[Port to C++] Blob Detection

4_cv_basics/7_blob_detection/Makefile

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+blob_detection: main.cpp
+	@echo "Building.."
+	@g++ main.cpp -o blob -lopencv_core -lopencv_imgcodecs -lopencv_highgui -lopencv_imgproc -lopencv_videoio -lsqlite3 -lgdal
+
+
+clean:
+	rm blob_detection

4_cv_basics/7_blob_detection/main.cpp

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+#include <bits/stdc++.h>
+#include <opencv2/opencv.hpp>
+using namespace std;
+using namespace cv;
+
+double median(vector<double> vec)
+{
+    int size = vec.size();
+    if (size == 0)
+    {
+        return 0; // Handle empty vector case
+    }
+    sort(vec.begin(), vec.end());
+    if (size % 2 == 0)
+    {
+        // Even number of elements, take average of middle 2
+        return (vec[size / 2 - 1] + vec[size / 2]) / 2.0;
+    }
+    else
+    {
+        // Odd number of elements, take middle element
+        return vec[size / 2];
+    }
+}
+
+tuple<double, double, double> getMedianPixelValues(Mat img)
+{
+    // Initialize vectors to store the pixel values of each color channel
+    vector<double> blue_pixels;
+    vector<double> green_pixels;
+    vector<double> red_pixels;
+
+    // Iterate over the rows and columns of the image
+    for (int i = 0; i < img.rows; i++)
+    {
+        for (int j = 0; j < img.cols; j++)
+        {
+            // Access the pixel value at (i, j)
+            Vec3b pixel = img.at<Vec3b>(i, j);
+
+            // Add the pixel values to the corresponding vectors
+            blue_pixels.push_back(pixel[0]);
+            green_pixels.push_back(pixel[1]);
+            red_pixels.push_back(pixel[2]);
+        }
+    }
+
+    // Calculate the median of each color channel
+    double blue_median = median(blue_pixels);
+    double green_median = median(green_pixels);
+    double red_median = median(red_pixels);
+
+    // Return the median values as a tuple
+    return make_tuple(blue_median, green_median, red_median);
+}
+
+int main()
+{
+    VideoCapture video(0);
+    int h = 0, s = 0, v = 0;
+    while (video.isOpened())
+    {
+        Mat frame;
+        video.read(frame);
+        imshow("Image", frame);
+
+        if (waitKey(10) == 'q')
+        {
+            Rect bbox = selectROI("Image", frame, false, false);
+            Mat hsv;
+            cvtColor(frame, hsv, COLOR_BGR2HSV);
+            Mat obj_img = hsv(Rect(bbox.x, bbox.y, bbox.width, bbox.height));
+            tuple<double, double, double> medians = getMedianPixelValues(obj_img);
+            h = get<0>(medians);
+            s = get<1>(medians);
+            v = get<2>(medians);
+            break;
+        }
+    }
+
+    while (video.isOpened())
+    {
+        Mat frame;
+        video.read(frame);
+
+        // Converting to hsv
+        Mat hsv;
+        cvtColor(frame, hsv, COLOR_BGR2HSV);
+
+
+        // Getting lower and upper bounds for h,s,v values
+        Scalar lower(h - 5, max(0, s - 50), max(0, v - 50));
+        Scalar upper(h + 5, min(s + 50, 255), min(v + 50, 255));
+
+        // Masking
+        Mat masked;
+        inRange(hsv, lower, upper, masked);
+
+        // Median Blur
+        Mat blur;
+        medianBlur(masked, blur, 5);
+
+        // Performing bitwise_and using the 'blur' mask
+        Mat blob_mask;
+        bitwise_and(frame, frame, blob_mask, blur);
+
+        imshow("blob_mask", blob_mask);
+
+        // Find contours
+        vector<vector<Point>> contours;
+        vector<Vec4i> hierarchy;
+        findContours(blur, contours, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE);
+
+
+        // Find largest contour
+        int idx = 0;
+        double current_max = 0;
+        int counter = 0;
+
+        for (const auto &n : contours)
+        {
+            double area = contourArea(n);
+            if (area > current_max)
+            {
+                current_max = area;
+                idx = counter;
+            }
+            counter++;
+        }
+
+        // Draw the largest contour detected
+        drawContours(frame, contours, idx, Scalar(0, 255, 255), 2);
+        imshow("Output", frame);
+
+        if (waitKey(10) == 'x')
+        {
+            destroyAllWindows();
+            video.release();
+            break;
+        }
+    }
+
+    return 0;
+}

