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I want to detect the bounding rectangle of an German ID card within an image by using OpenCV.
This is what my code looks like:
capture >> frame;
cv::resize(frame, frame, cv::Size(512,256));
cv::Mat grayScaledFrame, blurredFrame, cannyFrame;
cv::cvtColor(frame, grayScaledFrame, cv::COLOR_BGR2GRAY);
cv::GaussianBlur(grayScaledFrame, blurredFrame, cv::Size(9,9), 1);
cv::Canny(blurredFrame, cannyFrame, 40, 70);
// CONTOURS
std::vector<std::vector<cv::Point>> contours;
cv::findContours(cannyFrame, contours, cv::RETR_EXTERNAL, cv::CHAIN_APPROX_SIMPLE);
// SORT
int maxArea = 0;
std::vector<cv::Point> contour;
for(int i = 0; i < contours.size(); i++) {
int thisArea = cv::contourArea(contours.at(i));
if(thisArea > maxArea) {
maxArea = thisArea;
contour = contours.at(i);
}
}
cv::Rect borderBox = cv::boundingRect(contour);
cv::rectangle(cannyFrame, borderBox, cv::Scalar{255, 32, 32}, 8);
cv::imshow("Webcam", cannyFrame);
The result looks like this:
RESULT
There are some rectangles detected but not the big one I'm interested in.
I've already tried different thresholds for Canny and also different kernel sizes for Gaussian Blur.
Best regards
First of all, as the environmental conditions change, the parameters of the code change, so it is necessary to standardize the environment (light, distance to the object, etc.).
To get this detection right, put the card at a fixed distance from the camera and calculate the area of the rectangles.
When the card is at a certain distance from the camera, you get approximate reference values of the card's area. Then, when drawing a rectangle, you use values within a specified tolerance range.
I have to write a program that detect 3 types of road signs (speed limit, no parking and warnings). I know how to detect a circle using HoughCircles but I have several images and the parameters for HoughCircles are different for each image. There's a general way to detect circles without changing parameters for each image?
Moreover I need to detect triangle (warning signs) so I'm searching for a general shape detector. Have you any suggestions/code that can help me in this task?
Finally for detect the number on speed limit signs I thought to use SIFT and compare the image with some templates in order to identify the number on the sign. Could it be a good approach?
Thank you for the answer!
I know this is a pretty old question but I had been through the same problem and now I show you how I solved it.
The following images show some of the most accurate results that are displayed by the opencv program.
In the following images the street signs detected are circled with three different colors that distinguish the three kinds of street signs (warning, no parking, speed limit).
Red for warning signs
Blue for no parking signs
Fuchsia for speed limit signs
The speed limit value is written in green above the speed limit signs
[![example][1]][1]
[![example][2]][2]
[![example][3]][3]
[![example][4]][4]
As you can see the program performs quite well, it is able to detect and distinguish the three kinds of sign and to recognize the speed limit value in case of speed limit signs. Everything is done without computing too many false positives when, for instance, in the image there are some signs that do not belong to one of the three categories.
In order to achieve this result the software computes the detection in three main steps.
The first step involves a color based approach where the red objects in the image are detected and their region are extract to be analyzed. This step is particularly useful in order to prevent the detection of false positives, because only a small part of the image is processed.
The second step works with a machine learning algorithm: in particular we use a Cascade Classifier to compute the detection. This operation firstly requires to train the classifiers and on a later stage to use them to detect the signs.
In the last step the speed limit values inside the speed limit signs are read, also in this case through a machine learning algorithm but using the k-nearest neighbor algorithm.
Now we are going to see in detail each step.
COLOR BASED STEP
Since the street signs are always circled by a red frame, we can afford to take out and analyze only the regions where the red objects are detected.
In order to select the red objects, we consider all the ranges of the red color: even if this may produce some false positives, they will be easily discarded in the next steps.
inRange(image, Scalar(0, 70, 50), Scalar(10, 255, 255), mask1);
inRange(image, Scalar(170, 70, 50), Scalar(180, 255, 255), mask2);
In the image below we can see an example of the red objects detected with this method.
After having found the red pixels we can gather them to find the regions using a clustering algorithm, I use the method
partition(<#_ForwardIterator __first#>, _ForwardIterator __last, <#_Predicate __pred#>)
After the execution of this method we can save all the points in the same cluster in a vector (one for each cluster) and extract the bounding boxes which represent the
regions to be analyzed in the next step.
HAAR CASCADE CLASSIFIERS FOR SIGNS DETECTION
This is the real detection step where the street signs are detected. In order to perform a cascade classifier the first step consist in building a dataset of positives and negatives images. Now I explain how I have built my own datasets of images.
The first thing to note is that we need to train three different Haar cascades in order to distinguish between the three kind of signs that we have to detect, hence we must repeat the following steps for each of the three kinds of sign.
We need two datasets: one for the positive samples (which must be a set of images that contains the road signs that we are going to detect) and another one for the negative samples which can be any kind of image without street signs.
After collecting a set of 100 images for the positive samples and a set of 200 images for the negatives in two different folders, we need to write two text files:
Signs.info which contains a list of file names like the one below,
one for each positive sample in the positive folder.
pos/image_name.png 1 0 0 50 45
Here, the numbers after the name represent respectively the number
of street signs in the image, the coordinate of the upper left
corner of the street sign, his height and his width.
