How can I merge blobs / contours - c++

I use findContours for blob detection. Now I would merge close and similar blobs together.
Here are some sample images:
Is that possible with normal Opencv?

The input images you gave us are pretty easy to work with:
The first step is isolate the yellow blobs from everything else and a simple color segmentation technique can accomplish this task. You can take a look at Segmentation & Object Detection by color or Tracking colored objects in OpenCV to have an idea on how to do it.
Then, it's time to merge the blobs. One technique in particular that can be useful is the bounding box, to put all the blobs inside a rectangle. Notice in the images below, that there is a green rectangle surrounding the blobs:
After that, all you need to do is fill the rectangle with the color of your choice, thus connecting all the blobs. I'm leaving this last as homework for you.
This is the quickest and most simple approach I could think of. The following code demonstrates how to achieve what I just described:
#include <cv.h>
#include <highgui.h>
#include <iostream>
#include <vector>
int main(int argc, char* argv[])
{
cv::Mat img = cv::imread(argv[1]);
if (!img.data)
{
std::cout "!!! Failed to open file: " << argv[1] << std::endl;
return 0;
}
// Convert RGB Mat into HSV color space
cv::Mat hsv;
cv::cvtColor(img, hsv, CV_BGR2HSV);
// Split HSV Mat into HSV components
std::vector<cv::Mat> v;
cv::split(hsv,v);
// Erase pixels with low saturation
int min_sat = 70;
cv::threshold(v[1], v[1], min_sat, 255, cv::THRESH_BINARY);
/* Work with the saturated image from now on */
// Erode could provide some enhancement, but I'm not sure.
// cv::Mat element = cv::getStructuringElement(cv::MORPH_RECT, cv::Size(3, 3));
// cv::erode(v[1], v[1], element);
// Store the set of points in the image before assembling the bounding box
std::vector<cv::Point> points;
cv::Mat_<uchar>::iterator it = v[1].begin<uchar>();
cv::Mat_<uchar>::iterator end = v[1].end<uchar>();
for (; it != end; ++it)
{
if (*it) points.push_back(it.pos());
}
// Compute minimal bounding box
cv::RotatedRect box = cv::minAreaRect(cv::Mat(points));
// Display bounding box on the original image
cv::Point2f vertices[4];
box.points(vertices);
for (int i = 0; i < 4; ++i)
{
cv::line(img, vertices[i], vertices[(i + 1) % 4], cv::Scalar(0, 255, 0), 1, CV_AA);
}
cv::imshow("box", img);
//cv::imwrite(argv[2], img);
cvWaitKey(0);
return 0;
}

i think i did it, thanks to your program details i found this solution: (comments are welcome)
vector<vector<Point> > contours;
vector<vector<Point> > tmp_contours;
findContours(detectedImg, tmp_contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);
vector<vector<Point> >::iterator it1;
it1 = tmp_contours.begin();
Mat test;
test = Mat(FImage.size(), CV_32FC3);
while (it1 != tmp_contours.end()) {
vector<Point> approx1;
approxPolyDP(Mat(*it1), approx1, 3, true);
Rect box1 = boundingRect(approx1);
float area1 = contourArea(approx1);
if ((area1 > 50) && (area1 < 13000) && (box1.width < 100) && (box1.height < 120)) {
vector<vector<Point> >::iterator it2;
it2 = tmp_contours.begin();
while (it2 != tmp_contours.end()) {
vector<Point> approx2;
approxPolyDP(Mat(*it2), approx2, 3, true);
Moments m1 = moments(Mat(approx1), false);
Moments m2 = moments(Mat(approx2), false);
float x1 = m1.m10 / m1.m00;
float y1 = m1.m01 / m1.m00;
float x2 = m2.m10 / m2.m00;
float y2 = m2.m01 / m2.m00;
vector<Point> dist;
dist.push_back(Point(x1, y1));
dist.push_back(Point(x2, y2));
float d = arcLength(dist, false);
Rect box2 = boundingRect(approx2);
if (box1 != box2) {
if (d < 25) {
//Method to merge the vectors
approx1 = mergePoints(approx1, approx2);
}
}
++it2;
}
Rect b = boundingRect(approx1);
rectangle(test, b, CV_RGB(125, 255, 0), 2);
contours.push_back(approx1);
}
++it1;
}

Related

OpenCV Finding square center c++

To begin, I am a complete novice in OpenCV and am beginner/reasonable in c++ code.
But OpenCV is new to me and I try to learn by doing projects and stuff.
Now for my new project I am trying to find the centre of square in a picture.
In my case there is only 1 square in picture.
I would like to build further upon the square.cpp example of OpenCV.
