCV classes and functions are placed
into the cv namespace
 Mat
 the primary image structure in OpenCV 2.x
 overcomes the “old” IplImage/CvMat problems
(OpenCV 1.x/C API)
 automatic memory management (more or less)
 two data parts:
 matrix header (contains information about the matrix)
 a pointer to the matrix containing the pixel values

Data structures (II)
 Point_ (Point2f, Point, Point2d)
 2D point
 defined by x, y coordinates
 Point first(2, 3);
 Size_ (Size, Size2f)
 2D size structure
 specify the size (width and height) of an image or
rectangle
 Rect
 2D rectangle object


Image I/O
 Image I/O
 imread
 legge un’immagine da file e lo salva in un oggetto di tipo Mat
Mat imread(const string& filename, int flags=1)
 imwrite
 salva un’immagine su file
bool imwrite(const string& filename, InputArray img,
const vector<int>& params=vector<int>())
 imshow
 mostra un’immagine a schermo (in una finestra creata
precedentemente)
void imshow(const string& winname, InputArray mat)


Drawing operations
 Base drawing operations
 circle
 draws a simple or filled circle with a given center and radius on a
given image
 line
 draws a line between two point in the given image
 ellipse
 draws an ellipse outline, a filled ellipse, an elliptic arc, a filled
ellipse sector, …
 rectangle
 draws a rectangle outline or a filled rectangle
 note that negative thickness will fill the rectangle

Color spaces
 Converting color spaces
 cvtColor
 converts an input image from one color space to another
 examples:
 cvtColor(src, dest, CV_RGB2GRAY);
 cvtColor(src, dest, CV_HSV2BGR);
 cvtColor(src, dest, CV_RGB2BGR);
 important, since images in OpenCV uses BGR instead of
RGB


How we can use OpenCV?
 LABINF:
 already installed under Windows
 version 2.3.1

 Qt Creator (4.7.4) is the IDE to be used

 At home:
 feel free to install OpenCV version 2.4.3
 it should be more “stable”
 you can use whatever IDE you like
 but we give full support only for Qt Creator
 Installation:
 see the next set of slides for a step-by-step guide


What if I got problems?
 Small problems
 drop me a line Problems with Qt and a gray scale image

 luigi.derussis@polito.it
Awesome student to me

 Normal problems Hi,
[…] I’m using “cvtColor(image, gray, CV_BGR2GRAY);” and Indexed8 to convert an image in
gray scale but the image is not shown in Qt…

 come to office hours Can you help me?

Regards,
…
 every Tuesday, 9:00 - 11:00
 LAB6, second floor of DAUIN
 please send an e-mail beforehand
 Enormous problems
 come to the dedicated lessons
 Tuesday 30th October, 9:30-12:30, DAUIN
 Thursday 8th November, 15:00-18:00, DAUIN

OpenCV installation

 Small problems Not-So-Awesome student to me

 drop me a line Hi,
[…] I followed the guide for installing OpenCV on my Mac but I have an error after step 3. Can
we meet on next Tuesday to solve the problem?
 luigi.derussis@polito.it Thanks!

Regards,

 Normal problems …

 come to office hours
 every Tuesday, 9:00 - 11:00
 LAB6, second floor of DAUIN
 please send an e-mail beforehand

 Small problems
 drop me a line
 luigi.derussis@polito.it
 Normal problems Help with OpenCV

 come to office hours Good student to me

Hi,
 every Tuesday, 9:00 - 11:00 […] I see the solution of Exercise 2.1 but I don’t understand the following expressions:
- main();
- int* number;
 LAB6, second floor of DAUIN - &timer.
Can you explain to me what they are?

 please send an e-mail beforehand Regards,
…


An mail not to be sent!!!


Resources
 OpenCV Wiki
 http://code.opencv.org/projects/opencv/wiki
 OpenCV 2.x Official Documentation
 http://docs.opencv.org/
 User Q&A forum
 http://answers.opencv.org/questions/
 OpenCV 2.x Tutorials
 http://docs.opencv.org/opencv_tutorials.pdf

 Books:
 Robert Laganière, OpenCV 2 Computer Vision Application Programming
Cookbook, Packt Publishing, ISBN 978-1849513241
 Gary Bradsky, Adrian Kaehler, Learning OpenCV: Computer Vision in C++ with
the OpenCV Library, O'Reilly Media, ISBN 978-1449314651 (to be published)


License
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Introduction to OpenCV

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