Draw rectangles like voting on a heatmap - c++

Question

I want to create mat files in opencv and initialize them to zero(all the pixels to be black). Thus I use
for initialization purpose:
Mat img = Mat::zeros(image.rows, image.cols, CV_8UC1);
After that I have got some rectangles with locations inside that image and I want to draw the correspondent regions of rectangle white. How is it possible to draw a region in mat file?
I have the following function to draw rects. However I want to draw all the rectangle not just the boundaries.
static Mat image_draw(Mat image, vector<Rect> rect, CvScalar color){
for(int i = 0; i < faces.size(); i++)
{
Point pt1(rect[i].x + rect[i].width, rect[i].y + rect[i].height);
Point pt2(rect[i].x, rect[i].y);
rectangle(image, pt1, pt2, color, 5, 8, 0);
}
return image;
}
The exact thing I want to do is to create a heat map for my rectangles so the overlapped bounding boxes to have higher values(close to 255) that the simple non-overlapped rectangles. I change thickness:
img = image_draw( img, rects, cvScalar(255, 102, 255, 0), -1);
Variable rects contains from 0 to 10 rectangle. I want somehow to aggregate the rectangles drawing. Not just redraw again the rectangles.
If I want to functionize it, is somwthing like that: EDIT final solution:
static Mat heatmap2(Mat image1, vector<Rect> faces, CvScalar color, int thickness) {
cv::Mat heatmap(image1.rows, image1.cols, CV_8U,cv::Scalar(0));
for(int i = 0; i < faces.size(); i++)
{
cv::Mat temp(image1.rows, image1.cols , CV_8U, cv::Scalar(0));
Point pt1(faces[i].x + faces[i].width, faces[i].y + faces[i].height);
Point pt2(faces[i].x, faces[i].y);
rectangle(temp, pt1, pt2, color, thickness, 8, 0);
heatmap+=temp;
}
return heatmap;
}

Answer 1

Try this:
cv::Mat heatmap(200,300,CV_8U,cv::Scalar(0));
{
cv::Mat temp(200,300,CV_8U,cv::Scalar(0));
cv::Rect r(10,20,30,30);
cv::rectangle(temp,r,cv::Scalar(100),-1);
heatmap+=temp;
}
{
cv::Mat temp(200,300,CV_8U,cv::Scalar(0));
cv::Rect r(20,25,30,30);
cv::rectangle(temp,r,cv::Scalar(100),-1);
heatmap+=temp;
}
cv::imshow("Heatmap",heatmap);
cv::waitKey();
Result:

Answer 2

From the official OpenCV Documentation (check here), "Thickness of lines that make up the rectangle. Negative values, like CV_FILLED , mean that the function has to draw a filled rectangle."
So give thickness a negative value like -
rectangle(image, pt1, pt2, color, -1, 8, 0);
UPDATE
Use these lines in your code,
for(int i=0; i < rect.size(); i++)
for( int y = rect[i].y; y < rect[i].y + rect[i].height; y++ )
for( int x = rect[i].x; x < rect[i].x + rect[i].width; x++ )
{
image.at<uchar>(y,x) =
saturate_cast<uchar>( image.at<uchar>(y,x) + 50 );
}
Here each Rect will increase the intensity by 50, and when it reaches 255, it will stay 255.
Input Image
Output Image
2 overlapping rect

Answer 3

Just a slight modification to your code should work:
static void draw_rectangles(Mat image, vector<Rect> faces) {
cv::Mat heatmap(image.rows, image.cols, CV_8U,cv::Scalar(0));
for(int i = 0; i < faces.size(); i++)
{
cv::Mat temp = heatmat(faces[i]); // gives you a submatrix of your heatmap pointing at the location of your rectangle
temp += 10; // add 10 grey levels to the existing values. This also modifies heatmap as side-effect
}
imshow("heatmap", heatmap);
waitKey(0);