For my project there are 2 things I need some help with,
1: The edge of the window is detected as a square, I do not want this. Example
2: How could I get the centre of 1 square from the squares array?
This is the code from the example "square.cpp"
// The "Square Detector" program.
// It loads several images sequentially and tries to find squares in
// each image
#include "opencv2/core.hpp"
#include "opencv2/imgproc.hpp"
#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui.hpp"
#include <iostream>
using namespace cv;
using namespace std;
static void help(const char* programName)
{
cout <<
"\nA program using pyramid scaling, Canny, contours and contour simplification\n"
"to find squares in a list of images (pic1-6.png)\n"
"Returns sequence of squares detected on the image.\n"
"Call:\n"
"./" << programName << " [file_name (optional)]\n"
"Using OpenCV version " << CV_VERSION << "\n" << endl;
}
int thresh = 50, N = 11;
const char* wndname = "Square Detection Demo";
// helper function:
// finds a cosine of angle between vectors
// from pt0->pt1 and from pt0->pt2
static double angle(Point pt1, Point pt2, Point pt0)
{
double dx1 = pt1.x - pt0.x;
double dy1 = pt1.y - pt0.y;
double dx2 = pt2.x - pt0.x;
double dy2 = pt2.y - pt0.y;
return (dx1 * dx2 + dy1 * dy2) / sqrt((dx1 * dx1 + dy1 * dy1) * (dx2 * dx2 + dy2 * dy2) + 1e-10);
}
// returns sequence of squares detected on the image.
static void findSquares(const Mat& image, vector<vector<Point> >& squares)
{
squares.clear();
Mat pyr, timg, gray0(image.size(), CV_8U), gray;
// down-scale and upscale the image to filter out the noise
pyrDown(image, pyr, Size(image.cols / 2, image.rows / 2));
pyrUp(pyr, timg, image.size());
vector<vector<Point> > contours;
// find squares in every color plane of the image
for (int c = 0; c < 3; c++)
{
int ch[] = { c, 0 };
mixChannels(&timg, 1, &gray0, 1, ch, 1);
// try several threshold levels
for (int l = 0; l < N; l++)
{
// hack: use Canny instead of zero threshold level.
// Canny helps to catch squares with gradient shading
if (l == 0)
{
// apply Canny. Take the upper threshold from slider
// and set the lower to 0 (which forces edges merging)
Canny(gray0, gray, 0, thresh, 5);
// dilate canny output to remove potential
// holes between edge segments
dilate(gray, gray, Mat(), Point(-1, -1));
}
else
{
// apply threshold if l!=0:
// tgray(x,y) = gray(x,y) < (l+1)*255/N ? 255 : 0
gray = gray0 >= (l + 1) * 255 / N;
}
// find contours and store them all as a list
findContours(gray, contours, RETR_LIST, CHAIN_APPROX_SIMPLE);
vector<Point> approx;
// test each contour
for (size_t i = 0; i < contours.size(); i++)
{
// approximate contour with accuracy proportional
// to the contour perimeter
approxPolyDP(contours[i], approx, arcLength(contours[i], true) * 0.02, true);
// square contours should have 4 vertices after approximation
// relatively large area (to filter out noisy contours)
// and be convex.
// Note: absolute value of an area is used because
// area may be positive or negative - in accordance with the
// contour orientation
if (approx.size() == 4 &&
fabs(contourArea(approx)) > 1000 &&
isContourConvex(approx))
{
double maxCosine = 0;
for (int j = 2; j < 5; j++)
{
// find the maximum cosine of the angle between joint edges
double cosine = fabs(angle(approx[j % 4], approx[j - 2], approx[j - 1]));
maxCosine = MAX(maxCosine, cosine);
}
// if cosines of all angles are small
// (all angles are ~90 degree) then write quandrange
// vertices to resultant sequence
if (maxCosine < 0.3)
squares.push_back(approx);
}
}
}
}
}
int main(int argc, char** argv)
{
static const char* names[] = { "testimg.jpg", 0 };
help(argv[0]);
if (argc > 1)
{
names[0] = argv[1];
names[1] = "0";
}
for (int i = 0; names[i] != 0; i++)
{
string filename = samples::findFile(names[i]);
Mat image = imread(filename, IMREAD_COLOR);
if (image.empty())
{
cout << "Couldn't load " << filename << endl;
continue;
}
vector<vector<Point> > squares;
findSquares(image, squares);
polylines(image, squares, true, Scalar(0, 0, 255), 3, LINE_AA);
imshow(wndname, image);
int c = waitKey();
if (c == 27)
break;
}
return 0;
}
I would like some help to start off.
