Question

I am pushing Mat objects into a std::vector.

But, when I try to access the elements (Mat) after that, it did not give me the images pushed in earlier, depending on frameOrientation.

E.g., in the case below,i get “after_push_rotated2.jpg” same as “before_push_rotated3.jpg”, which should not be.

If I set frameOrientation to all “2s” or “4s”, “after_push_rotated1/2/3.jpg” all are identical and equals “before_push_rotated3.jpg”.

What is wrong here?

#include <cv.h>
#include <cxcore.h>
#include <highgui.h>
#include <iostream>
#include <vector>

using namespace cv;
using namespace std;

vector<Mat> checkOrientationAndRotate()
{
    Mat frame1 = imread("1.jpg" );
    Mat frame2 = imread("2.jpg" );
    Mat frame3 = imread("3.jpg" );



    vector<Mat> frameList;
    vector<Mat> frameList_rotated;

    frameList.push_back(frame1);
    frameList.push_back(frame2);
    frameList.push_back(frame3);

    Mat transposedFrame;
    Mat rotatedFrame;

    int i=0;
    for (vector<Mat>::iterator iter = frameList.begin(); iter != frameList.end(); iter++)
    {
        //Check for orientation of that frame
        int frameOrientation;
        if (i==0) frameOrientation = 1;
        if (i==1) frameOrientation = 2;
        if (i==2) frameOrientation = 4;

        switch (frameOrientation)
        {
        case 1: //1 - no rotate
            rotatedFrame = *iter;
            break;

        case 2: //2 - rotate it ACW 90 deg
            transpose(*iter, transposedFrame);
            flip(transposedFrame, rotatedFrame, 0);
            break;

        case 3: //3 - rotate it 180deg
            flip(*iter, rotatedFrame, -1);
            break;

        case 4: //4 - rotate it CW 90 deg
            transpose(*iter, transposedFrame);
            flip(transposedFrame, rotatedFrame, 1);
            break;
        default:
            break;
        }

        //Check frame before pushing into vector
        if (i==0) imwrite("before_push_rotated1.jpg",rotatedFrame);
        if (i==1) imwrite("before_push_rotated2.jpg",rotatedFrame);
        if (i==2) imwrite("before_push_rotated3.jpg",rotatedFrame);

        frameList_rotated.push_back(rotatedFrame);


        //Check frame after pushing into vector.
        //Depending on the frameorientation, the frames are not the frames pushed in earlier!
        int n=0;
        for (vector<Mat>::iterator iter = frameList_rotated.begin(); iter != frameList_rotated.end(); iter++)
        {
            Mat frame = *iter;
            if (n==0) imwrite("after_push_rotated1.jpg",frame);
            if (n==1) imwrite("after_push_rotated2.jpg",frame);
            if (n==2) imwrite("after_push_rotated3.jpg",frame);
            n++;
        }

        i++;
        } //for

        return frameList_rotated;

}

void main()
{
    vector<Mat> frameList_rotated = checkOrientationAndRotate();
}

Answer 1

[I didn’t use OpenCV anytime recently, so I could be wrong, please let me know if I am :)]

Mat represents (sort of) image header, not the image data. OpenCV maintains, internally, a reference counting mechanism to avoid large amounts of image-pixel-data being copied across different areas. Meaning to say that when you say:

Mat a = b; // Only headers are copied, both a and b refer to the same image.

In the above case, if you modify b, the change will be reflected in a as well. What you need to use in that case is:

Mat a = b.clone();

---

About your case, I really think the following line should fix the problem (and if not, there’s probably some other problem with the control flow, hehe):

Try replacing:

Mat transposedFrame;
Mat rotatedFrame;

with:

 //create a new image, not just the headers..
Mat transposedFrame(Size(image_width, image_height), image.type());
Mat rotatedFrame(Size(image_width, image_height), image.type());