OpenCV Cheat Sheet

Table of Contents

• Version
• Installation
• Eclipse
• Window
• Image
• Pixel
• Blur
• Contrast and Brightness
• Colors and Masks
• Channles
• Trackbar
• Drawing
• Geometry
• Contour
• Contributing

Version

• OpenCV 2.4

Installation

Ubuntu

• Required

sudo apt-get install build-essential
sudo apt-get install cmake git libgtk2.0-dev pkg-config libavcodec-dev libavformat-dev libswscale-dev

• Download OpenCV

wget https://github.com/Itseez/opencv/archive/2.4.13.zip 
mkdir -p /tmp/OpenCV
mv opencv-2.4.13.zip /tmp/OpenCV
cd /tmp/OpenCV
unzip opencv-2.4.13.zip

• Make OpenCV

cd opencv-2.4.13
mkdir release
cd release
cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local ..
make
sudo make install

Reference:

http://docs.opencv.org/2.4/doc/tutorials/introduction/linux_install/linux_install.html#linux-installation

---

Eclipse

• Project > Properties > C/C++ Build [Menu] > Tool Settings [Tab] > GCC C++ Compiler [Option] > Includes [Option] > Include paths (-I)

  Add:

/usr/local/include/opencv

• Project > Properties > C/C++ Build [Menu] > Tool Settings [Tab] > GCC C++ Linker [Option] > Libraries [Option] > Libraries (-l)

  Add:

opencv_core
opencv_imgproc
opencv_highgui
opencv_ml
opencv_video
opencv_features2d
opencv_calib3d
opencv_objdetect
opencv_imgcodecs
opencv_flann

• Project > Properties > C/C++ Build [Menu] > Tool Settings [Tab] > GCC C++ Linker [Option] > Libraries [Option] > Library search path (-L)

  Add:

/usr/local/lib

Reference:

• http://docs.opencv.org/2.4/doc/tutorials/introduction/linux_eclipse/linux_eclipse.html

---

Window

Create window

namedWindow("My window", WINDOW_AUTOSIZE);

Destroy window

destroyWindow("My window");

Destroy all windows

destroyAllWindows();

Move window

moveWindow("My window", 150 /*x*/, 160 /*y*/);

Resize window

resizeWindow("My window", 500 /*width*/, 500 /*height*/);

Set mouse call back

setMouseCallback("My Image", [](int event, int x, int y, int flags, void* userdata) {
 cout << "X: " << x << ", Y: " << y << endl;
}, 0);

Wait instead of terminating the program

waitKey(0);

Examples

• examples/window/

Reference:

• http://docs.opencv.org/2.4/modules/highgui/doc/user_interface.html

---

Image

Load and store image in a matrix

Mat image = imread("img.jpg", CV_LOAD_IMAGE_COLOR);

Save image from matrix to file

imwrite( "img.jpg", image);

Show image

imshow("My Image", image);

Create a matrix of zeros

Mat new_image = Mat::zeros( image.size(), image.type() );

Clone image

image.clone();

Rotate image

Mat matRotate = getRotationMatrix2D(Point(imageSrc.rows/2, imageSrc.cols/2) /*center*/, 180 /*angle*/, 1 /*scale*/);
warpAffine(imageSrc, imageDest, matRotate, imageSrc.size() /*new size*/);

Save part of an image in a new matrix

Mat imageDest = imageSrc(Rect(0, 0, 100, 100));  // Mat::operator() overloaded

Resize image

resize(imageSrc, imageDest, Size(imageSrc.rows/2, imageSrc.cols/2)); // Resize to width=50% and height=50%

Examples

• examples/image/

Reference:

• http://docs.opencv.org/2.4/modules/highgui/doc/reading_and_writing_images_and_video.html?highlight=imread#imread
• http://docs.opencv.org/2.4/modules/highgui/doc/user_interface.html

---

Pixel

Loop on all pixels for a matrix

for( int row = 0; row < imageSrc.rows; row++ ){
	for( int col = 0; col < imageSrc.cols; col++ ){
		for( int c = 0; c < imageSrc.channels(); c++ ) {
			// image.at<Vec3b>(row,col)[c] = ...;
		}
	}
}

---

Blur

Algorithms

Normalized Block Filter

int kernelSize=20;
blur( imageSrc, imageDest, Size(kernelSize, kernelSize), Point(-1,-1));

Gaussian Filter

int kernelSize=21; // Int: Positive & Odd
GaussianBlur( imageSrc, imageDest, Size(kernelSize, kernelSize), 0, 0);

Median Filter

int kernelSize=21; // Int: Odd
medianBlur(imageSrc, imageDest, kernelSize);