How could I get some information from 1 of the squares out of the array called "squares" (I am having a difficult time understand what exactly is in the array as well; is it an array of points?)
If something is not clear please let me know and I will try to re-explain.
Thank you in advance
Firstly, you are talking about squares but you are actually detecting rectangles. I provided a shorter code to be able to better answer your questions.
I read the image, apply a Canny filter for binarization and detect all contours. After that I iterate through the contours and find the ones which can be approximated by exactly four points and are convex:
int main(int argc, char** argv)
{
// Reading the images
cv::Mat img = cv::imread("squares_image.jpg", cv::IMREAD_GRAYSCALE);
cv::Mat edge_img;
std::vector <std::vector<cv::Point>> contours;
// Convert the image into a binary image using Canny filter - threshold could be automatically determined using OTSU method
cv::Canny(img, edge_img, 30, 100);
// Find all contours in the Canny image
findContours(edge_img, contours, cv::RETR_LIST, cv::CHAIN_APPROX_SIMPLE);
// Iterate through the contours and test if contours are square
std::vector<std::vector<cv::Point>> all_rectangles;
std::vector<cv::Point> single_rectangle;
for (size_t i = 0; i < contours.size(); i++)
{
// 1. Contours should be approximateable as a polygon
approxPolyDP(contours[i], single_rectangle, arcLength(contours[i], true) * 0.01, true);
// 2. Contours should have exactly 4 vertices and be convex
if (single_rectangle.size() == 4 && cv::isContourConvex(single_rectangle))
{
// 3. Determine if the polygon is really a square/rectangle using its properties (parallelity, angles etc.)
// Not necessary for the provided image
// Push the four points into your vector of squares (could be also std::vector<cv::Rect>)
all_rectangles.push_back(single_rectangle);
}
}
for (size_t num_contour = 0; num_contour < all_rectangles.size(); ++num_contour) {
cv::drawContours(img, all_rectangles, num_contour, cv::Scalar::all(-1));
}
cv::imshow("Detected rectangles", img);
cv::waitKey(0);
return 0;
}
1: The edge of the window is detected as a square, I do not want this.
There are several options depending on your applications. You can filter the outer boundary already using the Canny thresholds, using a different contour retrieval method for finding contours in findContours or by filtering single_rectangle using the area of the found contour (e.g. cv::contourArea(single_rectangle) < 1000).
2: How could I get the centre of 1 square from the squares array?
Since the code is already detecting the four corner points you could e.g. find the intersection of the diagonals. If you know that there are only rectangles in your image you could also try to detect all centroids of the detected contours using the Hu moments.
I am having a difficult time understand what exactly is in the array as well; is it an array of points?
One contour in OpenCV is always represented as a vector of single points. If you are adding multiple contours you are using a vector of vector of points. In the example you provided squares is a vector of a vector of exactly 4 points. You could also use a vector of cv::Rect in this case.

opencv, find a letter located at a specific location of a picture?

friends, could you please help with my questions?
I am using opencv in c++.
I am randomly cropping a small picture from a camera view. I want to find the word located at the bottom of this cropped picture, and this word should also be penetrated by the vertical center line (imaginary) of this cropped picture. please see the following code :
char* my_word = do_ocr(my_cropped_image);
and the do_ocr function is like this:
char* do_ocr(cv::Mat im)
{
cv::Mat gray;
cv::cvtColor(im, gray, CV_BGR2GRAY);
// ...other image pre-processing here...
// Pass it to Tesseract API
tesseract::TessBaseAPI tess;
tess.Init(NULL, "eng", tesseract::OEM_DEFAULT);
tess.SetPageSegMode(tesseract::PSM_SINGLE_BLOCK);
tess.SetImage((uchar*)gray.data, gray.cols, gray.rows, 1, gray.cols);
// Get the text
char* out = tess.GetUTF8Text();
std::cout << out << std::endl;
return out;
}
The following is the schematic diagram and some samples of my_cropped_image :
my_cropped_image sample # 1, the letter "preceding" should be detected:
my_cropped_image sample # 2, the letter "advantageous" should be detected:
my_cropped_image sample # 3, the letter "Correlation" should be detected:
my_cropped_image sample # 4, the letter "density" should be detected:
my_cropped_image sample # 5, the letter "time" should be detected:
I'll appreciate the helps from you to update my do_ocr function.
Thank you and have a great day!
Are these the results you were looking for?
Methodology:
1) Binaryze the image, white is foreground. Here is simply done with img = img < 150;. You can use more sophisticated methods, like adaptiveThreshold.
You get something like:
2) Apply a open morphological operation, so that all the letters in a single word for a single blob:
3) Find the rectangle of each connected component:
4) Take the bottom one, in the center.
