Algorithm

1. Save the reference frame(in gray scale and saturation scale) at the beginning of the video
2. Convert other coming frames to gray scale and also hsv channels
3. Compare each new frame with the reference frame in saturation and gray scale
4. If absolute value differences bigger than 25 than mark the pixel and save in a new Mat
5. Then erode + dilate to this frame (2 times)
6. Check non-zero pixels in gray channel domain and mark them for result image
7. Remained zero pixels in Step-6 check in saturation domain and mark them in result image
8. If still remained zero pixels which means they belong to background, you can make replace them with the desired background

Requirements:

1. At the beginning camera should take a reference frame so you shouldnt be in front of camera
2. Camera should be constant