Question

How do you implement a program using opencv to distinguish whether the object is triangle or square in the picture? I should use C++ in visual studio.

These are the two objects to be distinguished.

A

Answer 1

Here is the code that will detect the shapes and will also detect the squares and triangles differently

Make sure you install required packages like opencv etc

/**
 * Simple shape detector program.
 * It loads an image and tries to find simple shapes (rectangle, triangle, circle, squares etc) in it.

 */
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <cmath>
#include <iostream>
 
/**
 * Helper function to find a cosine of angle between vectors
 * from pt0->pt1 and pt0->pt2
 */
static 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);
}
 
/**
 * Helper function to display text in the center of a contour
 */
void setLabel(cv::Mat& im, const std::string label, std::vector<cv::Point>& contour)
{
        int fontface = cv::FONT_HERSHEY_SIMPLEX;
        double scale = 0.4;
        int thickness = 1;
        int baseline = 0;
 
        cv::Size text = cv::getTextSize(label, fontface, scale, thickness, &baseline);
        cv::Rect r = cv::boundingRect(contour);
 
        cv::Point pt(r.x + ((r.width - text.width) / 2), r.y + ((r.height + text.height) / 2));
        cv::rectangle(im, pt + cv::Point(0, baseline), pt + cv::Point(text.width, -text.height), CV_RGB(255,255,255), CV_FILLED);
        cv::putText(im, label, pt, fontface, scale, CV_RGB(0,0,0), thickness, 8);
}
 
int main()
{
        //cv::Mat src = cv::imread("polygon.png");
        cv::Mat src = cv::imread("assets/basic-shapes-2.png");
        if (src.empty())
                return -1;
 
        // Convert to grayscale
        cv::Mat gray;
        cv::cvtColor(src, gray, CV_BGR2GRAY);
 
        // Use Canny instead of threshold to catch squares with gradient shading
        cv::Mat bw;
        cv::Canny(gray, bw, 0, 50, 5);
 
        // Find contours
        std::vector<std::vector<cv::Point> > contours;
        cv::findContours(bw.clone(), contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);
 
        std::vector<cv::Point> approx;
        cv::Mat dst = src.clone();
 
        for (int i = 0; i < contours.size(); i++)
        {
                // Approximate contour with accuracy proportional
                // to the contour perimeter
                cv::approxPolyDP(cv::Mat(contours[i]), approx, cv::arcLength(cv::Mat(contours[i]), true)*0.02, true);
 
                // Skip small or non-convex objects 
                if (std::fabs(cv::contourArea(contours[i])) < 100 || !cv::isContourConvex(approx))
                        continue;
 
                if (approx.size() == 3)
                {
                        setLabel(dst, "TRI", contours[i]);    // Triangles
                }
                else if (approx.size() >= 4 && approx.size() <= 6)
                {
                        // Number of vertices of polygonal curve
                        int vtc = approx.size();
 
                        // Get the cosines of all corners
                        std::vector<double> cos;
                        for (int j = 2; j < vtc+1; j++)
                                cos.push_back(angle(approx[j%vtc], approx[j-2], approx[j-1]));
 
                        // Sort ascending the cosine values
                        std::sort(cos.begin(), cos.end());
 
                        // Get the lowest and the highest cosine
                        double mincos = cos.front();
                        double maxcos = cos.back();
 
                        // Use the degrees obtained above and the number of vertices
                        // to determine the shape of the contour
                        if (vtc == 4 && mincos >= -0.1 && maxcos <= 0.3)
                                setLabel(dst, "RECT", contours[i]);
                        else if (vtc == 5 && mincos >= -0.34 && maxcos <= -0.27)
                                setLabel(dst, "PENTA", contours[i]);
                        else if (vtc == 6 && mincos >= -0.55 && maxcos <= -0.45)
                                setLabel(dst, "HEXA", contours[i]);
                }
                else
                {
                        // Detect and label circles
                        double area = cv::contourArea(contours[i]);
                        cv::Rect r = cv::boundingRect(contours[i]);
                        int radius = r.width / 2;
 
                        if (std::abs(1 - ((double)r.width / r.height)) <= 0.2 &&
                            std::abs(1 - (area / (CV_PI * std::pow(radius, 2)))) <= 0.2)
                                setLabel(dst, "CIR", contours[i]);
                }
        }
 
        cv::imshow("src", src);
        cv::imshow("dst", dst);
        cv::waitKey(0);
        return 0;
}

Please comment if you have issues while running the code

Also if it shows the following error

fatal error: opencv2/highgui/highgui.hpp: No such file or directory

6 | #include <opencv2/highgui/highgui.hpp>

kindly do the following

if using linux install the package like this

~$ sudo apt-get install libopencv-highgui-dev

Answer 2

Here is the code that will detect the shapes and will also detect the squares and triangles differently

Make sure you install required packages like opencv etc

/**
 * Simple shape detector program.
 * It loads an image and tries to find simple shapes (rectangle, triangle, circle, squares etc) in it.