Here the full code:
#include <opencv2\opencv.hpp>
#include <vector>
using namespace std;
using namespace cv;
Mat3b dbg;
int main()
{
Mat1b img = imread("path_to_image", IMREAD_GRAYSCALE);
cvtColor(img, dbg, COLOR_GRAY2BGR);
Mat3b result;
cvtColor(img, result, COLOR_GRAY2BGR);
Mat1b img2;
img2 = img < 150;
Mat kernel = getStructuringElement(MORPH_ELLIPSE, Size(5,3));
morphologyEx(img2, img2, MORPH_DILATE, kernel);
// Apply a small border
copyMakeBorder(img2, img2, 5, 5, 5, 5, BORDER_CONSTANT, Scalar(0));
// Take the bounding boxes of all connected components
vector<vector<Point>> contours;
findContours(img2.clone(), contours, CV_RETR_LIST, CHAIN_APPROX_NONE);
int minArea = 60;
vector<Rect> rects;
for (int i = 0; i < contours.size(); ++i)
{
Rect r = boundingRect(contours[i]);
if (r.area() >= minArea)
{
// Account for border
r -= Point(5,5);
rects.push_back(r);
}
}
int middle = img.cols / 2;
// Keep bottom rect, containig middle point
if (rects.empty()) return -1;
Rect word;
for (int i = 1; i < rects.size(); ++i)
{
Point pt(middle, rects[i].y + rects[i].height/2);
if (rects[i].contains(pt))
{
if (rects[i].y > word.y)
{
word = rects[i];
}
}
}
// Show results
Mat3b res;
cvtColor(img, res, COLOR_GRAY2BGR);
for (int i = 0; i < rects.size(); ++i)
{
rectangle(res, rects[i], Scalar(0, 255, 0));
}
rectangle(result, word, Scalar(0, 0, 255), 2);
imshow("Rects", res);
imshow("Result", result);
waitKey();
return 0;
}

How to detect squares in video with OpenCV?

So I combined squares.cpp with cvBoundingRect.cpp code to detect squares in video. I therefore, had to convert from IplImage to Mat type so that findSquares and drawSquares methods could run (By using cvarrToMat function). But unfortunately, after successful compilation I get this error when running:
OpenCV Error: Assertion failed (j < nsrcs && src[j].depth() == depth) in mixChannels, file /Users/Desktop/opencv-3.0.0-rc1/modules/core/src/convert.cpp, line 1205
libc++abi.dylib: terminating with uncaught exception of type cv::Exception: /Users/Desktop/opencv-3.0.0-rc1/modules/core/src/convert.cpp:1205: error: (-215) j < nsrcs && src[j].depth() == depth in function mixChannels
Abort trap: 6
Here's the code:
#include "opencv2/core/core.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui/highgui.hpp"
#include <iostream>
#include <math.h>
#include <string.h>
using namespace cv;
using namespace std;
int thresh = 50, N = 11;
const char* wndname = "Square Detection Demo";
// finds a cosine of angle between vectors
// from pt0->pt1 and from pt0->pt2
static double angle( Point pt1, Point pt2, Point pt0 )
{
double dx1 = pt1.x - pt0.x;
double dy1 = pt1.y - pt0.y;
double dx2 = pt2.x - pt0.x;
double dy2 = pt2.y - pt0.y;
return (dx1*dx2 + dy1*dy2)/sqrt((dx1*dx1 + dy1*dy1)*(dx2*dx2 + dy2*dy2) + 1e-10);
}
// returns sequence of squares detected on the image.
// the sequence is stored in the specified memory storage
static void findSquares( const Mat& image, vector<vector<Point> >& squares )
{
squares.clear();
Mat pyr, timg, gray0(image.size(), CV_8U), gray;
// down-scale and upscale the image to filter out the noise
pyrDown(image, pyr, Size(image.cols/2, image.rows/2));
pyrUp(pyr, timg, image.size());
vector<vector<Point> > contours;
// find squares in every color plane of the image
for( int c = 0; c < 3; c++ )
{
int ch[] = {c, 0};
mixChannels(&timg, 1, &gray0, 1, ch, 1);
// try several threshold levels
for( int l = 0; l < N; l++ )
{
// hack: use Canny instead of zero threshold level.
