OpenCV Image Manipulation - c++

Question

I am trying to find out the difference in 2 images.
Scenario: Suppose that i have 2 images, one of a background and the other of a person in front of the background, I want to subtract the two images in such a way that I get the position of the person, that is the program can detect where the person was standing and give the subtracted image as the output.
The code that I have managed to come up with is taking two images from the camera and re-sizing them and is converting both the images to gray scale. I wanted to know what to do after this. I checked the subtract function provided by OpenCV but it takes arrays as inputs so I don't know how to progress.
The code that I have written is:
cap>>frame; //gets the first image
cv::cvtColor(frame,frame,CV_RGB2GRAY); //converts it to gray scale
cv::resize(frame,frame,Size(30,30)); //re-sizes it
cap>>frame2;//gets the second image
cv::cvtColor(frame2,frame2,CV_RGB2GRAY); //converts it to gray scale
cv::resize(frame2,frame2,Size(30,30)); //re-sizes it
Now do I simply use the subtract function like:
cv::subtract(frame_gray,frame,frame);
or do I apply some filters first and then use the subtract function?

Answer 1

As others have noticed, it's a tricky problem: easy to come up with a hack that will work sometimes, hard to come up with a solution that will work most of the time with minimal human intervention. Also, much easier to do if you can control tightly the material and illumination of the background. The professional applications are variously known as "chromakeying" (esp. in the TV industry), "bluescreening", "matting" or "traveling matte" (in cinematography), "background removal" in computer vision.
The groundbreaking work for matting quasi-uniform backdrops was done by Petro Vlahos many years ago. The patents on its basic algorithms have already expired, so you can go to town with them (and find open source implementations of various quality). Needless to say, IANAL, so do your homework on the patent subject.
Matting out more complex backgrounds is still an active research area, especially for the case when no 3D information is available. You may want to look into a few research papers that have come out of MS Research in the semi-recent past (A. Criminisi did some work in that area).

Answer 2

Using the subtract would not be appropriate because, it might result in some values becoming negative and will work only if you are trying to see if there is a difference or not( a boolean true/false).
If you need to get the pixels where it is differing, you should do a pixel by pixel comparison - something like:
int rows = frame.rows;
int cols = frame.cols;
cv::Mat diffImage = cv::Mat::zeros(rows, cols, CV_8UC1);
for(int i = 0; i < rows; ++i)
{
for(int j = 0; j < cols; ++j)
{
if(frame.at<uchar>(i,j) != frame2.at<uchar>(i,j))
diffImage.at<uchar>(i, j) = 255;
}
}
now, you can either show or save diffImage. All pixels that differ will be white while the similar ones will be in black