Bg.txt which contains a list of file names like the one below, one
for each sign in the negative folder.
neg/street15.png
With the command line below we generate the .vect file which contains all the information that the software retrieves from the positive samples.
opencv_createsamples -info sign.info -num 100 -w 50 -h 50 -vec signs.vec
Afterwards we train the cascade classifier with the following command:
opencv_traincascade -data data -vec signs.vec -bg bg.txt -numPos 60 -numNeg 200 -numStages 15 -w 50 -h 50 -featureType LBP
where the number of stages indicates the number of classifiers that will be generated in order to build the cascade.
At the end of this process we gain a file cascade.xml which will be used from the CascadeClassifier program in order to detect the objects in the image.
Now we have trained our algorithm and we can declare a CascadeClassifier for each kind of street sign, than we detect the signs in the image through
detectMultiScale(<#InputArray image#>, <#std::vector<Rect> &objects#>)
this method creates a Rect around each object that has been detected.
It is important to note that exactly as every machine learning algorithm, in order to perform well, we need a large number of samples in the dataset. The dataset that I have built, is not extremely large, thus in some situations it is not able to detect all the signs. This mostly happens when a small part of the street sign is not visible in the image like in the warning sign below:
I have expanded my dataset up to the point where I have obtained a fairly accurate result without
too many errors.
SPEED LIMIT VALUE DETECTION
Like for the street signs detection also here I used a machine learning algorithm but with a different approach. After some work, I realized that an OCR (tesseract) solution does not perform well, so I decided to build my own ocr software.
For the machine learning algorithm I took the image below as training data which contains some speed limit values:
The amount of training data is small. But, since in speed limit signs all letters have the same font, it is not a huge problem.
To prepare the data for training, I made a small code in OpenCV. It does the following things:
It loads the image on the left;
It selects the digits (obviously by contour finding and applying constraints on area and height of letters to avoid false detections).
It draws the bounding rectangle around one letter and it waits for the key to be manually pressed. This time the user presses the digit key corresponding to the letter in box by himself.
Once the corresponding digit key is pressed, it saves 100 pixel values in an array and the correspondent manually entered digit in another array.
Eventually it saves both the arrays in separate txt files.
Following the manual digit classification all the digits in the train data( train.png) are manually labeled, and the image will look like the one below.
Now we enter into training and testing part.
For training we do as follows:
Load the txt files we already saved earlier
Create an instance of classifier that we are going to use ( KNearest)
Then we use KNearest.train function to train the data
Now the detection:
We load the image with the speed limit sign detected
Process the image as before and extract each digit using contour methods
Draw bounding box for it, then resize to 10x10, and store its pixel values in an array as done earlier.
Then we use KNearest.find_nearest() function to find the nearest item to the one we gave.
And it recognizes the correct digit.
I tested this little OCR on many images, and just with this small dataset I have obtained an accuracy of about 90%.
CODE
Below I post all my openCv c++ code in a single class, following my instruction you should be able to achive my result.
#include "opencv2/objdetect/objdetect.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>
#include <cmath>
#include <stdlib.h>
#include "opencv2/core/core.hpp"
#include "opencv2/highgui.hpp"
#include <string.h>
#include <opencv2/ml/ml.hpp>
using namespace std;
using namespace cv;
std::vector<cv::Rect> getRedObjects(cv::Mat image);
vector<Mat> detectAndDisplaySpeedLimit( Mat frame );
vector<Mat> detectAndDisplayNoParking( Mat frame );
vector<Mat> detectAndDisplayWarning( Mat frame );
void trainDigitClassifier();
string getDigits(Mat image);
vector<Mat> loadAllImage();
int getSpeedLimit(string speed);
//path of the haar cascade files
String no_parking_signs_cascade = "/Users/giuliopettenuzzo/Desktop/cascade_classifiers/no_parking_cascade.xml";
String speed_signs_cascade = "/Users/giuliopettenuzzo/Desktop/cascade_classifiers/speed_limit_cascade.xml";
String warning_signs_cascade = "/Users/giuliopettenuzzo/Desktop/cascade_classifiers/warning_cascade.xml";
CascadeClassifier speed_limit_cascade;
CascadeClassifier no_parking_cascade;
CascadeClassifier warning_cascade;
int main(int argc, char** argv)
{
//train the classifier for digit recognition, this require a manually train, read the report for more details
trainDigitClassifier();
cv::Mat sceneImage;
vector<Mat> allImages = loadAllImage();
for(int i = 0;i<=allImages.size();i++){
sceneImage = allImages[i];