// Canny helps to catch squares with gradient shading
if( l == 0 )
{
// apply Canny. Take the upper threshold from slider
// and set the lower to 0 (which forces edges merging)
Canny(gray0, gray, 0, thresh, 5);
// dilate canny output to remove potential
// holes between edge segments
dilate(gray, gray, Mat(), Point(-1,-1));
}
else
{
// apply threshold if l!=0:
// tgray(x,y) = gray(x,y) < (l+1)*255/N ? 255 : 0
gray = gray0 >= (l+1)*255/N;
}
// find contours and store them all as a list
findContours(gray, contours, RETR_LIST, CHAIN_APPROX_SIMPLE);
vector<Point> approx;
// test each contour
for( size_t i = 0; i < contours.size(); i++ )
{
// approximate contour with accuracy proportional
// to the contour perimeter
approxPolyDP(Mat(contours[i]), approx, arcLength(Mat(contours[i]), true)*0.02, true);
// square contours should have 4 vertices after approximation
// relatively large area (to filter out noisy contours)
// and be convex.
// Note: absolute value of an area is used because
// area may be positive or negative - in accordance with the
// contour orientation
if( approx.size() == 4 &&
fabs(contourArea(Mat(approx))) > 1000 &&
isContourConvex(Mat(approx)) )
{
double maxCosine = 0;
for( int j = 2; j < 5; j++ )
{
// find the maximum cosine of the angle between joint edges
double cosine = fabs(angle(approx[j%4], approx[j-2], approx[j-1]));
maxCosine = MAX(maxCosine, cosine);
}
// if cosines of all angles are small
// (all angles are ~90 degree) then write quandrange
// vertices to resultant sequence
if( maxCosine < 0.3 )
squares.push_back(approx);
}
}
}
}
}
// the function draws all the squares in the image
static void drawSquares( Mat& image, const vector<vector<Point> >& squares )
{
for( size_t i = 0; i < squares.size(); i++ )
{
const Point* p = &squares[i][0];
int n = (int)squares[i].size();
polylines(image, &p, &n, 1, true, Scalar(255,0,0), 3, LINE_AA);
}
imshow(wndname, image);
}
CvRect rect;
CvSeq* contours = 0;
CvMemStorage* storage = NULL;
CvCapture *cam;
IplImage *currentFrame, *currentFrame_grey, *differenceImg, *oldFrame_grey;
bool first = true;
int main(int argc, char* argv[])
{
//Create a new movie capture object.
cam = cvCaptureFromCAM(0);
//create storage for contours
storage = cvCreateMemStorage(0);
//capture current frame from webcam
currentFrame = cvQueryFrame(cam);
//Size of the image.
CvSize imgSize;
imgSize.width = currentFrame->width;
imgSize.height = currentFrame->height;
//Images to use in the program.
currentFrame_grey = cvCreateImage( imgSize, IPL_DEPTH_8U, 1);
namedWindow( wndname, 1 );
vector<vector<Point> > squares;
while(1)
{
currentFrame = cvQueryFrame( cam );
if( !currentFrame ) break;
//Convert the image to grayscale.
cvCvtColor(currentFrame,currentFrame_grey,CV_RGB2GRAY);
if(first) //Capturing Background for the first time
{
differenceImg = cvCloneImage(currentFrame_grey);
oldFrame_grey = cvCloneImage(currentFrame_grey);
cvConvertScale(currentFrame_grey, oldFrame_grey, 1.0, 0.0);
first = false;
continue;
}
//Minus the current frame from the moving average.
cvAbsDiff(oldFrame_grey,currentFrame_grey,differenceImg);
//bluring the differnece image
cvSmooth(differenceImg, differenceImg, CV_BLUR);
//apply threshold to discard small unwanted movements
cvThreshold(differenceImg, differenceImg, 25, 255, CV_THRESH_BINARY);
//find contours
cv::Mat diffImg = cv::cvarrToMat(differenceImg);
cv::Mat currFrame = cv::cvarrToMat(currentFrame);
findSquares(diffImg, squares);
//draw bounding box around each contour
drawSquares(currFrame, squares);
//display colour image with bounding box
cvShowImage("Output Image", currentFrame);
//display threshold image
cvShowImage("Difference image", differenceImg);
//New Background
cvConvertScale(currentFrame_grey, oldFrame_grey, 1.0, 0.0);
//clear memory and contours
cvClearMemStorage( storage );
contours = 0;
//press Esc to exit
char c = cvWaitKey(33);
if( c == 27 ) break;
}
// Destroy the image & movies objects
cvReleaseImage(&oldFrame_grey);
cvReleaseImage(&differenceImg);
cvReleaseImage(&currentFrame);
cvReleaseImage(&currentFrame_grey);
return 0;
}
As the error message says, your problem is in cv::mixChannels(). See documentation.
Or you could simply do something like
cv::Mat channels[3];
cv::split(multiChannelImage, channels);
and then access each channel using
cv::Mat currChannel = channels[channelNumber]

OpenCV C++/Obj-C: Advanced square detection

A while ago I asked a question about square detection and karlphillip came up with a decent result.