 */
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <cmath>
#include <iostream>
 
/**
 * Helper function to find a cosine of angle between vectors
 * from pt0->pt1 and pt0->pt2
 */
static 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);
}
 
/**
 * Helper function to display text in the center of a contour
 */
void setLabel(cv::Mat& im, const std::string label, std::vector<cv::Point>& contour)
{
        int fontface = cv::FONT_HERSHEY_SIMPLEX;
        double scale = 0.4;
        int thickness = 1;
        int baseline = 0;
 
        cv::Size text = cv::getTextSize(label, fontface, scale, thickness, &baseline);
        cv::Rect r = cv::boundingRect(contour);
 
        cv::Point pt(r.x + ((r.width - text.width) / 2), r.y + ((r.height + text.height) / 2));
        cv::rectangle(im, pt + cv::Point(0, baseline), pt + cv::Point(text.width, -text.height), CV_RGB(255,255,255), CV_FILLED);
        cv::putText(im, label, pt, fontface, scale, CV_RGB(0,0,0), thickness, 8);
}
 
int main()
{
        //cv::Mat src = cv::imread("polygon.png");
        cv::Mat src = cv::imread("assets/basic-shapes-2.png");
        if (src.empty())
                return -1;
 
        // Convert to grayscale
        cv::Mat gray;
        cv::cvtColor(src, gray, CV_BGR2GRAY);
 
        // Use Canny instead of threshold to catch squares with gradient shading
        cv::Mat bw;
        cv::Canny(gray, bw, 0, 50, 5);
 
        // Find contours
        std::vector<std::vector<cv::Point> > contours;
        cv::findContours(bw.clone(), contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);
 
        std::vector<cv::Point> approx;
        cv::Mat dst = src.clone();
 
        for (int i = 0; i < contours.size(); i++)
        {
                // Approximate contour with accuracy proportional
                // to the contour perimeter
                cv::approxPolyDP(cv::Mat(contours[i]), approx, cv::arcLength(cv::Mat(contours[i]), true)*0.02, true);
 
                // Skip small or non-convex objects 
                if (std::fabs(cv::contourArea(contours[i])) < 100 || !cv::isContourConvex(approx))
                        continue;
 
                if (approx.size() == 3)
                {
                        setLabel(dst, "TRI", contours[i]);    // Triangles
                }
                else if (approx.size() >= 4 && approx.size() <= 6)
                {
                        // Number of vertices of polygonal curve
                        int vtc = approx.size();
 
                        // Get the cosines of all corners
                        std::vector<double> cos;
                        for (int j = 2; j < vtc+1; j++)
                                cos.push_back(angle(approx[j%vtc], approx[j-2], approx[j-1]));
 
                        // Sort ascending the cosine values
                        std::sort(cos.begin(), cos.end());
 
                        // Get the lowest and the highest cosine
                        double mincos = cos.front();
                        double maxcos = cos.back();
 
                        // Use the degrees obtained above and the number of vertices
                        // to determine the shape of the contour
                        if (vtc == 4 && mincos >= -0.1 && maxcos <= 0.3)
                                setLabel(dst, "RECT", contours[i]);
                        else if (vtc == 5 && mincos >= -0.34 && maxcos <= -0.27)
                                setLabel(dst, "PENTA", contours[i]);
                        else if (vtc == 6 && mincos >= -0.55 && maxcos <= -0.45)
                                setLabel(dst, "HEXA", contours[i]);
                }
                else
                {
                        // Detect and label circles
                        double area = cv::contourArea(contours[i]);
                        cv::Rect r = cv::boundingRect(contours[i]);
                        int radius = r.width / 2;
 
                        if (std::abs(1 - ((double)r.width / r.height)) <= 0.2 &&
                            std::abs(1 - (area / (CV_PI * std::pow(radius, 2)))) <= 0.2)
                                setLabel(dst, "CIR", contours[i]);
                }
        }
 
        cv::imshow("src", src);
        cv::imshow("dst", dst);
        cv::waitKey(0);
        return 0;
}
 

Please comment if you have issues while running the code

Also if it shows the following error

fatal error: opencv2/highgui/highgui.hpp: No such file or directory

6 | #include <opencv2/highgui/highgui.hpp>

kindly do the following

if using linux install the package like this

~$ sudo apt-get install libopencv-highgui-dev