//load the haar cascade files
if( !speed_limit_cascade.load( speed_signs_cascade ) ){ printf("--(!)Error loading\n"); return -1; };
if( !no_parking_cascade.load( no_parking_signs_cascade ) ){ printf("--(!)Error loading\n"); return -1; };
if( !warning_cascade.load( warning_signs_cascade ) ){ printf("--(!)Error loading\n"); return -1; };
Mat scene = sceneImage.clone();
//detect the red objects
std::vector<cv::Rect> allObj = getRedObjects(scene);
//use the three cascade classifier for each object detected by the getRedObjects() method
for(int j = 0;j<allObj.size();j++){
Mat img = sceneImage(Rect(allObj[j]));
vector<Mat> warningVec = detectAndDisplayWarning(img);
if(warningVec.size()>0){
Rect box = allObj[j];
}
vector<Mat> noParkVec = detectAndDisplayNoParking(img);
if(noParkVec.size()>0){
Rect box = allObj[j];
}
vector<Mat> speedLitmitVec = detectAndDisplaySpeedLimit(img);
if(speedLitmitVec.size()>0){
Rect box = allObj[j];
for(int i = 0; i<speedLitmitVec.size();i++){
//get speed limit and skatch it in the image
int digit = getSpeedLimit(getDigits(speedLitmitVec[i]));
if(digit > 0){
Point point = box.tl();
point.y = point.y + 30;
cv::putText(sceneImage,
"SPEED LIMIT " + to_string(digit),
point,
cv::FONT_HERSHEY_COMPLEX_SMALL,
0.7,
cv::Scalar(0,255,0),
1,
cv::CV__CAP_PROP_LATEST);
}
}
}
}
imshow("currentobj",sceneImage);
waitKey(0);
}
}
/*
* detect the red object in the image given in the param,
* return a vector containing all the Rect of the red objects
*/
std::vector<cv::Rect> getRedObjects(cv::Mat image)
{
Mat3b res = image.clone();
std::vector<cv::Rect> result;
cvtColor(image, image, COLOR_BGR2HSV);
Mat1b mask1, mask2;
//ranges of red color
inRange(image, Scalar(0, 70, 50), Scalar(10, 255, 255), mask1);
inRange(image, Scalar(170, 70, 50), Scalar(180, 255, 255), mask2);
Mat1b mask = mask1 | mask2;
Mat nonZeroCoordinates;
vector<Point> pts;
findNonZero(mask, pts);
for (int i = 0; i < nonZeroCoordinates.total(); i++ ) {
cout << "Zero#" << i << ": " << nonZeroCoordinates.at<Point>(i).x << ", " << nonZeroCoordinates.at<Point>(i).y << endl;
}
int th_distance = 2; // radius tolerance
// Apply partition
// All pixels within the radius tolerance distance will belong to the same class (same label)
vector<int> labels;
// With lambda function (require C++11)
int th2 = th_distance * th_distance;
int n_labels = partition(pts, labels, [th2](const Point& lhs, const Point& rhs) {
return ((lhs.x - rhs.x)*(lhs.x - rhs.x) + (lhs.y - rhs.y)*(lhs.y - rhs.y)) < th2;
});
// You can save all points in the same class in a vector (one for each class), just like findContours
vector<vector<Point>> contours(n_labels);
for (int i = 0; i < pts.size(); ++i){
contours[labels[i]].push_back(pts[i]);
}
// Get bounding boxes
vector<Rect> boxes;
for (int i = 0; i < contours.size(); ++i)
{
Rect box = boundingRect(contours[i]);
if(contours[i].size()>500){//prima era 1000
boxes.push_back(box);
Rect enlarged_box = box + Size(100,100);
enlarged_box -= Point(30,30);
if(enlarged_box.x<0){
enlarged_box.x = 0;
}
if(enlarged_box.y<0){
enlarged_box.y = 0;
}
if(enlarged_box.height + enlarged_box.y > res.rows){
enlarged_box.height = res.rows - enlarged_box.y;
}
if(enlarged_box.width + enlarged_box.x > res.cols){
enlarged_box.width = res.cols - enlarged_box.x;
}
Mat img = res(Rect(enlarged_box));
result.push_back(enlarged_box);
}
}
Rect largest_box = *max_element(boxes.begin(), boxes.end(), [](const Rect& lhs, const Rect& rhs) {
return lhs.area() < rhs.area();
});
//draw the rects in case you want to see them
for(int j=0;j<=boxes.size();j++){
if(boxes[j].area() > largest_box.area()/3){
rectangle(res, boxes[j], Scalar(0, 0, 255));
Rect enlarged_box = boxes[j] + Size(20,20);
enlarged_box -= Point(10,10);
rectangle(res, enlarged_box, Scalar(0, 255, 0));
}
}
rectangle(res, largest_box, Scalar(0, 0, 255));
Rect enlarged_box = largest_box + Size(20,20);
enlarged_box -= Point(10,10);
rectangle(res, enlarged_box, Scalar(0, 255, 0));
return result;
}
/*
* code for detect the speed limit sign , it draws a circle around the speed limit signs
*/
vector<Mat> detectAndDisplaySpeedLimit( Mat frame )
{
std::vector<Rect> signs;
vector<Mat> result;
Mat frame_gray;
cvtColor( frame, frame_gray, CV_BGR2GRAY );
//normalizes the brightness and increases the contrast of the image
equalizeHist( frame_gray, frame_gray );
//-- Detect signs
speed_limit_cascade.detectMultiScale( frame_gray, signs, 1.1, 3, 0|CV_HAAR_SCALE_IMAGE, Size(30, 30) );
cout << speed_limit_cascade.getFeatureType();
for( size_t i = 0; i < signs.size(); i++ )
{
Point center( signs[i].x + signs[i].width*0.5, signs[i].y + signs[i].height*0.5 );
ellipse( frame, center, Size( signs[i].width*0.5, signs[i].height*0.5), 0, 0, 360, Scalar( 255, 0, 255 ), 4, 8, 0 );
Mat resultImage = frame(Rect(center.x - signs[i].width*0.5,center.y - signs[i].height*0.5,signs[i].width,signs[i].height));
result.push_back(resultImage);
}
return result;
}
/*
* code for detect the warning sign , it draws a circle around the warning signs
*/
vector<Mat> detectAndDisplayWarning( Mat frame )
{
std::vector<Rect> signs;
vector<Mat> result;
Mat frame_gray;