Now I want to take this a step further and find squares which edge aren't fully visible. Take a look at this example:
Any ideas? I'm working with karlphillips code:
void find_squares(Mat& image, vector<vector<Point> >& squares)
{
// blur will enhance edge detection
Mat blurred(image);
medianBlur(image, blurred, 9);
Mat gray0(blurred.size(), CV_8U), gray;
vector<vector<Point> > contours;
// find squares in every color plane of the image
for (int c = 0; c < 3; c++)
{
int ch[] = {c, 0};
mixChannels(&blurred, 1, &gray0, 1, ch, 1);
// try several threshold levels
const int threshold_level = 2;
for (int l = 0; l < threshold_level; l++)
{
// Use Canny instead of zero threshold level!
// Canny helps to catch squares with gradient shading
if (l == 0)
{
Canny(gray0, gray, 10, 20, 3); //
// Dilate helps to remove potential holes between edge segments
dilate(gray, gray, Mat(), Point(-1,-1));
}
else
{
gray = gray0 >= (l+1) * 255 / threshold_level;
}
// Find contours and store them in a list
findContours(gray, contours, CV_RETR_LIST, CV_CHAIN_APPROX_SIMPLE);
// Test contours
vector<Point> approx;
for (size_t i = 0; i < contours.size(); i++)
{
// approximate contour with accuracy proportional
// to the contour perimeter
approxPolyDP(Mat(contours[i]), approx, arcLength(Mat(contours[i]), true)*0.02, true);
// Note: absolute value of an area is used because
// area may be positive or negative - in accordance with the
// contour orientation
if (approx.size() == 4 &&
fabs(contourArea(Mat(approx))) > 1000 &&
isContourConvex(Mat(approx)))
{
double maxCosine = 0;
for (int j = 2; j < 5; j++)
{
double cosine = fabs(angle(approx[j%4], approx[j-2], approx[j-1]));
maxCosine = MAX(maxCosine, cosine);
}
if (maxCosine < 0.3)
squares.push_back(approx);
}
}
}
}
}
You might try using HoughLines to detect the four sides of the square. Next, locate the four resulting line intersections to detect the corners. The Hough transform is fairly robust to noise and occlusions, so it could be useful here. Also, here is an interactive demo showing how the Hough transform works (I thought it was cool at least :). Here is one of my previous answers that detects a laser cross center showing most of the same math (except it just finds a single corner).
You will probably have multiple lines on each side, but locating the intersections should help to determine the inliers vs. outliers. Once you've located candidate corners, you can also filter these candidates by area or how "square-like" the polygon is.
EDIT : All these answers with code and images were making me think my answer was a bit lacking :) So, here is an implementation of how you could do this:
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <iostream>
#include <vector>
using namespace cv;
using namespace std;
Point2f computeIntersect(Vec2f line1, Vec2f line2);
vector<Point2f> lineToPointPair(Vec2f line);
bool acceptLinePair(Vec2f line1, Vec2f line2, float minTheta);
int main(int argc, char* argv[])
{
Mat occludedSquare = imread("Square.jpg");
resize(occludedSquare, occludedSquare, Size(0, 0), 0.25, 0.25);
Mat occludedSquare8u;
cvtColor(occludedSquare, occludedSquare8u, CV_BGR2GRAY);
Mat thresh;
threshold(occludedSquare8u, thresh, 200.0, 255.0, THRESH_BINARY);
GaussianBlur(thresh, thresh, Size(7, 7), 2.0, 2.0);
Mat edges;
Canny(thresh, edges, 66.0, 133.0, 3);
vector<Vec2f> lines;
HoughLines( edges, lines, 1, CV_PI/180, 50, 0, 0 );
cout << "Detected " << lines.size() << " lines." << endl;
// compute the intersection from the lines detected...
vector<Point2f> intersections;
for( size_t i = 0; i < lines.size(); i++ )
{
for(size_t j = 0; j < lines.size(); j++)
{
Vec2f line1 = lines[i];
Vec2f line2 = lines[j];
if(acceptLinePair(line1, line2, CV_PI / 32))
{
Point2f intersection = computeIntersect(line1, line2);
intersections.push_back(intersection);
}
}
}
if(intersections.size() > 0)
{
vector<Point2f>::iterator i;
for(i = intersections.begin(); i != intersections.end(); ++i)
{
cout << "Intersection is " << i->x << ", " << i->y << endl;
circle(occludedSquare, *i, 1, Scalar(0, 255, 0), 3);
}
}
imshow("intersect", occludedSquare);
waitKey();
return 0;
}
bool acceptLinePair(Vec2f line1, Vec2f line2, float minTheta)
{
float theta1 = line1[1], theta2 = line2[1];
if(theta1 < minTheta)
{
theta1 += CV_PI; // dealing with 0 and 180 ambiguities...