4_cv_basics/7_blob_detection/readme.md

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+# Blob Detection
+
+BLOB stands for Binary Large Object
+
+Informally a blob is a region of an image in which some properties like intensity or color are approximately constant.
+
+## Understanding the code
+
+---
+
+* The code below captures a video using the webcam and stores the video by the name 'frame'
+
+```cpp
+int main()
+{
+    VideoCapture video(0);
+    int h = 0, s = 0, v = 0;
+    while (video.isOpened())
+    {
+        Mat frame;
+        video.read(frame);
+        imshow("Image", frame);
+
+```
+
+---
+
+* Pressing 'q' would select the frame at that instance.
+* `selectROI()` allows you to select that part of the captured frame in which the blob exists (and has to be detected later).
+* Next, convert the frame to hsv.
+* obj_img is hsv image of the size of the selected ROI.
+<br>Why converting to HSV?<br>
+   Since we are using the web cam the intensity and illumination of consecutive frame does not remain same.
+   Hence to find the color in range instead of particular color. HSV format is useful as H value denotes specific color and S, V can be used for illumination and intensity.
+
+* Next, we want the median values of each of hue, saturation and value, of the <b>selected ROI</b>.
+
+```cpp
+ while (1)
+    {
+        if (waitKey(10) == 'q')
+        {
+            Rect bbox = selectROI("Image", frame, false, false);
+            Mat hsv;
+            cvtColor(frame, hsv, COLOR_BGR2HSV);
+            Mat obj_img = hsv(Rect(bbox.x, bbox.y, bbox.width, bbox.height));
+            tuple<double, double, double> medians = getMedianPixelValues(obj_img);
+            h = get<0>(medians);
+            s = get<1>(medians);
+            v = get<2>(medians);
+            break;
+        }
+    }
+```
+
+---
+
+* We keep capturing the video via webcam until the program is running.
+* The captured video is coverted to hsv
+* Using the median values of h,s,v obtained in the previous section, we calculate the upper and lower bounds of each (for masking done later).
+
+```cpp
+    while (video.isOpened())
+    {
+        Mat frame;
+        video.read(frame);
+
+        // Converting to hsv
+        Mat hsv;
+        cvtColor(frame, hsv, COLOR_BGR2HSV);
+
+        // Getting lower and upper bounds for h,s,v values
+        Scalar lower(h - 5, max(0, s - 50), max(0, v - 50));
+        Scalar upper(h + 5, min(s + 50, 255), min(v + 50, 255));
+
+```
+* We make a mask using `inRange()` function with the lower and upper scalars obatined.
+<br>
+    What is a mask?<br>
+    A mask is a binary image consisting of zero and non-zero values. If a mask is applied to another image of the same size, all pixels which are zero in the mask are set to zero in the output image. All others remain unchanged.
+* We blur the mask to remove noises. 
+* Placing blurred mask over frame to find `bitwise_and()`.
+
+```cpp
+        // Masking
+        Mat masked;
+        inRange(hsv, lower, upper, masked);
+
+        // Median Blur
+        Mat blur;
+        medianBlur(masked, blur, 5);
+
+        // Performing bitwise_and using the 'blur' mask
+        Mat blob_mask;
+        bitwise_and(frame, frame, blob_mask, blur);
+
+        imshow("blob_mask", blob_mask);
+```
+
+---
+
+## Detecting the blob
+* Find the contour from the generated mask using `findContours()`
+
+   - What is a contour?<br>
+     Contours can be explained simply as a curve joining all the continuous points (along the boundary), having same color or intensity. The contours are a useful tool for shape analysis and object detection and recognition
+
+* Find the contour having the maximum area using `contourArea()`
+* Draw the largest contour on the frame using `drawContours()`
+
+```cpp
+        // Find contours
+        vector<vector<Point>> contours;
+        vector<Vec4i> hierarchy;
+        findContours(blur, contours, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE);
+
+        // Find largest contour
+        int idx = 0;
+        double current_max = 0;
+        int counter = 0;
+
+        for (const auto &n : contours)
+        {
+            double area = contourArea(n);
+            if (area > current_max)
+            {
+                current_max = area;
+                idx = counter;
+            }
+            counter++;
+        }
+
+        // Draw the largest contour detected
+        drawContours(frame, contours, idx, Scalar(0, 255, 255), 2);
+        imshow("Output", frame);
+
+        if (waitKey(10) == 'x')
+        {
+            destroyAllWindows();
+            video.release();
+            break;
+        }
+    }
+
+    return 0;
+}
+```