cvtColor( frame, frame_gray, CV_BGR2GRAY );
equalizeHist( frame_gray, frame_gray );
//-- Detect signs
warning_cascade.detectMultiScale( frame_gray, signs, 1.1, 3, 0|CV_HAAR_SCALE_IMAGE, Size(30, 30) );
cout << warning_cascade.getFeatureType();
Rect previus;
for( size_t i = 0; i < signs.size(); i++ )
{
Point center( signs[i].x + signs[i].width*0.5, signs[i].y + signs[i].height*0.5 );
Rect newRect = Rect(center.x - signs[i].width*0.5,center.y - signs[i].height*0.5,signs[i].width,signs[i].height);
if((previus & newRect).area()>0){
previus = newRect;
}else{
ellipse( frame, center, Size( signs[i].width*0.5, signs[i].height*0.5), 0, 0, 360, Scalar( 0, 0, 255 ), 4, 8, 0 );
Mat resultImage = frame(newRect);
result.push_back(resultImage);
previus = newRect;
}
}
return result;
}
/*
* code for detect the no parking sign , it draws a circle around the no parking signs
*/
vector<Mat> detectAndDisplayNoParking( Mat frame )
{
std::vector<Rect> signs;
vector<Mat> result;
Mat frame_gray;
cvtColor( frame, frame_gray, CV_BGR2GRAY );
equalizeHist( frame_gray, frame_gray );
//-- Detect signs
no_parking_cascade.detectMultiScale( frame_gray, signs, 1.1, 3, 0|CV_HAAR_SCALE_IMAGE, Size(30, 30) );
cout << no_parking_cascade.getFeatureType();
Rect previus;
for( size_t i = 0; i < signs.size(); i++ )
{
Point center( signs[i].x + signs[i].width*0.5, signs[i].y + signs[i].height*0.5 );
Rect newRect = Rect(center.x - signs[i].width*0.5,center.y - signs[i].height*0.5,signs[i].width,signs[i].height);
if((previus & newRect).area()>0){
previus = newRect;
}else{
ellipse( frame, center, Size( signs[i].width*0.5, signs[i].height*0.5), 0, 0, 360, Scalar( 255, 0, 0 ), 4, 8, 0 );
Mat resultImage = frame(newRect);
result.push_back(resultImage);
previus = newRect;
}
}
return result;
}
/*
* train the classifier for digit recognition, this could be done only one time, this method save the result in a file and
* it can be used in the next executions
* in order to train user must enter manually the corrisponding digit that the program shows, press space if the red box is just a point (false positive)
*/
void trainDigitClassifier(){
Mat thr,gray,con;
Mat src=imread("/Users/giuliopettenuzzo/Desktop/all_numbers.png",1);
cvtColor(src,gray,CV_BGR2GRAY);
threshold(gray,thr,125,255,THRESH_BINARY_INV); //Threshold to find contour
imshow("ci",thr);
waitKey(0);
thr.copyTo(con);
// Create sample and label data
vector< vector <Point> > contours; // Vector for storing contour
vector< Vec4i > hierarchy;
Mat sample;
Mat response_array;
findContours( con, contours, hierarchy,CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE ); //Find contour
for( int i = 0; i< contours.size(); i=hierarchy[i][0] ) // iterate through first hierarchy level contours
{
Rect r= boundingRect(contours[i]); //Find bounding rect for each contour
rectangle(src,Point(r.x,r.y), Point(r.x+r.width,r.y+r.height), Scalar(0,0,255),2,8,0);
Mat ROI = thr(r); //Crop the image
Mat tmp1, tmp2;
resize(ROI,tmp1, Size(10,10), 0,0,INTER_LINEAR ); //resize to 10X10
tmp1.convertTo(tmp2,CV_32FC1); //convert to float
imshow("src",src);
int c=waitKey(0); // Read corresponding label for contour from keyoard
c-=0x30; // Convert ascii to intiger value
response_array.push_back(c); // Store label to a mat
rectangle(src,Point(r.x,r.y), Point(r.x+r.width,r.y+r.height), Scalar(0,255,0),2,8,0);
sample.push_back(tmp2.reshape(1,1)); // Store sample data
}
// Store the data to file
Mat response,tmp;
tmp=response_array.reshape(1,1); //make continuous
tmp.convertTo(response,CV_32FC1); // Convert to float
FileStorage Data("TrainingData.yml",FileStorage::WRITE); // Store the sample data in a file
Data << "data" << sample;
Data.release();
FileStorage Label("LabelData.yml",FileStorage::WRITE); // Store the label data in a file
Label << "label" << response;
Label.release();
cout<<"Training and Label data created successfully....!! "<<endl;
imshow("src",src);
waitKey(0);
}
/*
* get digit from the image given in param, using the classifier trained before
*/
string getDigits(Mat image)
{
Mat thr1,gray1,con1;
Mat src1 = image.clone();
cvtColor(src1,gray1,CV_BGR2GRAY);
threshold(gray1,thr1,125,255,THRESH_BINARY_INV); // Threshold to create input
thr1.copyTo(con1);
// Read stored sample and label for training
Mat sample1;
Mat response1,tmp1;
FileStorage Data1("TrainingData.yml",FileStorage::READ); // Read traing data to a Mat
Data1["data"] >> sample1;
Data1.release();
FileStorage Label1("LabelData.yml",FileStorage::READ); // Read label data to a Mat
Label1["label"] >> response1;
Label1.release();
Ptr<ml::KNearest> knn(ml::KNearest::create());
knn->train(sample1, ml::ROW_SAMPLE,response1); // Train with sample and responses
cout<<"Training compleated.....!!"<<endl;
vector< vector <Point> > contours1; // Vector for storing contour
vector< Vec4i > hierarchy1;
//Create input sample by contour finding and cropping
findContours( con1, contours1, hierarchy1,CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE );
Mat dst1(src1.rows,src1.cols,CV_8UC3,Scalar::all(0));
string result;
for( int i = 0; i< contours1.size(); i=hierarchy1[i][0] ) // iterate through each contour for first hierarchy level .
