Detect location(s) of objects in an image - opencv

Question

I have an input image that looks like this:
Notice that there are 6 boxes with black borders. I need to detect the location (upper-left hand corder) of each box. Normally I would use something like template matching but the contents (the colored area inside the black border) of each box is distinct.
Is there a version of template matching that can configured to ignore the inner area of each box? Is the an algorithm better suited to this situation?
Also note, that I have to deal with several different resolutions... thus the actual size of the boxes will be different from image to image. That said, the ratio (length to width) will always be the same.
Real-world example/input image per request:

Answer 1

You can do this finding the bounding box of connected components.
To find connected components you can convert to grayscale, and keep all pixels with value 0, i.e. the black border of the rectangles.
Then you can find the contours of each connected component, and compute its bounding box. Here the red bounding boxes found:
Code:
#include <opencv2/opencv.hpp>
#include <vector>
using namespace cv;
using namespace std;
int main()
{
// Load the image, as BGR
Mat3b img = imread("path_to_image");
// Convert to gray scale
Mat1b gray;
cvtColor(img, gray, COLOR_BGR2GRAY);
// Get binary mask
Mat1b binary = (gray == 0);
// Find contours of connected components
vector<vector<Point>> contours;
findContours(binary.clone(), contours, RETR_EXTERNAL, CHAIN_APPROX_SIMPLE);
// For each contour
for (int i = 0; i < contours.size(); ++i)
{
// Get the bounding box
Rect box = boundingRect(contours[i]);
// Draw the box on the original image in red
rectangle(img, box, Scalar(0, 0, 255), 5);
}
// Show result
imshow("Result", img);
waitKey();
return 0;
}
From the image posted in chat, this code produces:
In general, this code will correctly detect the cards, as well as noise. You just need to remove noise according to some criteria. Among others: size or aspect ratio of boxes, colors inside boxes, some texture information.