how to subtract one frame from another using opencv in android

Question

i am working on frames of a video and i want to subtract one frame from other to find out the difference but i dont know how to proceed. i tried converting my bitmap frames into mat and then subtracting them but its not working. i am using opencv 2.4.3 for mat function. can anybody tell me how to do that. if possible explain with code snippets.

i tried something like this

     Bitmap myBitmap1 = BitmapFactory.decodeFile("/mnt/sdcard/Frames/mpgFrames/image001.jpg");  
    Bitmap myBitmap2 = BitmapFactory.decodeFile("/mnt/sdcard/Frames/mpgFrames/image002.jpg");  

    int width = myBitmap1.getWidth();
    int height = myBitmap1.getHeight();

    Mat  imgToProcess1 = new Mat(height, width, CvType.CV_8UC4);
    Mat  imgToProcess2 = new Mat(height, width, CvType.CV_8UC4);
    Mat  imgToProcess = new Mat(height, width, CvType.CV_8UC4);

    Utils.bitmapToMat(myBitmap1, imgToProcess1); 
    Utils.bitmapToMat(myBitmap2, imgToProcess1);        
    imgToProcess = imgToProcess1-imgToProcess2;

Answer 1

If you just subtract one frame from the other, you are going to end up with an image that only contains the areas where the second frame has higher values than the first frame.

To get differences, you need to use absdiff:

absdiff(imgToProcess1, imgToProcess2, imgToProcess);

This will give you a matrix of the actual differences, but if you want a mask of the areas of difference, you can apply a threshold to the result:

Mat mask(imgToProcess.rows, imgToProcess.cols, CV_8U);
cvtColor(imgToProcess, mask, CV_RGB2GRAY, 1); //your conversion specifier may vary
threshold(mask, mask, 0, 255, CV_THRESH_BINARY);

The minimum threshold value (0 above) can be adjusted. If you are using JPEG images, then you may need to increase it a bit to account for encoding noise.