A brief introduction to OpenCV 2.x (with Java) for the Computer Vision course at Politecnico di Torino (academic year 2013/2014).

1. Computer Vision
Luigi De Russis
Introduction to
OpenCV

2. 2
OpenC… what?
OpenCV in brief
Introduction to OpenCV
07/11/2013

3. What is OpenCV?
3

Open source Computer Vision library
BSD License
 http://opencv.org




Originally developed by Intel
Has more than 2500 optimized algorithms
Supports a lot of different languages
C, C++, Python, Java
 but is written natively in C++


Cross platform

also available for Android and iOS
Introduction to OpenCV
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4. What it is used for?
4









Human-Computer Interaction (HCI)
Object Identification
Object Recognition
Face Recognition
Gesture Recognition
Motion Tracking
Image Processing
Mobile Robotics
… and so on
Introduction to OpenCV
07/11/2013

5. 5
Motivations - OpenCV
Always ask “why?”
Introduction to OpenCV
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6. Why OpenCV?
6
vs.
Introduction to OpenCV
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7. OpenCV: pros
7

Specificity
 OpenCV
was made for image processing
 Matlab is quite generic

Speed
 30+
frames processed per seconds in real time image
processing with OpenCV
 around 4-5 frames processed per seconds in real time
image processing with Matlab

Efficient
 Matlab
Introduction to OpenCV
needs more system resources than OpenCV
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8. OpenCV: cons
8

Easy of use
 Matlab

won hands down!
Integrated Development Environment (IDE)
 you
can use Eclipse, Netbeans, Visual Studio, Qt,
XCode, … even a simple text editor for OpenCV
 Matlab has is own IDE

Memory management
Introduction to OpenCV
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9. OpenCV: pros (final)
9
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
Price (!)
OpenCV Wrappers
 SimpleCV,

Emgu CV, ruby-opencv…
More similar to industry-level frameworks
Introduction to OpenCV
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10. 10
Motivation - Java
When Java meets OpenCV
Introduction to OpenCV
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11. Some facts…
11

The OpenCV APIs have few capabilities for user
interfaces
 i.e.
you can open a window with an image inside. Do
you want a button? You cannot have it!
 to have a “decent” user interface you need to install
“something else” (e.g., Qt for C++)…
 … and then compile OpenCV for adding Qt support

C++ learning curve is quite… steep
 source:
experiences from teaching this course in the last
years
Introduction to OpenCV
07/11/2013

12. Some (other) facts…
12

OpenCV C++ APIs exists since OpenCV 1.0
 It
is possible, but not recommended, to “mix” different
OpenCV version
 Typically, it is a good way to get into trouble!

It is possible to use OpenCV C APIs together with
C++ APIs
 Typically
Introduction to OpenCV
it is a good way to get into trouble!
07/11/2013

13. Some (other) facts…
13

OpenCV C++ API exists since OpenCV 1.0
 It
is possible, but not recommended, to “mix” different
OpenCV version
 Typically, it is a good way to get into trouble!
Yeah, some students did it!

It is possible to use OpenCV C APIs together with
C++ APIs
 Typically
Introduction to OpenCV
it is a good way to get into trouble!
07/11/2013

14. A better world is possible?
14

OpenCV provides also the Java APIs
nothing to compile (on Windows)
 they works on Android, also


Java 7 has a first-grade user interface
not Swing, but JavaFX
 http://www.oracle.com/technetwork/java/javafx



Almost everybody here should have some (basic)
experience with Java
Performance between the C++ APIs and the Java APIs
are comparable
Introduction to OpenCV
07/11/2013

15. A better world is possible?
15

The OpenCV Java APIs are almost identical to the C++
version
knowledge can be transferred!
 examples:

to store an image in memory both use the Mat object
 to write an image on disk both use the imwrite method


Something change a bit

examples:

CV_RGB2GRAY (C++) becomes COLOR_RGB2GRAY (Java)

Point, Point2d, Point2f (C++) becomes 3 overloaded
constructor of Point (Java)
Introduction to OpenCV
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16. A better world is possible?
16

Question:
 is
OpenCV with Java/JavaFX the “best” solution for
every application?
Introduction to OpenCV
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17. A better world is possible?
17

Question:
 is
OpenCV with Java/JavaFX the “best” solution for
every application?

