frame subtraction in video - opencv

Question

I am trying to do Frame subtraction in a video.Steps I am following
Get a image , convert it into grayscale.
Subtract it from previous frame grayscale.
All I see in diff2(and diff also) a complete black image.One observation I made is that pixel value of gray1 and gray2 become equal.
My code
#include "opencv2/core/core.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/objdetect/objdetect.hpp"
#include <opencv2/video/background_segm.hpp>
#include <iostream>
using namespace cv;
using namespace std;
RNG rng(12345);
int main( int argc, const char** argv )
{
VideoCapture cap(0);
if ( !cap.isOpened() )
{
cout << "Cannot open the web cam" << endl;
return -1;
}
Mat img1,img2,diff,gray1,gray2,diff2;
bool bSuccess = cap.read(img1); // read a new frame from video
if (!bSuccess) //if not success, break loop
{
cout << "Cannot read a frame from video stream" << endl;
return -1;
}
cvtColor( img1,gray1, CV_BGR2GRAY );
while (true)
{
bSuccess = cap.read(img2); // read a new frame from video
if (!bSuccess) //if not success, break loop
{
cout << "Cannot read a frame from video stream" << endl;
break;
}
cvtColor( img2,gray2, CV_BGR2GRAY );
absdiff(gray2,gray1,diff);
threshold(diff, diff2, 150, 255, CV_THRESH_BINARY);
cout<<gray2.at<uchar>(100,200) <<endl;
cout<<gray1.at<uchar>(100,200) <<endl;
gray1=gray2;
imshow("1",gray1);
imshow("2",diff2);
if (waitKey(1000) == 27) //wait for 'esc' key press for 30ms. If 'esc' key is pressed, break loop
{
cout << "esc key is pressed by user" << endl;
break;
}
}
return -1;
}

Answer 1

please try this code. It looks like you're overwriting gray1 so that gray1 and gray2 use the very same data memory positions.
You could either use gray1=gray2.clone(); instead or use some real "swapping" of the buffers instead of overwriting. My code should perform a simple buffer swapping and has some comments about the problem.
#include "opencv2/core/core.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/objdetect/objdetect.hpp"
#include <opencv2/video/background_segm.hpp>
#include <iostream>
using namespace cv;
using namespace std;
RNG rng(12345);
int main( int argc, const char** argv )
{
VideoCapture cap(0);
if ( !cap.isOpened() )
{
cout << "Cannot open the web cam" << endl;
return -1;
}
Mat img1,img2,diff,gray1,gray2,diff2;
Mat tmp; // used to swap the buffers
bool bSuccess = cap.read(img1); // read a new frame from video
if (!bSuccess) //if not success, break loop
{
cout << "Cannot read a frame from video stream" << endl;
return -1;
}
// this will allocate memory of gray1 if not allocated yet
cvtColor( img1,gray1, CV_BGR2GRAY );
while (true)
{
bSuccess = cap.read(img2); // read a new frame from video
if (!bSuccess) //if not success, break loop
{
cout << "Cannot read a frame from video stream" << endl;
break;
}
// memory for gray2 won't be allocated if it is present already => if gray2 and gray1 use the same data memory, you'll overwrite gray1's pixels here and obviously gray1 and gray2 will have the same pixel values then
cvtColor( img2,gray2, CV_BGR2GRAY );
absdiff(gray2,gray1,diff);
threshold(diff, diff2, 150, 255, CV_THRESH_BINARY);
cout<<gray2.at<uchar>(100,200) <<endl;
cout<<gray1.at<uchar>(100,200) <<endl;
// don't lose the memory of gray1
tmp = gray1;
// this means gray1 and gray2 will use the same data memory location
gray1=gray2;
// give gray2 a new data memory location. Since previous gray1 memory is still present but wont be used anymore, use it here.
gray2=tmp;
imshow("1",gray1);
imshow("2",diff2);
if (waitKey(1000) == 27) //wait for 'esc' key press for 30ms. If 'esc' key is pressed, break loop
{
cout << "esc key is pressed by user" << endl;
break;
}
}
return -1;
}
in addition, a thres difference threshold of 150 might be very high for common tasks, but it might be ok for your special task. Typical difference values/thresholds in background subtraction for foreground extraction are around 20 to 30 from my experience, but at the end it depends on your task/problem/domain.