Subtract blue background from image by OpenCV C++ - opencv

Question

I am a beginner in OpenCV and C++, but now I have to find a solution for this problem:
I have an image of a person with blue background, now I have to subtract background from image then replace it by another image.
Now I think there are 2 ways to resolve this problem, but I don't know which is better:
Solution 1:
Convert image to B&W
Use it as a mask to subtract background.
Solution 2:
Using coutour to find the background,
and then subtract it.
I have already implemented as solution 1, but the result is not as my expect.
Do you know there's another better solution or somebody already implement it as source code?
I will appreciate your help.
I update my source code here, please give me some comment
//Get the image with person
cv::Mat imgRBG = imread("test.jpg");
//Convert this image to grayscale
cv::Mat imgGray = imread("test.jpg",CV_LOAD_IMAGE_GRAYSCALE);
//Get the background from image
cv::Mat background = imread("paris.jpg");
cv::Mat imgB, imgW;
//Image with black background but inside have some area black
threshold(imgGray, imgB, 200, 255, CV_THRESH_BINARY_INV);
cv::Mat imgTemp;
cv::Mat maskB, maskW;
cv::Mat imgDisplayB, imgDisplayW;
cv::Mat imgDisplay1, imgDisplay2, imgResult;
//Copy image with black background, overide the original image
//Now imgTemp has black background wrap the human image, and inside the person, if there're some white area, they will be replace by black area
imgRBG.copyTo(imgTemp, imgB);
//Now replace the black background with white color
cv::floodFill(imgTemp, cv::Point(imgTemp.cols -10 ,10), cv::Scalar(255.0, 255.0, 255.0));
cv::floodFill(imgTemp, cv::Point(10,10), cv::Scalar(255.0, 255.0, 255.0));
cv::floodFill(imgTemp, cv::Point(10,imgTemp.rows -10), cv::Scalar(255.0, 255.0, 255.0));
cv::floodFill(imgTemp, cv::Point(imgTemp.cols -10,imgTemp.rows -10), cv::Scalar(255.0, 255.0, 255.0));
//Convert to grayscale
cvtColor(imgTemp,imgGray,CV_RGB2GRAY);
//Convert to B&W image, now background is black, other is white
threshold(imgGray, maskB, 200, 255, CV_THRESH_BINARY_INV);
//Convert to B&W image, now background is white, other is black
threshold(imgGray, maskW, 200, 255, CV_THRESH_BINARY);
//Replace background of image by the black mask
imgRBG.copyTo(imgDisplayB, maskB);
//Clone the background image
cv::Mat overlay = background.clone();
//Create ROI
cv::Mat overlayROI = overlay(cv::Rect(0,0,imgDisplayB.cols,imgDisplayB.rows));
//Replace the area which will be human image by white color
overlayROI.copyTo(imgResult, maskW);
//Add the person image
cv::addWeighted(imgResult,1,imgDisplayB,1,0.0,imgResult);
imshow("Image Result", imgResult);
waitKey();
return 0;

Answer 1

Check this project
https://sourceforge.net/projects/cvchromakey
void chromakey(const Mat under, const Mat over, Mat *dst, const Scalar& color) {
// Create the destination matrix
*dst = Mat(under.rows,under.cols,CV_8UC3);
for(int y=0; y<under.rows; y++) {
for(int x=0; x<under.cols; x++) {
if (over.at<Vec3b>(y,x)[0] >= red_l && over.at<Vec3b>(y,x)[0] <= red_h && over.at<Vec3b>(y,x)[1] >= green_l && over.at<Vec3b>(y,x)[1] <= green_h && over.at<Vec3b>(y,x)[2] >= blue_l && over.at<Vec3b>(y,x)[2] <= blue_h)
{
dst->at<Vec3b>(y,x)[0]= under.at<Vec3b>(y,x)[0];
dst->at<Vec3b>(y,x)[1]= under.at<Vec3b>(y,x)[1];
dst->at<Vec3b>(y,x)[2]= under.at<Vec3b>(y,x)[2];}
else{
dst->at<Vec3b>(y,x)[0]= over.at<Vec3b>(y,x)[0];
dst->at<Vec3b>(y,x)[1]= over.at<Vec3b>(y,x)[1];
dst->at<Vec3b>(y,x)[2]= over.at<Vec3b>(y,x)[2];}
}
}
}

Answer 2

If you know that the background is blue, you are losing valuable information by converting the image to B/W.
If the person is not wearing blue (at least not one that is very close to the background color), you don't have to use contours. just replace the blue pixels with the pixels from the other image. You can use cvScalar data type with, cvGet2D and cvSet2D functions to achieve this.
Edit:
Your code looks a lot more complicated than the original problem you stated. Having a blue background (also called "blue screen" and "chroma key") is a common method used by TV channels to change backgrounds of news readers. The reason for selecting blue was that the human skin has less dominance in the blue component.
Assuming that the person is not wearing blue, the following code should work. Let me know if you need something different.
//Read the image with person
IplImage* imgPerson = cvLoadImage("person.jpg");
//Read the image with background
IplImage* imgBackground = cvLoadImage("paris.jpg");
// assume that the blue background is quite even
// here is a possible range of pixel values
// note that I did not use all of them :-)
unsigned char backgroundRedMin = 0;
unsigned char backgroundRedMax = 10;
unsigned char backgroundGreenMin = 0;
unsigned char backgroundGreenMax = 10;
unsigned char backgroundBlueMin = 245;
unsigned char backgroundBlueMax = 255;
// for simplicity, I assume that both images are of the same resolution
// run a loop to replace pixels
for (int i=0; i<imgPerson->width; i++)
{
for (int j=0; j< imgPerson->height; j++)
{
CvScalar currentPixel = cvGet2D(imgPerson, j, i);
// compare the RGB values of the pixel, with the range
if (curEdgePixel.val[0] > backgroundBlueMin && curEdgePixel.val[1] <
backgroundGreenMax && curEdgePixel.val[2] < backgroundRedMax)
{
// copy the corresponding pixel from background
CvScalar currentBackgroundPixel = cvGet2D(imgBackground, j, i);
cvSet2D(imgPerson, j, i, currentBackgroundPixel);
}
}
}
imshow("Image Result", imgPerson);
waitKey();
return 0;