object detection of various shapes in opencv - opencv

Question

I have an image and want to detect various objects at a time using opencv methods.
I have tried detecting one object using contouring and using the area to filter other counters. But I need to detect other objects too but they vary in area and length.
Can anyone help me to use any methods for detecting it.
This is the original image:
this is the code that I have tried for detection:
int main()
{
Mat msrc = imread("Task6_Resources/Scratch.jpg", 1);
Mat src = imread("Task6_Resources/Scratch.jpg", 0);
Mat imgblur;
GaussianBlur(src, imgblur, Size(13,13), 0);
cv::Ptr<cv::CLAHE> clahe = cv::createCLAHE();
clahe->setClipLimit(8);
cv::Mat gcimg;
clahe->apply(imgblur, gcimg);
Mat thresh;
threshold(gcimg, thresh, 55, 255, THRESH_BINARY_INV);
Mat th_mina = minareafilter(thresh, 195); //function used to filter small and large blobs
Mat th_maxa = maxareafilter(th_mina, 393);
Mat imdilate;
dilate(th_maxa, imdilate, getStructuringElement(MORPH_RECT, Size(3, 1)), Point(-1, -1), 7);
int largest_area = 0;
int largest_contour_index = 0;
Rect bounding_rect;
vector<vector<Point>> contours;
vector<Vec4i> hierarchy;
findContours(imdilate, contours, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE);
cout << "Number of contours" << contours.size() << endl;
//Largest contour according to area
for (int i = 0; i < contours.size(); i++){
double a = contourArea(contours[i], false);
if (a > largest_area) {
largest_area = a;
largest_contour_index = i;
bounding_rect = boundingRect(contours[i]);
}
}
for (int c = 0; c < contours.size(); c++){
printf(" * Contour[%d] Area OpenCV: %.2f - Length: %.2f \n",
c,contourArea(contours[c]), arcLength(contours[c], true));
}
rectangle(msrc, bounding_rect, Scalar(0, 255, 0), 2, 8, 0);
imshow("largest contour", msrc);
waitKey(0);
destroyAllWindows();
return 0;
}
This is the image on which I am applying contouring
After the code I am able to detect the green box using largest area in contouring, but I need to detect those red boxes too. (only the region of red boxes)
The problem is here I cannot apply again area parameter to filter the contours as some other contours have same area as the resultant contour.
The image result required: