Introduction to OpenCV (with Java)

Computer Vision

Luigi De Russis

Introduction to
OpenCV

2

OpenC… what?
OpenCV in brief

Introduction to OpenCV

07/11/2013

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


07/11/2013

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


07/11/2013

5

Motivations - OpenCV
Always ask “why?”


07/11/2013

Why OpenCV?
6

vs.


07/11/2013

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


needs more system resources than OpenCV
07/11/2013

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


07/11/2013

OpenCV: pros (final)
9





Price (!)
OpenCV Wrappers
 SimpleCV,



Emgu CV, ruby-opencv…

More similar to industry-level frameworks


07/11/2013

10

Motivation - Java
When Java meets OpenCV


07/11/2013

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

07/11/2013

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


it is a good way to get into trouble!

07/11/2013

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


it is a good way to get into trouble!

07/11/2013

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


07/11/2013

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)


07/11/2013

16



Question:
 is

OpenCV with Java/JavaFX the “best” solution for
every application?


07/11/2013

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!

07/11/2013

18

OpenCV meets Java
Getting started with OpenCV and Java


07/11/2013

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)


07/11/2013

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, …)


07/11/2013

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


algorithms
07/11/2013

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



07/11/2013

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


07/11/2013

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)


07/11/2013

Basic Drawing Operations
25









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


07/11/2013

Color Spaces
26



Imgproc.cvtColor



converts an input image from one color space to another
examples:







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


07/11/2013

27

Let’s get practical!
Getting started (without going crazy)


07/11/2013

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


07/11/2013

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”



07/11/2013

30

OpenCV installation



Small problems


drop me a line





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,
…





Enormous problems
pray?



07/11/2013

31



Small problems


drop me a line





Normal problems


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?



07/11/2013

An e-mail not to be sent!
32


07/11/2013

Useful Resources…
33



OpenCV Wiki




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


07/11/2013

34

Demo Hour
Put everything together


07/11/2013

License
35





This work is licensed under the Creative Commons “AttributionNonCommercial-ShareAlike Unported (CC BY-NC-SA 3,0)” License.
You are free:





Under the following conditions:







to Share - to copy, distribute and transmit the work
to Remix - to adapt the work

Attribution - You must attribute the work in the manner specified by the
author or licensor (but not in any way that suggests that they endorse
you or your use of the work).
Noncommercial - You may not use this work for commercial purposes.
Share Alike - If you alter, transform, or build upon this work, you may
distribute the resulting work only under the same or similar license to this
one.

To view a copy of this license, visit
http://creativecommons.org/licenses/by-nc-sa/3.0/


07/11/2013

Introduction to OpenCV (with Java)

Recomendados

Recomendados

Mais conteúdo relacionado

Mais procurados

Mais procurados (20)

Destaque

Destaque (8)

Semelhante a Introduction to OpenCV (with Java)

Semelhante a Introduction to OpenCV (with Java) (20)

Mais de Luigi De Russis

Mais de Luigi De Russis (20)

Último

Último (20)

Introduction to OpenCV (with Java)