opencv how to visualize a non-rectangular region (roi) in a performant way

Question

I have an image (cv::Mat) and a ROI that can be seen as a mask. I want to show the original image with the ROI blended over it.

My mask is smaller than my origiginal image: each element represents a block in the image. Suppose my mask is this (note that my mask is NOT a rectangle)

0  0  1
1  1  1
0  0  0

then I would like to have the parts where (mask == 1) untouched and the rest blended with a color. This is the code I have

cv::Mat blocks = image.clone;
uint npixcol = 32;
uint npixrow = 32;
for (uint ri = 0; ri < 480; ++ri)
    for (uint ci = 0; ci < 640; ++ci)
        {
        if (mask[ri * 640 + ci])
            cv::rectangle(blocks, cv::Rect(ci * npixcol, ri * npixrow, npixcol, npixrow), cv::Scalar(0, 0, 0), CV_FILLED, 8, 0);
        }
cv::addWeighted(image, 0.5, blocks, 0.5, 0, image, -1);

How can I do this without the extra "clone" command since that is not very performant...

to make it more clear; this is an example of what I want (the color doesn't really matter)!

Answer 1

Is your mask of constant colour? Assuming mask is the same dimension as the image(you can easily scale it) :-

//Manually instead of addWeighted()
for (uint ri = 0; ri < 480; ++ri)
    for (uint ci = 0; ci < 640; ++ci)
        {
        if (mask[ri * 640 + ci])
        {
            image.at<uchar>(ri,ci) [0] = image.at<uchar>(ri,ci) [0] * weight_blue;
            image.at<uchar>(ri,ci) [1] = image.at<uchar>(ri,ci) [1] * weight_green;
            image.at<uchar>(ri,ci) [2] = image.at<uchar>(ri,ci) [2] * weight_red;
        }

        }    

Answer 2

Based on your comment, if you can make a mask with the same dimensions as the original image, you could directly modify original image pixel values using iterators. Here is a standalone example:

#include <cstdlib>
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>


int
main(int argc, char *argv[])
{
  cv::Mat image = cv::imread(argv[1]);
  cv::Mat mask = cv::Mat::zeros(image.size(), CV_8U);
  // let's put some 1 in my test mask.
  cv::Mat roi = mask(cv::Rect(0,0,mask.cols/2, mask.rows/2));
  roi = 1;

  cv::Vec3b blue(255,0,0); // (B,G,R)
  float alpha = 0.5;
  // Let's have fun with iterators
  cv::MatConstIterator_<unsigned char> maskIter = mask.begin<unsigned char>();
  const cv::MatConstIterator_<unsigned char> maskIterEnd = mask.end<unsigned char>();
  cv::MatIterator_<cv::Vec3b> imageIter = image.begin<cv::Vec3b>();
  for (; maskIter != maskIterEnd; ++maskIter, ++imageIter) {
    if (*maskIter) {// mask == 1
      *imageIter = (1-alpha)*(*imageIter) + alpha*blue; // same as addWeighted 
    }
  }

  cv::namedWindow("image", 0);
  cv::imshow("image", image);
  cv::waitKey(0);
  return EXIT_SUCCESS;
}

Answer 3

Basically you want to have a check if your are inside the roi. Then it should return a pointer to your original image. If your are not inside you want to have some kind of colour.

Your could do that with your own wrapper for Mat.

MyMat::at(int x, int y){
    if(inRoi(x,y)){
        return original.at(x,y);
    else
        return color(0,0,0);
}

I don't think you can point a subImage of an image onto another image. (That means that i don't think you can redirect the pixel in your blue image onto your original image)

Mat blueImage;
Rect roi;
Mat roiInImage = blueImage(roi);
roiInImage.redirect = originalImage(roi); //don't think something like this is possible