{
Rect r= boundingRect(contours1[i]);
Mat ROI = thr1(r);
Mat tmp1, tmp2;
resize(ROI,tmp1, Size(10,10), 0,0,INTER_LINEAR );
tmp1.convertTo(tmp2,CV_32FC1);
Mat bestLabels;
float p=knn -> findNearest(tmp2.reshape(1,1),4, bestLabels);
char name[4];
sprintf(name,"%d",(int)p);
cout << "num = " << (int)p;
result = result + to_string((int)p);
putText( dst1,name,Point(r.x,r.y+r.height) ,0,1, Scalar(0, 255, 0), 2, 8 );
}
imwrite("dest.jpg",dst1);
return result ;
}
/*
* from the digits detected, it returns a speed limit if it is detected correctly, -1 otherwise
*/
int getSpeedLimit(string numbers){
if ((numbers.find("30") != std::string::npos) || (numbers.find("03") != std::string::npos)) {
return 30;
}
if ((numbers.find("50") != std::string::npos) || (numbers.find("05") != std::string::npos)) {
return 50;
}
if ((numbers.find("80") != std::string::npos) || (numbers.find("08") != std::string::npos)) {
return 80;
}
if ((numbers.find("70") != std::string::npos) || (numbers.find("07") != std::string::npos)) {
return 70;
}
if ((numbers.find("90") != std::string::npos) || (numbers.find("09") != std::string::npos)) {
return 90;
}
if ((numbers.find("100") != std::string::npos) || (numbers.find("001") != std::string::npos)) {
return 100;
}
if ((numbers.find("130") != std::string::npos) || (numbers.find("031") != std::string::npos)) {
return 130;
}
return -1;
}
/*
* load all the image in the file with the path hard coded below
*/
vector<Mat> loadAllImage(){
vector<cv::String> fn;
glob("/Users/giuliopettenuzzo/Desktop/T1/dataset/*.jpg", fn, false);
vector<Mat> images;
size_t count = fn.size(); //number of png files in images folder
for (size_t i=0; i<count; i++)
images.push_back(imread(fn[i]));
return images;
}
maybe you should try implementing the ransac algorithm, if you are using color images, migt be a good idea (if you are in europe) to get the red channel only since the speed limits are surrounded by a red cricle (or a thin white i think also).
For that you need to filter the image to get the edges, (canny filter).
Here are some useful links:
OpenCV detect partial circle with noise
https://hal.archives-ouvertes.fr/hal-00982526/document
Finally for the numbers detection i think its ok. Other approach is to use something like Viola-Jones algorithm to detect the signals, with pretrained existing models... It's up to you!
I have the following code to detect contours in an image using cvThreshold and cvFindContours:
CvMemStorage* storage = cvCreateMemStorage(0);
CvSeq* contours = 0;
cvThreshold( processedImage, processedImage, thresh1, 255, CV_THRESH_BINARY );
nContours = cvFindContours(processedImage, storage, &contours, sizeof(CvContour), CV_RETR_EXTERNAL, CV_CHAIN_APPROX_NONE, cvPoint(0,0) );
I would like to somehow extend this code to filter/ignore/remove any contours that touch the image boundaries. However I am unsure how to go about this. Should I filter the threshold image or can I filter the contours afterwards? Hope somebody knows an elegant solution, since surprisingly I could not come up with a solution by googling.
Update 2021-11-25
updates code example
fixes bugs with image borders
adds more images
adds Github repo with CMake support to build example app
Full out-of-the-box example can be found here:
C++ application with CMake
General info
I am using OpenCV 3.0.0
Using cv::findContours actually alters the input image, so make sure that you work either on a separate copy specifically for this function or do not further use the image at all
Update 2019-03-07: "Since opencv 3.2 source image is not modified by this function." (see corresponding OpenCV documentation)
General solution
All you need to know of a contour is if any of its points touches the image border. This info can be extracted easily by one of the following two procedures:
Check each point of your contour regarding its location. If it lies at the image border (x = 0 or x = width - 1 or y = 0 or y = height - 1), simply ignore it.
Create a bounding box around the contour. If the bounding box lies along the image border, you know the contour does, too.
Code for the second solution (CMake):
cmake_minimum_required(VERSION 2.8)
project(SolutionName)
find_package(OpenCV REQUIRED)
set(TARGETNAME "ProjectName")
add_executable(${TARGETNAME} ./src/main.cpp)
include_directories(${CMAKE_CURRENT_BINARY_DIR} ${OpenCV_INCLUDE_DIRS} ${OpenCV2_INCLUDE_DIR})
target_link_libraries(${TARGETNAME} ${OpenCV_LIBS})
Code for the second solution (C++):
bool contourTouchesImageBorder(const std::vector<cv::Point>& contour, const cv::Size& imageSize)
{
cv::Rect bb = cv::boundingRect(contour);
bool retval = false;
int xMin, xMax, yMin, yMax;
xMin = 0;
yMin = 0;
xMax = imageSize.width - 1;
yMax = imageSize.height - 1;
// Use less/greater comparisons to potentially support contours outside of
// image coordinates, possible future workarounds with cv::copyMakeBorder where
// contour coordinates may be shifted and just to be safe.
// However note that bounding boxes of size 1 will have their start point
// included (of course) but also their and with/height values set to 1
// but should not contain 2 pixels.
// Which is why we have to -1 the "search grid"
int bbxEnd = bb.x + bb.width - 1;
int bbyEnd = bb.y + bb.height - 1;
if (bb.x <= xMin ||
bb.y <= yMin ||
bbxEnd >= xMax ||
bbyEnd >= yMax)
{
retval = true;
}
return retval;
}
Call it via:
...
cv::Size imageSize = processedImage.size();
for (auto c: contours)
{
if(contourTouchesImageBorder(c, imageSize))
{
// Do your thing...
int asdf = 0;
}
}
...