Response:
 No,
obviously
 Do you need a GUI? Go with JavaFX!
 Do you have memory constraint? Go with C/C++!
…
 Please, no extremism!
Introduction to OpenCV
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18. 18
OpenCV meets Java
Getting started with OpenCV and Java
Introduction to OpenCV
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19. Modules
19
OpenCV has a modular structure:
the package includes several shared or static libraries

core


imgproc


basic structures and algorithms
image processing algorithms (such as image filtering,
geometrical image transformations, histograms, etc.)
video

video analysis (such as motion estimation and object
tracking)
Introduction to OpenCV
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20. Modules
20

highgui


calib3d


camera calibration and 3D reconstruction
features2d


basic operation to read/write/encode images; in C, C++
and Python it provides also basic UI capabilities
2D features framework (feature detectors, descriptors, and
descriptor matchers)
objdetect

detection of objects and other items (e.g., faces, eyes, mugs,
people, …)
Introduction to OpenCV
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21. Modules
21

ml
 machine
learning classes used for statistical
classification, regression and clustering of data

gpu
 GPU-accelerated

photo
 computational

algorithms
photography
ccl
 OpenCL-accelerated
Introduction to OpenCV
algorithms
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22. Data Structures
22



We speak about Java API
All the OpenCV classes and methods are placed into
the org.opencv.* packages
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 in C++)
 two data parts:
matrix header (contains information about the matrix)
 a pointer to the matrix containing the pixel values

Introduction to OpenCV
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23. Data Structures
23

Point
 2D point
 defined by x, y coordinates



Point first = new Point(2, 3);
Size
 2D size structure
 specify the size (width and height) of an image or
rectangle
Rect
 2D rectangle object
Introduction to OpenCV
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24. Basic Image I/O
24

Highgui.imread
 loads an image from file and return the corresponding Mat
object
Mat Highgui.imread(String filename,
int flags)

Highui.imwrite

save an image on disk
bool Highgui.imwrite(String filename,
Mat img, MatOfInt params)
Introduction to OpenCV
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25. Basic Drawing Operations
25
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


Core.circle
 draws a simple or filled circle with a given center and radius on a
given image
Core.line
 draws a line between two point in the given image
Core.ellipse
 draws an ellipse outline, a filled ellipse, an elliptic arc, a filled
ellipse sector, …
Core.rectangle
 draws a rectangle outline or a filled rectangle
 note that negative thickness will fill the rectangle
Introduction to OpenCV
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26. Color Spaces
26

Imgproc.cvtColor


converts an input image from one color space to another
examples:


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cvtColor(src, dest, Imgproc.COLOR_RGB2GRAY);
cvtColor(src, dest, Imgproc.COLOR_HSV2BGR);
cvtColor(src, dest, Imgproc.COLOR_RGB2BGR);
Important! Images in OpenCV uses BGR instead of RGB
Introduction to OpenCV
07/11/2013

27. 27
Let’s get practical!
Getting started (without going crazy)
Introduction to OpenCV
07/11/2013

28. How can we use OpenCV?
28

LABINF:
 already
installed under Windows
 version 2.4.6
 Eclipse (Kepler) is the IDE

At home:
 you
can use whatever IDE you like
 but we give full support only for Eclipse

Installation:
 see
the PDF document in the teaching portal
Introduction to OpenCV
07/11/2013

29. What if I got problems?
29

Small problems

drop me a line


luigi.derussis@polito.it
Normal problems
Problems with JavaFX and a gray scale image
Awesome student to me
Hi,
[…] I’m using “cvtColor(image, gray, COLOR_BGR2GRAY);” but it give this exception: […]
Can you help me?
come to office hours
 every Wednesday, 9:00 - 11:00
 LAB6, second floor of DAUIN
 please send an e-mail beforehand


Regards,
…
Enormous problems
pray?
 no, seriously, we can schedule an extra “lesson”

Introduction to OpenCV
07/11/2013

30. What if I got problems?
30
OpenCV installation

Small problems

drop me a line


luigi.derussis@polito.it
Normal problems
Not-So-Awesome student to me
Hi,
[…] I followed the guide for installing OpenCV on my Mac but I have an error after step 3. Can
we meet on next Wednesday to solve the problem?
Thanks!
Regards,
…
come to office hours
 every Wednesday, 9:00 - 11:00
 LAB6, second floor of DAUIN
 please send an e-mail beforehand


Enormous problems
pray?
 no, seriously, we can schedule an extra “lesson”

Introduction to OpenCV
07/11/2013

31. What if I got problems?
31

Small problems

drop me a line


luigi.derussis@polito.it
Normal problems
come to office hours
 every Wednesday, 9:00 - 11:00
 LAB6, second floor of DAUIN
 please send an e-mail beforehand

Help with OpenCV
Good student to me
Hi,
[…] I see the solution of Exercise 2.1 but I don’t understand the following expressions:
main();
System.out.println();
@Override.
Can you explain to me what they are?

Enormous problems
Regards,
…
pray?
 no, seriously, we can schedule an extra “lesson”

Introduction to OpenCV
07/11/2013

32. An e-mail not to be sent!
32
Introduction to OpenCV
07/11/2013

33. Useful Resources…
33

OpenCV Wiki

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OpenCV Official Documentation


http://answers.opencv.org/questions/
OpenCV Javadocs


http://docs.opencv.org/
User Q&A forum


http://code.opencv.org/projects/opencv/wiki
http://docs.opencv.org/java/
JavaFX Documentation

http://www.oracle.com/technetwork/java/javafx/document
ation
Introduction to OpenCV
07/11/2013

34. 34
Demo Hour
Put everything together
Introduction to OpenCV
07/11/2013

35. License
35
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
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Introduction to OpenCV
07/11/2013