如何屏蔽cv :: Mat中的所有通道？

Question

I'm using a HOGDescriptor to detect pedestrians in an image. My intention is to mask all pixels outside of the person rectangles. I start by creating a mask image of all zeros and for each rectangle would like to overwrite all three channels at an ROI with 0xFF.

The result ends up being that only a single channel is set to the mask. What I want in effect is to have a black mask with white rectangles. I've inspected the matrix output as well and can see that only one component is 255 while the others are 0.

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0 255 0 0

void ImageUtil::detectPersonRectangles(cv::Mat image)
{
    cv::HOGDescriptor HoG;
    HoG.setSVMDetector(cv::HOGDescriptor::getDefaultPeopleDetector());

    cv::Mat mask = cv::Mat::zeros(image.size(), image.type());
    std::vector<cv::Rect> found;
    HoG.detectMultiScale(image, found, 0, cv::Size(8,8), cv::Size(32,32), 1.05, 2);
    for (int i = 0; i < found.size(); i++) {
        cv::Rect r = found[i];
        std::cout << " HoG detected rectangle " << i << " : " << r << "\n";
        mask(r) |= 0xFF;
    }

    static int index = 0;
    cv::Mat maskedImage = image & mask;
    std::stringstream name;
    name << "masked_" << index++;
    ImageUtil::dumpDebugImage(mask, name.str());
}

Answer 1

I'll answer my own question, because I didn't think it was directly obvious at first. We can write a single matrix to the ROI that will be used for each pixel in the ROI.

// Use 0xFF for all channels of the mask.
mask(r) |= cv::Mat(1, image.channels(), CV_8U, {0xFF});