Full C++ example:
void testContourBorderCheck()
{
std::vector<std::string> filenames =
{
"0_single_pixel_top_left.png",
"1_left_no_touch.png",
"1_left_touch.png",
"2_right_no_touch.png",
"2_right_touch.png",
"3_top_no_touch.png",
"3_top_touch.png",
"4_bot_no_touch.png",
"4_bot_touch.png"
};
// Load example image
//std::string path = "C:/Temp/!Testdata/ContourBorderDetection/test_1/";
std::string path = "../Testdata/ContourBorderDetection/test_1/";
for (int i = 0; i < filenames.size(); ++i)
{
//std::string filename = "circle3BorderDistance0.png";
std::string filename = filenames.at(i);
std::string fqn = path + filename;
cv::Mat img = cv::imread(fqn, cv::IMREAD_GRAYSCALE);
cv::Mat processedImage;
img.copyTo(processedImage);
// Create copy for contour extraction since cv::findContours alters the input image
cv::Mat workingCopyForContourExtraction;
processedImage.copyTo(workingCopyForContourExtraction);
std::vector<std::vector<cv::Point>> contours;
// Extract contours
cv::findContours(workingCopyForContourExtraction, contours, cv::RetrievalModes::RETR_EXTERNAL, cv::ContourApproximationModes::CHAIN_APPROX_SIMPLE);
// Prepare image for contour drawing
cv::Mat drawing;
processedImage.copyTo(drawing);
cv::cvtColor(drawing, drawing, cv::COLOR_GRAY2BGR);
// Draw contours
cv::drawContours(drawing, contours, -1, cv::Scalar(255, 255, 0), 1);
//cv::imwrite(path + "processedImage.png", processedImage);
//cv::imwrite(path + "workingCopyForContourExtraction.png", workingCopyForContourExtraction);
//cv::imwrite(path + "drawing.png", drawing);
const auto imageSize = img.size();
bool liesOnBorder = contourTouchesImageBorder(contours.at(0), imageSize);
// std::cout << "lies on border: " << std::to_string(liesOnBorder);
std::cout << filename << " lies on border: "
<< liesOnBorder;
std::cout << std::endl;
std::cout << std::endl;
cv::imshow("processedImage", processedImage);
cv::imshow("workingCopyForContourExtraction", workingCopyForContourExtraction);
cv::imshow("drawing", drawing);
cv::waitKey();
//cv::Size imageSize = workingCopyForContourExtraction.size();
for (auto c : contours)
{
if (contourTouchesImageBorder(c, imageSize))
{
// Do your thing...
int asdf = 0;
}
}
for (auto c : contours)
{
if (contourTouchesImageBorder(c, imageSize))
{
// Do your thing...
int asdf = 0;
}
}
}
}
int main(int argc, char** argv)
{
testContourBorderCheck();
return 0;
}
Problem with contour detection near image borders
OpenCV seems to have a problem with correctly finding contours near image borders.
For both objects, the detected contour is the same (see images). However, in image 2 the detected contour is not correct since a part of the object lies along x = 0, but the contour lies in x = 1.
This seem like a bug to me.
There is an open issue regarding this here: https://github.com/opencv/opencv/pull/7516
There also seems to be a workaround with cv::copyMakeBorder (https://github.com/opencv/opencv/issues/4374), however it seems a bit complicated.
If you can be a bit patient, I'd recommend waiting for the release of OpenCV 3.2 which should happen within the next 1-2 months.
New example images:
Single pixel top left, objects left, right, top, bottom, each touching and not touching (1px distance)
Example images
Object touching image border
Object not touching image border
Contour for object touching image border
Contour for object not touching image border
Although this question is in C++, the same issue affects openCV in Python. A solution to the openCV '0-pixel' border issue in Python (and which can likely be used in C++ as well) is to pad the image with 1 pixel on each border, then call openCV with the padded image, and then remove the border afterwards. Something like:
img2 = np.pad(img.copy(), ((1,1), (1,1), (0,0)), 'edge')
# call openCV with img2, it will set all the border pixels in our new pad with 0
# now get rid of our border
img = img2[1:-1,1:-1,:]
# img is now set to the original dimensions, and the contours can be at the edge of the image
If anyone needs this in MATLAB, here is the function.
function [touch] = componentTouchesImageBorder(C,im_row_max,im_col_max)
%C is a bwconncomp instance
touch=0;
S = regionprops(C,'PixelList');
c_row_max = max(S.PixelList(:,1));
c_row_min = min(S.PixelList(:,1));
c_col_max = max(S.PixelList(:,2));
c_col_min = min(S.PixelList(:,2));
if (c_row_max==im_row_max || c_row_min == 1 || c_col_max == im_col_max || c_col_min == 1)
touch = 1;
end
end
I'm using opencv 2.4.13
I'm trying to find the perimeter of a connected component, I was thinking of using ConnectedComponentWithStats but it doesn't return the perimeter, only the area, width, etc...
There is a method to find the area with the contour but not the opposite (with one component i mean, not the entire image).
The method arcLength doesn't work as well beause i have all the points of the component, not only the contour.
I know there is a BF way to find it by iterating through each pixel of the component and see if he has neighbors which aren't in the same component. But I'd like a function which costs less.
Otherwise, if you know a way to link a component with the contours found by the method findContours, it suits me as well.
