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I had some extra time at work so I whipped up a quick and dirty implementation of my comment above:

Code: cv::Mat image = cv::imread("image.jpg", 0);

cv::Mat image_thresh;
cv::threshold(image, image_thresh, 125, 255, cv::THRESH_BINARY);

// Loop through the border pixels and if they're black, floodFill from there
cv::Mat mask;
image_thresh.copyTo(mask);
for (int i = 0; i < mask.cols; i++) {
    if (mask.at<char>(0, i) == 0) {
        cv::floodFill(mask, cv::Point(i, 0), 255, 0, 10, 10);
    }   
    if (mask.at<char>(mask.rows-1, i) == 0) {
        cv::floodFill(mask, cv::Point(i, mask.rows-1), 255, 0, 10, 10);
    }
}
for (int i = 0; i < mask.rows; i++) {
    if (mask.at<char>(i, 0) == 0) {
        cv::floodFill(mask, cv::Point(0, i), 255, 0, 10, 10);
    }
    if (mask.at<char>(i, mask.cols-1) == 0) {
        cv::floodFill(mask, cv::Point(mask.cols-1, i), 255, 0, 10, 10);
    }
}


// Compare mask with original.
cv::Mat newImage;
image.copyTo(newImage);
for (int row = 0; row < mask.rows; ++row) {
    for (int col = 0; col < mask.cols; ++col) {
        if (mask.at<char>(row, col) == 0) {
            newImage.at<char>(row, col) = 255;
        }           
    }
}
cv::imshow("filled image", mask);
cv::imshow("Final image", newImage);
cv::imwrite("final.jpg", newImage);
cv::waitKey(0);

return 0;

And the result:

I had some extra time at work so I whipped up a quick and dirty implementation of my comment above:

Code: Code:

cv::Mat image = cv::imread("image.jpg", 0);
 
0);

cv::Mat image_thresh;
cv::threshold(image, image_thresh, 125, 255, cv::THRESH_BINARY);

// Loop through the border pixels and if they're black, floodFill from there
cv::Mat mask;
image_thresh.copyTo(mask);
for (int i = 0; i < mask.cols; i++) {
    if (mask.at<char>(0, i) == 0) {
        cv::floodFill(mask, cv::Point(i, 0), 255, 0, 10, 10);
    }   
    if (mask.at<char>(mask.rows-1, i) == 0) {
        cv::floodFill(mask, cv::Point(i, mask.rows-1), 255, 0, 10, 10);
    }
}
for (int i = 0; i < mask.rows; i++) {
    if (mask.at<char>(i, 0) == 0) {
        cv::floodFill(mask, cv::Point(0, i), 255, 0, 10, 10);
    }
    if (mask.at<char>(i, mask.cols-1) == 0) {
        cv::floodFill(mask, cv::Point(mask.cols-1, i), 255, 0, 10, 10);
    }
}


// Compare mask with original.
cv::Mat newImage;
image.copyTo(newImage);
for (int row = 0; row < mask.rows; ++row) {
    for (int col = 0; col < mask.cols; ++col) {
        if (mask.at<char>(row, col) == 0) {
            newImage.at<char>(row, col) = 255;
        }           
    }
}
cv::imshow("filled image", mask);
cv::imshow("Final image", newImage);
cv::imwrite("final.jpg", newImage);
cv::waitKey(0);

return 0;
 
And the result: