Inverse fill image in OpenCV - c

Question

I'm new to OpenCv and have been using it for a small project.
I intend to fill a single channel image all over, except a rectangle region within the image.
I have two problems.
1) Filling a single channel image with black. (cvSet wont work on single channel)
2) Carrying out the fill all over the image except a rectangle region within the image.
Any solutions?

Answer 1

Here's a program that shows how to fill a single channel with black and also how to set the image to black with a mask.
#include <iostream>
#include "opencv2/core/core.hpp"
#include "opencv2/highgui/highgui.hpp"
int main(int argc, const char * argv[]) {
cv::Mat image;
image = cv::imread("../../lena.jpg", CV_LOAD_IMAGE_GRAYSCALE);
if (!image.data) {
std::cout << "Image file not found\n";
return 1;
}
cv::namedWindow("original");
cv::imshow("original", image);
//Define the ROI rectangle
cv::Rect ROIrect(100, 100, 200, 200);
//Create a deep copy of the image
cv::Mat fill(image.clone());
//Specify the ROI
cv::Mat fillROI = fill(ROIrect);
//Fill the ROI with black
fillROI = cv::Scalar(0);
cv::namedWindow("fill");
cv::imshow("fill", fill);
cvMoveWindow("fill", 500, 40);
//create a deep copy of the image
cv::Mat inverseFill(image.clone());
//create a single-channel mask the same size as the image filled with 1
cv::Mat inverseMask(inverseFill.size(), CV_8UC1, cv::Scalar(1));
//Specify the ROI in the mask
cv::Mat inverseMaskROI = inverseMask(ROIrect);
//Fill the mask's ROI with 0
inverseMaskROI = cv::Scalar(0);
//Set the image to 0 in places where the mask is 1
inverseFill.setTo(cv::Scalar(0), inverseMask);
cv::namedWindow("inverseFill");
cv::imshow("inverseFill", inverseFill);
cvMoveWindow("inverseFill", 1000, 40);
// wait for key
cv::waitKey(0);
return 0;
}

Answer 2

Nested for loops would indeed be the quickest way.
Otherwise, consider making a buffer of identical size that's cleared using cvZero (all black). Then, setROI to the region that you care about, and cvCopy into the temporary buffer.
A bit mask with cvAnd is also a nice and clean solution.