Thanks
Adding to #Miki's answer, This is a faster way to find the perimeter of the connected component
//getting the connected components with statistics
cv::Mat1i labels, stats;
cv::Mat centroids;
int lab = connectedComponentsWithStats(img, labels, stats, centroids);
for (int i = 1; i < lab; ++i)
{
//Rectangle around the connected component
cv::Rect rect(stats(i, 0), stats(i, 1), stats(i, 2), stats(i, 3));
// Get the mask for the i-th contour
Mat1b mask_i = labels(rect) == i;
// Compute the contour
vector<vector<Point>> contours;
findContours(mask_i, contours, RETR_EXTERNAL, CHAIN_APPROX_NONE);
if(contours.size() <= 0)
continue;
//Finding the perimeter
double perimeter = contours[0].size();
//you can use this as well for measuring perimeter
//double perimeter = arcLength(contours[0], true);
}
The easiest thing is probably to use findContours.
You can compute the contour on the i-th component computed by connectedComponents(WithStats) , so they are aligned with your labels. Using CHAIN_APPROX_NONE you'll get all the points in the contour, so the size() of the vector is already a measure of the perimeter. You can eventually use arcLength(...) to get a more accurate result:
Mat1i labels;
int n_labels = connectedComponents(img, labels);
for (int i = 1; i < n_labels; ++i)
{
// Get the mask for the i-th contour
Mat1b mask_i = labels == i;
// Compute the contour
vector<vector<Point>> contours;
findContours(mask_i.clone(), contours, RETR_EXTERNAL, CHAIN_APPROX_NONE);
if (!contours.empty())
{
// The first contour (and probably the only one)
// is the one you're looking for
// Compute the perimeter
double perimeter_i = contours[0].size();
}
}
What I'm trying to do is measure the thickness of the eyeglasses frames. I had the idea to measure the thickness of the frame's contours (may be a better way?). I have so far outlined the frame of the glasses, but there are gaps where the lines don't meet. I thought about using HoughLinesP, but I'm not sure if this is what I need.
So far I have conducted the following steps:
Convert image to grayscale
Create ROI around the eye/glasses area
Blur the image
Dilate the image (have done this to remove any thin framed glasses)
Conduct Canny edge detection
Found contours
These are the results:
This is my code so far:
//convert to grayscale
cv::Mat grayscaleImg;
cv::cvtColor( img, grayscaleImg, CV_BGR2GRAY );
//create ROI
cv::Mat eyeAreaROI(grayscaleImg, centreEyesRect);
cv::imshow("roi", eyeAreaROI);
//blur
cv::Mat blurredROI;
cv::blur(eyeAreaROI, blurredROI, Size(3,3));
cv::imshow("blurred", blurredROI);
//dilate thin lines
cv::Mat dilated_dst;
int dilate_elem = 0;
int dilate_size = 1;
int dilate_type = MORPH_RECT;
cv::Mat element = getStructuringElement(dilate_type,
cv::Size(2*dilate_size + 1, 2*dilate_size+1),
cv::Point(dilate_size, dilate_size));
cv::dilate(blurredROI, dilated_dst, element);
cv::imshow("dilate", dilated_dst);
//edge detection
int lowThreshold = 100;
int ratio = 3;
int kernel_size = 3;
cv::Canny(dilated_dst, dilated_dst, lowThreshold, lowThreshold*ratio, kernel_size);
//create matrix of the same type and size as ROI
Mat dst;
dst.create(eyeAreaROI.size(), dilated_dst.type());
dst = Scalar::all(0);
dilated_dst.copyTo(dst, dilated_dst);
cv::imshow("edges", dst);
//join the lines and fill in
vector<Vec4i> hierarchy;
vector<vector<Point>> contours;
cv::findContours(dilated_dst, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE);
cv::imshow("contours", dilated_dst);
I'm not entirely sure what the next steps would be, or as I said above, if I should use HoughLinesP and how to implement it. Any help is very much appreciated!
I think there are 2 main problems.
segment the glasses frame
find the thickness of the segmented frame
I'll now post a way to segment the glasses of your sample image. Maybe this method will work for different images too, but you'll probably have to adjust parameters, or you might be able to use the main ideas.
Main idea is:
First, find the biggest contour in the image, which should be the glasses. Second, find the two biggest contours within the previous found biggest contour, which should be the glasses within the frame!
I use this image as input (which should be your blurred but not dilated image):
// this functions finds the biggest X contours. Probably there are faster ways, but it should work...
std::vector<std::vector<cv::Point>> findBiggestContours(std::vector<std::vector<cv::Point>> contours, int amount)
{
std::vector<std::vector<cv::Point>> sortedContours;
if(amount <= 0) amount = contours.size();
if(amount > contours.size()) amount = contours.size();
for(int chosen = 0; chosen < amount; )
{
double biggestContourArea = 0;
int biggestContourID = -1;
for(unsigned int i=0; i<contours.size() && contours.size(); ++i)
{
double tmpArea = cv::contourArea(contours[i]);
if(tmpArea > biggestContourArea)
{
biggestContourArea = tmpArea;
biggestContourID = i;
}
}
if(biggestContourID >= 0)
{
//std::cout << "found area: " << biggestContourArea << std::endl;
// found biggest contour
// add contour to sorted contours vector:
sortedContours.push_back(contours[biggestContourID]);
chosen++;
// remove biggest contour from original vector:
contours[biggestContourID] = contours.back();
contours.pop_back();
}
else
{
// should never happen except for broken contours with size 0?!?