}
if(theta2 < minTheta)
{
theta2 += CV_PI; // dealing with 0 and 180 ambiguities...
}
return abs(theta1 - theta2) > minTheta;
}
// the long nasty wikipedia line-intersection equation...bleh...
Point2f computeIntersect(Vec2f line1, Vec2f line2)
{
vector<Point2f> p1 = lineToPointPair(line1);
vector<Point2f> p2 = lineToPointPair(line2);
float denom = (p1[0].x - p1[1].x)*(p2[0].y - p2[1].y) - (p1[0].y - p1[1].y)*(p2[0].x - p2[1].x);
Point2f intersect(((p1[0].x*p1[1].y - p1[0].y*p1[1].x)*(p2[0].x - p2[1].x) -
(p1[0].x - p1[1].x)*(p2[0].x*p2[1].y - p2[0].y*p2[1].x)) / denom,
((p1[0].x*p1[1].y - p1[0].y*p1[1].x)*(p2[0].y - p2[1].y) -
(p1[0].y - p1[1].y)*(p2[0].x*p2[1].y - p2[0].y*p2[1].x)) / denom);
return intersect;
}
vector<Point2f> lineToPointPair(Vec2f line)
{
vector<Point2f> points;
float r = line[0], t = line[1];
double cos_t = cos(t), sin_t = sin(t);
double x0 = r*cos_t, y0 = r*sin_t;
double alpha = 1000;
points.push_back(Point2f(x0 + alpha*(-sin_t), y0 + alpha*cos_t));
points.push_back(Point2f(x0 - alpha*(-sin_t), y0 - alpha*cos_t));
return points;
}
NOTE : The main reason I resized the image was so I could see it on my screen, and speed-up processing.
Canny
This uses Canny edge detection to help greatly reduce the number of lines detected after thresholding.
Hough transform
Then the Hough transform is used to detect the sides of the square.
Intersections
Finally, we compute the intersections of all the line pairs.
Hope that helps!
I tried to use convex hull method which is pretty simple.
Here you find convex hull of the contour detected. It removes the convexity defects at the bottom of paper.
Below is the code (in OpenCV-Python):
import cv2
import numpy as np
img = cv2.imread('sof.jpg')
img = cv2.resize(img,(500,500))
gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
ret,thresh = cv2.threshold(gray,127,255,0)
contours,hier = cv2.findContours(thresh,cv2.RETR_LIST,cv2.CHAIN_APPROX_SIMPLE)
for cnt in contours:
if cv2.contourArea(cnt)>5000: # remove small areas like noise etc
hull = cv2.convexHull(cnt) # find the convex hull of contour
hull = cv2.approxPolyDP(hull,0.1*cv2.arcLength(hull,True),True)
if len(hull)==4:
cv2.drawContours(img,[hull],0,(0,255,0),2)
cv2.imshow('img',img)
cv2.waitKey(0)
cv2.destroyAllWindows()
(Here, i haven't found square in all planes. Do it yourself if you want.)
Below is the result i got:
I hope this is what you needed.
1st: start experimenting with threshold techniques to isolate the white paper sheet from the rest of the image. This is a simple way:
Mat new_img = imread(argv[1]);
double thres = 200;
double color = 255;
threshold(new_img, new_img, thres, color, CV_THRESH_BINARY);
imwrite("thres.png", new_img);
but there are other alternatives that could provide better result. One is to investigate inRange(), and another is to detect through color by converting the image to the HSV color space.
This thread also provides an interest discussion on the subject.
2nd: after you execute one of this procedures, you could try to feed the result directly into find_squares():
An alternative to find_squares() is to implement the bounding box technique, which has the potential to provide a more accurate detection of the rectangular area (provided that you have a perfect result of threshold). I've used it here and here. It's worth noting that OpenCV has it's own bounding box tutorial.
Another approach besides find_squares(), as pointed by Abid on his answer, is to use the convexHull method. Check OpenCV's C++ tutorial on this method for code.
convert to lab space
use kmeans for 2 clusters
detect suqares one internal cluster it will solve many thing in the rgb space

OpenCV Line Detection

I am trying to find the edges of the centered box in this image:
I have tried using a HoughLines using dRho=img_width/1000, dTheta=pi/180, and threshold=250
It works great on this image, scaled to 1/3 of the size, but on the full size image it just gets lines everywhere in every direction...
What can I do to tune this to be more accurate?
