OpenCV  3.2.0
Open Source Computer Vision
Image Moments

.2.0+dfsg_doc_tutorials_imgproc_shapedescriptors_moments_moments

Goal

In this tutorial you will learn how to:

Theory

Code

This tutorial code's is shown lines below. You can also download it from here

#include <iostream>
using namespace cv;
using namespace std;
Mat src; Mat src_gray;
int thresh = 100;
int max_thresh = 255;
RNG rng(12345);
void thresh_callback(int, void* );
int main( int, char** argv )
{
src = imread( argv[1], IMREAD_COLOR );
cvtColor( src, src_gray, COLOR_BGR2GRAY );
blur( src_gray, src_gray, Size(3,3) );
const char* source_window = "Source";
namedWindow( source_window, WINDOW_AUTOSIZE );
imshow( source_window, src );
createTrackbar( " Canny thresh:", "Source", &thresh, max_thresh, thresh_callback );
thresh_callback( 0, 0 );
waitKey(0);
return(0);
}
void thresh_callback(int, void* )
{
Mat canny_output;
vector<vector<Point> > contours;
vector<Vec4i> hierarchy;
Canny( src_gray, canny_output, thresh, thresh*2, 3 );
findContours( canny_output, contours, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE, Point(0, 0) );
vector<Moments> mu(contours.size() );
for( size_t i = 0; i < contours.size(); i++ )
{ mu[i] = moments( contours[i], false ); }
vector<Point2f> mc( contours.size() );
for( size_t i = 0; i < contours.size(); i++ )
{ mc[i] = Point2f( static_cast<float>(mu[i].m10/mu[i].m00) , static_cast<float>(mu[i].m01/mu[i].m00) ); }
Mat drawing = Mat::zeros( canny_output.size(), CV_8UC3 );
for( size_t i = 0; i< contours.size(); i++ )
{
Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
drawContours( drawing, contours, (int)i, color, 2, 8, hierarchy, 0, Point() );
circle( drawing, mc[i], 4, color, -1, 8, 0 );
}
namedWindow( "Contours", WINDOW_AUTOSIZE );
imshow( "Contours", drawing );
printf("\t Info: Area and Contour Length \n");
for( size_t i = 0; i< contours.size(); i++ )
{
printf(" * Contour[%d] - Area (M_00) = %.2f - Area OpenCV: %.2f - Length: %.2f \n", (int)i, mu[i].m00, contourArea(contours[i]), arcLength( contours[i], true ) );
Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
drawContours( drawing, contours, (int)i, color, 2, 8, hierarchy, 0, Point() );
circle( drawing, mc[i], 4, color, -1, 8, 0 );
}
}

Explanation

Result

Here it is:

imgproc.hpp
cv::IMREAD_COLOR
@ IMREAD_COLOR
If set, always convert image to the 3 channel BGR color image.
Definition: imgcodecs.hpp:67
cv::Mat::zeros
static MatExpr zeros(int rows, int cols, int type)
Returns a zero array of the specified size and type.
cv::cvtColor
void cvtColor(InputArray src, OutputArray dst, int code, int dstCn=0)
Converts an image from one color space to another.
cv::Point2f
Point_< float > Point2f
Definition: types.hpp:181
cv::waitKey
int waitKey(int delay=0)
Waits for a pressed key.
cv::Canny
void Canny(InputArray image, OutputArray edges, double threshold1, double threshold2, int apertureSize=3, bool L2gradient=false)
Finds edges in an image using the Canny algorithm .
highgui.hpp
cv::namedWindow
void namedWindow(const String &winname, int flags=WINDOW_AUTOSIZE)
Creates a window.
cv::Scalar_< double >
cv::Size
Size2i Size
Definition: types.hpp:315
cv::imread
Mat imread(const String &filename, int flags=IMREAD_COLOR)
Loads an image from a file.
cv::COLOR_BGR2GRAY
@ COLOR_BGR2GRAY
convert between RGB/BGR and grayscale, color conversions
Definition: imgproc.hpp:538
CV_8UC3
#define CV_8UC3
Definition: interface.h:84
cv::contourArea
double contourArea(InputArray contour, bool oriented=false)
Calculates a contour area.
cv::Mat::size
MatSize size
Definition: mat.hpp:1978
imgcodecs.hpp
cv::arcLength
double arcLength(InputArray curve, bool closed)
Calculates a contour perimeter or a curve length.
cv::imshow
void imshow(const String &winname, InputArray mat)
Displays an image in the specified window.
cv::Scalar
Scalar_< double > Scalar
Definition: types.hpp:606
cv::RNG
Random Number Generator.
Definition: core.hpp:2690
cv::drawContours
void drawContours(InputOutputArray image, InputArrayOfArrays contours, int contourIdx, const Scalar &color, int thickness=1, int lineType=LINE_8, InputArray hierarchy=noArray(), int maxLevel=INT_MAX, Point offset=Point())
Draws contours outlines or filled contours.
cv::blur
void blur(InputArray src, OutputArray dst, Size ksize, Point anchor=Point(-1,-1), int borderType=BORDER_DEFAULT)
Blurs an image using the normalized box filter.
cv::Point
Point2i Point
Definition: types.hpp:183
cv::Mat
n-dimensional dense array class
Definition: mat.hpp:741
i
for i
Definition: modelConvert.m:63
cv::createTrackbar
int createTrackbar(const String &trackbarname, const String &winname, int *value, int count, TrackbarCallback onChange=0, void *userdata=0)
Creates a trackbar and attaches it to the specified window.
cv
Definition: affine.hpp:52
cv::WINDOW_AUTOSIZE
@ WINDOW_AUTOSIZE
the user cannot resize the window, the size is constrainted by the image displayed.
Definition: highgui.hpp:184
cv::moments
Moments moments(InputArray array, bool binaryImage=false)
Calculates all of the moments up to the third order of a polygon or rasterized shape.
cv::CHAIN_APPROX_SIMPLE
@ CHAIN_APPROX_SIMPLE
Definition: imgproc.hpp:448
cv::findContours
void findContours(InputOutputArray image, OutputArrayOfArrays contours, OutputArray hierarchy, int mode, int method, Point offset=Point())
Finds contours in a binary image.
cv::RETR_TREE
@ RETR_TREE
Definition: imgproc.hpp:436
cv::circle
void circle(InputOutputArray img, Point center, int radius, const Scalar &color, int thickness=1, int lineType=LINE_8, int shift=0)
Draws a circle.