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apologies, forgot to add some karma for this ..

"shaped thing over white background," - since opencv wants white contours on dark bg, you probably have to use THRESH_INVERT. also visualizing the binary image using imshow() might be helpful.

~~apologies, forgot to add some karma for this ..~~

"shaped thing over white background," - since opencv wants white contours on dark bg, you probably have to use THRESH_INVERT. also visualizing the binary image using imshow() might be helpful.

then, to see the outcome of your contours finding / warping, you'd need to run it in a loop, not only once:

 thresh_callback( 0, 0 );
// rect is the RotatedRect (I got it from a contour...)
while(true) {
        RotatedRect rect;
        // matrices we'll use
        Mat M, rotated, cropped;
        // get angle and size from the bounding box
        float angle = rect.angle;
        Size rect_size = rect.size;
        if (rect.angle < -45.) {
            angle += 90.0;
            swap(rect_size.width, rect_size.height);
        }
        // get the rotation matrix
        M = getRotationMatrix2D(rect.center, angle, 1.0);
        // perform the affine transformation
        warpAffine(src, rotated, M, src.size(), INTER_CUBIC);
        // crop the resulting image
        getRectSubPix(rotated, rect_size, rect.center, cropped);

        if (waitKey(10) > 0 ) break;
}