The code to achieve the result below is a slight modification of the one presented in this answer: how to detect a square:
The original program can be found inside OpenCV, it's called squares.cpp. The code below was modified to search squares only in the first color plane, but as it still detects many squares, at the end of the program I discard all of them except the first, and then call draw_squares() to show what was detected. You can change this easilly to draw all of them and see everything that was detected.
You can do all sorts of thing from now own, including setting a (ROI) region of interest to extract the area that's inside the square (ignore everything else around it).
You can see that the detected rectangle is not perfectly aligned with the lines in the image. You should perform some pre-processing (erode?) operations in the image to decrease the thickness of lines and improve the detection. But from here on it's all on you:
#include <cv.h>
#include <highgui.h>
using namespace cv;
double angle( cv::Point pt1, cv::Point pt2, cv::Point pt0 ) {
double dx1 = pt1.x - pt0.x;
double dy1 = pt1.y - pt0.y;
double dx2 = pt2.x - pt0.x;
double dy2 = pt2.y - pt0.y;
return (dx1*dx2 + dy1*dy2)/sqrt((dx1*dx1 + dy1*dy1)*(dx2*dx2 + dy2*dy2) + 1e-10);
}
void find_squares(Mat& image, vector<vector<Point> >& squares)
{
// TODO: pre-processing
// blur will enhance edge detection
Mat blurred(image);
medianBlur(image, blurred, 9);
Mat gray0(blurred.size(), CV_8U), gray;
vector<vector<Point> > contours;
// find squares in the first color plane.
for (int c = 0; c < 1; c++)
{
int ch[] = {c, 0};
mixChannels(&blurred, 1, &gray0, 1, ch, 1);
// try several threshold levels
const int threshold_level = 2;
for (int l = 0; l < threshold_level; l++)
{
// Use Canny instead of zero threshold level!
// Canny helps to catch squares with gradient shading
if (l == 0)
{
Canny(gray0, gray, 10, 20, 3); //
// Dilate helps to remove potential holes between edge segments
dilate(gray, gray, Mat(), Point(-1,-1));
}
else
{
gray = gray0 >= (l+1) * 255 / threshold_level;
}
// Find contours and store them in a list
findContours(gray, contours, CV_RETR_LIST, CV_CHAIN_APPROX_SIMPLE);
// Test contours
vector<Point> approx;
for (size_t i = 0; i < contours.size(); i++)
{
// approximate contour with accuracy proportional
// to the contour perimeter
approxPolyDP(Mat(contours[i]), approx, arcLength(Mat(contours[i]), true)*0.02, true);
// Note: absolute value of an area is used because
// area may be positive or negative - in accordance with the
// contour orientation
if (approx.size() == 4 &&
fabs(contourArea(Mat(approx))) > 1000 &&
isContourConvex(Mat(approx)))
{
double maxCosine = 0;
for (int j = 2; j < 5; j++)
{
double cosine = fabs(angle(approx[j%4], approx[j-2], approx[j-1]));
maxCosine = MAX(maxCosine, cosine);
}
if (maxCosine < 0.3)
squares.push_back(approx);
}
}
}
}
}
void draw_squares(Mat& img, vector<vector<Point> > squares)
{
for (int i = 0; i < squares.size(); i++)
{
for (int j = 0; j < squares[i].size(); j++)
{
cv::line(img, squares[i][j], squares[i][(j+1) % 4], cv::Scalar(0, 255, 0), 1, CV_AA);
}
}
}
int main(int argc, char* argv[])
{
Mat img = imread(argv[1]);
vector<vector<Point> > squares;
find_squares(img, squares);
std::cout << "* " << squares.size() << " squares were found." << std::endl;
// Ignore all the detected squares and draw just the first found
vector<vector<Point> > tmp;
if (squares.size() > 0)
{
tmp.push_back(squares[0]);
draw_squares(img, tmp);
}
//imshow("squares", img);
//cvWaitKey(0);
imwrite("out.png", img);
return 0;
}
when resizing the image, the image is normally first blurred with a filter, e.g. Gaussian, in order to get rid of high frequencies. The fact that resized one works better is likely because your original image is somehow noisy.
Try blur the image first, e.g. with cv::GaussianBlur(src, target, Size(0,0), 1.5), then it should be equivalent to resizing. (It forgot the theory, if it does not work, try 3 and 6 as well)
Try using a preprocessing pass with the erosion filter. It will give you the same effect as the downscaling - the lines will become thinner and will not disappear at the same time.
The "Blur" filter is also a good idea, as chaiy says.
This way (with blur) it will become something like http://www.ic.uff.br/~laffernandes/projects/kht/index.html (Kernel Based Hough Transform)