return sortedContours;
}
}
return sortedContours;
}
int main()
{
cv::Mat input = cv::imread("../Data/glass2.png", CV_LOAD_IMAGE_GRAYSCALE);
cv::Mat inputColors = cv::imread("../Data/glass2.png"); // used for displaying later
cv::imshow("input", input);
//edge detection
int lowThreshold = 100;
int ratio = 3;
int kernel_size = 3;
cv::Mat canny;
cv::Canny(input, canny, lowThreshold, lowThreshold*ratio, kernel_size);
cv::imshow("canny", canny);
// close gaps with "close operator"
cv::Mat mask = canny.clone();
cv::dilate(mask,mask,cv::Mat());
cv::dilate(mask,mask,cv::Mat());
cv::dilate(mask,mask,cv::Mat());
cv::erode(mask,mask,cv::Mat());
cv::erode(mask,mask,cv::Mat());
cv::erode(mask,mask,cv::Mat());
cv::imshow("closed mask",mask);
// extract outermost contour
std::vector<cv::Vec4i> hierarchy;
std::vector<std::vector<cv::Point>> contours;
//cv::findContours(mask, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE);
cv::findContours(mask, contours, hierarchy, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);
// find biggest contour which should be the outer contour of the frame
std::vector<std::vector<cv::Point>> biggestContour;
biggestContour = findBiggestContours(contours,1); // find the one biggest contour
if(biggestContour.size() < 1)
{
std::cout << "Error: no outer frame of glasses found" << std::endl;
return 1;
}
// draw contour on an empty image
cv::Mat outerFrame = cv::Mat::zeros(mask.rows, mask.cols, CV_8UC1);
cv::drawContours(outerFrame,biggestContour,0,cv::Scalar(255),-1);
cv::imshow("outer frame border", outerFrame);
// now find the glasses which should be the outer contours within the frame. therefore erode the outer border ;)
cv::Mat glassesMask = outerFrame.clone();
cv::erode(glassesMask,glassesMask, cv::Mat());
cv::imshow("eroded outer",glassesMask);
// after erosion if we dilate, it's an Open-Operator which can be used to clean the image.
cv::Mat cleanedOuter;
cv::dilate(glassesMask,cleanedOuter, cv::Mat());
cv::imshow("cleaned outer",cleanedOuter);
// use the outer frame mask as a mask for copying canny edges. The result should be the inner edges inside the frame only
cv::Mat glassesInner;
canny.copyTo(glassesInner, glassesMask);
// there is small gap in the contour which unfortunately cant be closed with a closing operator...
cv::dilate(glassesInner, glassesInner, cv::Mat());
//cv::erode(glassesInner, glassesInner, cv::Mat());
// this part was cheated... in fact we would like to erode directly after dilation to not modify the thickness but just close small gaps.
cv::imshow("innerCanny", glassesInner);
// extract contours from within the frame
std::vector<cv::Vec4i> hierarchyInner;
std::vector<std::vector<cv::Point>> contoursInner;
//cv::findContours(glassesInner, contoursInner, hierarchyInner, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE);
cv::findContours(glassesInner, contoursInner, hierarchyInner, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);
// find the two biggest contours which should be the glasses within the frame
std::vector<std::vector<cv::Point>> biggestInnerContours;
biggestInnerContours = findBiggestContours(contoursInner,2); // find the one biggest contour
if(biggestInnerContours.size() < 1)
{
std::cout << "Error: no inner frames of glasses found" << std::endl;
return 1;
}
// draw the 2 biggest contours which should be the inner glasses
cv::Mat innerGlasses = cv::Mat::zeros(mask.rows, mask.cols, CV_8UC1);
for(unsigned int i=0; i<biggestInnerContours.size(); ++i)
cv::drawContours(innerGlasses,biggestInnerContours,i,cv::Scalar(255),-1);
cv::imshow("inner frame border", innerGlasses);
// since we dilated earlier and didnt erode quite afterwards, we have to erode here... this is a bit of cheating :-(
cv::erode(innerGlasses,innerGlasses,cv::Mat() );
// remove the inner glasses from the frame mask
cv::Mat fullGlassesMask = cleanedOuter - innerGlasses;
cv::imshow("complete glasses mask", fullGlassesMask);
// color code the result to get an impression of segmentation quality
cv::Mat outputColors1 = inputColors.clone();
cv::Mat outputColors2 = inputColors.clone();
for(int y=0; y<fullGlassesMask.rows; ++y)
for(int x=0; x<fullGlassesMask.cols; ++x)
{
if(!fullGlassesMask.at<unsigned char>(y,x))
outputColors1.at<cv::Vec3b>(y,x)[1] = 255;
else
outputColors2.at<cv::Vec3b>(y,x)[1] = 255;
}
cv::imshow("output", outputColors1);
/*
cv::imwrite("../Data/Output/face_colored.png", outputColors1);
cv::imwrite("../Data/Output/glasses_colored.png", outputColors2);
cv::imwrite("../Data/Output/glasses_fullMask.png", fullGlassesMask);
*/
cv::waitKey(-1);
return 0;
}
I get this result for segmentation:
the overlay in original image will give you an impression of quality:
and inverse:
There are some tricky parts in the code and it's not tidied up yet. I hope it's understandable.
The next step would be to compute the thickness of the the segmented frame. My suggestion is to compute the distance transform of the inversed mask. From this you will want to compute a ridge detection or skeletonize the mask to find the ridge. After that use the median value of ridge distances.
Anyways I hope this posting can help you a little, although it's not a solution yet.
Depending on lighting, frame color etc this may or may not work but how about simple color detection to separate the frame ? Frame color will usually be a lot darker than human skin. You'll end up with a binary image (just black and white) and by calculating the number (area) of black pixels you get the area of the frame.
Another possible way is to get better edge detection, by adjusting/dilating/eroding/both until you get better contours. You will also need to differentiate the contour from the lenses and then apply cvContourArea.