Other

Blur part of the image

Rect region(0 /*x*/, 0 /*y*/, 200 /*width*/, 200 /*height*/);
int kernelSize = 25;
GaussianBlur(imageSrc(region), imageDest(region), Size(kernelSize,kernelSize), 0, 0); // Mat::operator() overloaded

Save only the blurred region in a matrix

Rect region(0 /*x*/, 0 /*y*/, 200 /*width*/, 200 /*height*/);
int kernelSize = 25;
Mat imageDest;
GaussianBlur(imageSrc(region), imageDest, Size(kernelSize,kernelSize), 0, 0); // Mat::operator() overloaded

Examples

• examples/blur/

Reference:

• http://docs.opencv.org/2.4.13/doc/tutorials/imgproc/gausian_median_blur_bilateral_filter/gausian_median_blur_bilateral_filter.html#goal

---

Contrast and Brightness

Information

Contrast and brightness can be controlled by the following formula: g(row, col) = alpha * f(row, col) + beta, where:

• g(row, col) is the result pixel value
• f(row, col) is the original pixel value
• alpha controls the contrast
• beta controls the brightness

Manually

Using nested loops

double alpha = 1;
int beta = 100;

// Loop on matrix
for( int row = 0; row < imageSrc.rows; row++ ){
	for( int col = 0; col < imageSrc.cols; col++ ){

		// Loop on channels (0: Blue, 1: Green, 2: Red)
		for( int c = 0; c < imageSrc.channels(); c++ ) {
			
			// g(row, col) = alpha * f(row, col) + beta
			imageDest.at<Vec3b>(row,col)[c] = saturate_cast<uchar>(alpha *( imageSrc.at<Vec3b>(row,col)[c] ) + beta);
		}
	}
}

saturate_cast: Template function for accurate conversion from one primitive type to another.

Automatically

Using Mat::convertTo() which applies the same formula g(row, col) explained above

double alpha = 1;
int beta = 100;
imageSrc.convertTo(imageDest, imageSrc.type(), alpha, beta);

Brightness can also be increased using Scalar

Mat imageDest = imageSrc + Scalar(80, 80, 80); // Add to each channel for all pixels of a matrix the amount specified in the arguments

Examples

• examples/contrast_and_brightness/

Reference:

• http://docs.opencv.org/2.4/doc/tutorials/core/basic_linear_transform/basic_linear_transform.html
• http://docs.opencv.org/2.4/modules/core/doc/utility_and_system_functions_and_macros.html?highlight=saturate_cast#saturate-cast
• http://docs.opencv.org/2.4/modules/core/doc/basic_structures.html#mat-convertto

---

Colors and Masks

Convert image to grayscale

cvtColor(imageSrc, imageDest, CV_BGR2GRAY);

Mark pixels in specified range by value 255 and all others by 0 in the destination matrix

inRange(imageSrc, Scalar(50, 50, 50) /*low*/, Scalar(180, 180, 180) /*high*/, imageDest);

Shrink bright regions within an image

erode(imageSrc, imageDest, getStructuringElement(MORPH_ELLIPSE, Size(5, 5)));

Grow bright regions within an image

dilate(imageSrc, imageDest, getStructuringElement(MORPH_ELLIPSE, Size(5, 5)));

Threshold

threshold(imageDest, imageDest, 150 /*threshold*/, 255 /*max*/, CV_THRESH_BINARY);

Examples

• examples/colors/

Reference:

• http://docs.opencv.org/2.4/modules/imgproc/doc/miscellaneous_transformations.html#cvtcolor
• http://docs.opencv.org/2.4/doc/tutorials/imgproc/erosion_dilatation/erosion_dilatation.html
• http://docs.opencv.org/2.4/modules/core/doc/operations_on_arrays.html#inrange
• http://opencv-srf.blogspot.ca/2010/09/object-detection-using-color-seperation.html

---

Channels

Get the number of channels for a matrix

image.channels();

Split multi-channel array into several single-channel arrays
Note: matrices created have one channel each

vector<Mat> bgr;
split(imageSrc, bgr);

Merge several single-channel into one multichannel array

merge(bgr /*vector<Mat> bgr*/, imageDest);

Examples

• examples/channels/

Reference:

• http://docs.opencv.org/2.4/modules/core/doc/operations_on_arrays.html

---

Trackbar

Create trackbar (min=0 always)

int brightnessValue = 50;
createTrackbar("Brightness" /*label*/, "My window" /*existing window*/, &brightnessValue, 255 /*max*/);

Create trackbar with callback function (on change event)

int brightnessValue = 0;
createTrackbar("Brightness", "My window 1", &brightnessValue, 255, [](int brightnessValue, void *userData){
	imshow("My window 1", *(Mat*)userData);
}, &imageSrc);

Examples

• examples/trackbar/

Reference:

• http://docs.opencv.org/2.4/modules/highgui/doc/user_interface.html?highlight=createtrackbar#createtrackbar

