Move every pixel to right (by 1px) in OpenCV without using remap?

Question

I want to move every pixel in an image to right by 1px, and below is the map I use to do the remap transformation.

This approach require much more time than it should to do such a simple transform. Is there a cv function I can use? Or do I just split the image into 2 images, one is src.cols-1 pixels wide, the other is 1 px wide, and then copy them to the new image?

void update_map()
{
    for( int j = 0; j < src.cols; j++ ){
       for( int i = 0; i < src.rows; i++ ){
         if (j == src.cols-1)
             mat_x_Rotate.at<float>(i,j) = 0;
         else
            mat_x_Rotate.at<float>(i,j) = j + 1;
            mat_y_Rotate.at<float>(i,j) = i;
         }
     }
}

Answer 1

Things you can do to improve your performance:

• remap is overkill for this purpose. It is more efficient to copy the pixels directly than to define an entire remap transformation and then use it.
• switch your loop order: iterate over rows, then columns. (OpenCV's Mat is stored in row-major order, so iterating over columns first is very cache-unfriendly)
• use Mat::ptr() to access pixels in the same row directly, as a C-style array. (this is a big performance win over using at<>(), which probably does stuff like check indices for each access)
• take your if statement out of the inner loop, and handle column 0 separately.

---

As an alternative: yes, splitting the image into parts and copying to the new image might be about as efficient as copying directly, as described above.

Answer 2

Mat Shift_Image_to_Right( Mat src_in, int num_pixels)
{
    Size sz_src_in = src_in.size();
    Mat img_out(sz_src_in.height, sz_src_in.width, CV_8UC3);

    Rect  roi;
    roi.x = 0;
    roi.y = 0;
    roi.width = sz_src_in.width-num_pixels;
    roi.height = sz_src_in.height;

    Mat crop;

    crop = src_in(roi);

    // Move the left boundary to the right
    img_out =  Scalar::all(0);
    img_out.adjustROI(0, 0, -num_pixels, 0);
    crop.copyTo(img_out);
    img_out.adjustROI(0, 0, num_pixels, 0);

    return img_out;
}