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OpenCV
3.2.0
Open Source Computer Vision
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.2.0+dfsg_doc_tutorials_imgproc_shapedescriptors_bounding_rects_circles_bounding_rects_circles
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 std;
int thresh = 100;
int max_thresh = 255;
void thresh_callback(int, void* );
int main( int, char** argv )
{
const char* source_window = "Source";
createTrackbar(
" Threshold:",
"Source", &thresh, max_thresh, thresh_callback );
thresh_callback( 0, 0 );
return(0);
}
void thresh_callback(int, void* )
{
vector<vector<Point> > contours;
vector<Vec4i> hierarchy;
vector<vector<Point> > contours_poly( contours.size() );
vector<Rect> boundRect( contours.size() );
vector<Point2f>center( contours.size() );
vector<float>radius( contours.size() );
for(
size_t i = 0;
i < contours.size();
i++ )
{
}
for(
size_t i = 0;
i< contours.size();
i++ )
{
Scalar color =
Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
rectangle( drawing, boundRect[
i].tl(), boundRect[
i].br(), color, 2, 8, 0 );
circle( drawing, center[
i], (
int)radius[
i], color, 2, 8, 0 );
}
imshow(
"Contours", drawing );
}
Explanation
The main function is rather simple, as follows from the comments we do the following:
- Open the image, convert it into grayscale and blur it to get rid of the noise.
- Create a window with header "Source" and display the source file in it.
const char* source_window = "Source";
- Create a trackbar on the source_window and assign a callback function to it In general callback functions are used to react to some kind of signal, in our case it's trackbar's state change.
createTrackbar(
" Threshold:",
"Source", &thresh, max_thresh, thresh_callback );
- Explicit one-time call of
thresh_callback is necessary to display the "Contours" window simultaniously with the "Source" window.
- Wait for user to close the windows.
The callback function thresh_callback does all the interesting job.
- Writes to
threshold_output the threshold of the grayscale picture (you can check out about thresholding here).
- Finds contours and saves them to the vectors
contour and hierarchy.
For every found contour we now apply approximation to polygons with accuracy +-3 and stating that the curve must me closed.
After that we find a bounding rect for every polygon and save it to boundRect.
At last we find a minimum enclosing circle for every polygon and save it to center and radius vectors.
for(
size_t i = 0;
i < contours.size();
i++ )
{
}
We found everything we need, all we have to do is to draw.
- Create new Mat of unsigned 8-bit chars, filled with zeros. It will contain all the drawings we are going to make (rects and circles).
Mat drawing = Mat::zeros( threshold_output.
size(),
CV_8UC3 );
- For every contour: pick a random color, draw the contour, the bounding rectangle and the minimal enclosing circle with it,
for(
size_t i = 0;
i< contours.size();
i++ )
{
Scalar color =
Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
rectangle( drawing, boundRect[
i].tl(), boundRect[
i].br(), color, 2, 8, 0 );
circle( drawing, center[
i], (
int)radius[
i], color, 2, 8, 0 );
}
- Display the results: create a new window "Contours" and show everything we added to drawings on it.
imshow(
"Contours", drawing );
Result
Here it is:
@ IMREAD_COLOR
If set, always convert image to the 3 channel BGR color image.
Definition: imgcodecs.hpp:67
@ THRESH_BINARY
Definition: imgproc.hpp:329
static MatExpr zeros(int rows, int cols, int type)
Returns a zero array of the specified size and type.
void cvtColor(InputArray src, OutputArray dst, int code, int dstCn=0)
Converts an image from one color space to another.
void minEnclosingCircle(InputArray points, Point2f ¢er, float &radius)
Finds a circle of the minimum area enclosing a 2D point set.
double threshold(InputArray src, OutputArray dst, double thresh, double maxval, int type)
Applies a fixed-level threshold to each array element.
int waitKey(int delay=0)
Waits for a pressed key.
void namedWindow(const String &winname, int flags=WINDOW_AUTOSIZE)
Creates a window.
Size2i Size
Definition: types.hpp:315
void rectangle(InputOutputArray img, Point pt1, Point pt2, const Scalar &color, int thickness=1, int lineType=LINE_8, int shift=0)
Draws a simple, thick, or filled up-right rectangle.
Mat imread(const String &filename, int flags=IMREAD_COLOR)
Loads an image from a file.
@ COLOR_BGR2GRAY
convert between RGB/BGR and grayscale, color conversions
Definition: imgproc.hpp:538
#define CV_8UC3
Definition: interface.h:84
MatSize size
Definition: mat.hpp:1978
void imshow(const String &winname, InputArray mat)
Displays an image in the specified window.
void approxPolyDP(InputArray curve, OutputArray approxCurve, double epsilon, bool closed)
Approximates a polygonal curve(s) with the specified precision.
Rect boundingRect(InputArray points)
Calculates the up-right bounding rectangle of a point set.
Scalar_< double > Scalar
Definition: types.hpp:606
Random Number Generator.
Definition: core.hpp:2690
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.
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.
Point2i Point
Definition: types.hpp:183
n-dimensional dense array class
Definition: mat.hpp:741
for i
Definition: modelConvert.m:63
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.
Definition: affine.hpp:52
@ WINDOW_AUTOSIZE
the user cannot resize the window, the size is constrainted by the image displayed.
Definition: highgui.hpp:184
@ CHAIN_APPROX_SIMPLE
Definition: imgproc.hpp:448
void findContours(InputOutputArray image, OutputArrayOfArrays contours, OutputArray hierarchy, int mode, int method, Point offset=Point())
Finds contours in a binary image.
@ RETR_TREE
Definition: imgproc.hpp:436
@ circle
Definition: gr_skig.hpp:62
void circle(InputOutputArray img, Point center, int radius, const Scalar &color, int thickness=1, int lineType=LINE_8, int shift=0)
Draws a circle.