---

Drawing

Draw circle

circle(imageSrc, Point(imageSrc.rows/2, imageSrc.cols/2) /*circle center*/, 100 /*radius*/, Scalar(255, 0, 0) /*color*/, 10/*thickness*/);

Draw ellipse

ellipse(imageSrc, Point(imageSrc.rows/2, imageSrc.cols/2) /*ellipse center*/, Size(100, 50) /*min-max radius*/, 40 /*ellipse angle*/, 0 /*start angle*/, 359 /*end angle*/, Scalar(255,0,0) /*color*/, 10 /*thickness*/);

Draw line

line(imageSrc, Point(0, 0) /*from*/, Point(imageSrc.rows/2, imageSrc.cols/2) /*to*/, Scalar(255,0,0) /*color*/, 10 /*thickness*/);

Draw arrowed line

arrowedLine(imageSrc, Point(0, 0) /*from*/, Point(imageSrc.rows/2, imageSrc.cols/2) /*to*/, Scalar(255,0,0) /*color*/, 10 /*thickness*/);

Draw rectangle

rectangle(imageSrc, Point(0, 0) /*top-left*/, Point(imageSrc.rows/2, imageSrc.cols/2) /*bottom-right*/, Scalar(255,0,0) /*color*/, 10 /*thickness*/);

Draw polylines

vector<Point> points;
points.push_back(Point(0, 0));
points.push_back(Point(50, 50));
points.push_back(Point(200, 50));
polylines(imageSrc, points, true /*closed*/, Scalar(255,0,0) /*color*/, 10 /*thickness*/);

Draw text

putText(imageSrc, "Hello world", Point(50, 50), FONT_HERSHEY_PLAIN, 2.5 /*scale*/, Scalar(255,0,0) /*color*/, 5 /*thickness*/);

Reference:

• http://docs.opencv.org/2.4/modules/core/doc/drawing_functions.html

---

Geometry

Get center of a Binary matrix or contour

Moments oMoments = moments(imageSrc); // Mat should have 1 channel
// Moments oMoments = moments(contour /*vector<Point>*/);
double x = mmts.m10 / mmts.m00; // center x
double y = mmts.m01 / mmts.m00; // center y

Examples

• examples/geometry/
• examples/contour/06_Multi_contour_center.cpp

Reference:

• http://docs.opencv.org/2.4/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html?highlight=moments#moments
• https://en.wikipedia.org/wiki/Image_moment

---

Contour

Find and draw edges in 1 channel image

int thresh = 100;
Canny(imageSrc, imageDest, thresh /*threshold1*/, thresh*2 /*threshold2*/, 3/*apertureSize*/);

Find contours in a binary image
Note: Shape must be filled in white color, otherwise it will count it as more than one contour

vector<vector<Point> > contours;
vector<Vec4i> hierarchy;
findContours(imageSrc, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0));

Parameter: mode

• CV_RETR_EXTERNAL retrieves only the extreme outer contours.
• CV_RETR_LIST retrieves all of the contours without establishing any hierarchical relationships.
• CV_RETR_CCOMP retrieves all of the contours and organizes them into a two-level hierarchy.
• CV_RETR_TREE retrieves all of the contours and reconstructs a full hierarchy of nested contours.

Draw contours

drawContours(imageDest, contours, i /*contours[i]*/, Scalar(0, 0, 255) /*color*/, 2/*thickness*/, 8/*type*/, hierarchy, 0, Point());

Approximates a polygonal curve(s) with the specified precision

approxPolyDP(Mat(contours[i]), approx_contours[i] /*don't overwrite*/, 3 /*accuracy*/, true /*closed*/);

Contour area

contourArea(contour /*vector<Point>*/);

Draw rectangle around contour

Rect rect = boundingRect(Mat(contours[i]));
rectangle(tmpImage, rect.tl(), rect.br(), Scalar(0,255,0), 2);

Draw circle around contour

Point2f center;
float radius;
minEnclosingCircle(contours[i], center, radius);
circle( tmpImage, center, radius, Scalar(255, 0, 0), 2/*thickness*/);

Examples

• examples/contour/

Reference:

• http://docs.opencv.org/2.4/modules/imgproc/doc/feature_detection.html?highlight=canny#canny
• http://docs.opencv.org/2.4/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html
• http://docs.opencv.org/2.4/doc/tutorials/imgproc/shapedescriptors/find_contours/find_contours.html
• https://en.wikipedia.org/wiki/Ramer%E2%80%93Douglas%E2%80%93Peucker_algorithm

---

Contributing

• Edit README.md
• Add your changes
• Write description of your changes in the Commit changes form
• Select the radio button "Create a new branch for this commit and start a pull request."
• Specify a name for your commit branch
• Click on "Propose file change" and then on "Create pull request"