Crop and rotate picture OpenCV

Question

I am new in OpenCV so please to be lenient. I am doing an Android application to recognize the squares/rectangles and crop them. Function which looks for the squares/rectangles puts the found objects to vector> squares. I just wonder how to crop the picture according to the data in points stored in vector> squares and how to compute an angle on which the picture should be rotated. Thank you for any help

Solution 1

This post is citing from OpenCV QA: Extract a RotatedRect area.

There's a great article by Felix Abecassis on rotating and deskewing images. This also shows you how to extract the data in the RotatedRect:

• http://felix.abecassis.me/2011/10/opencv-rotation-deskewing/

You basically only need cv::getRotationMatrix2D to get the rotation matrix for the affine transformation with cv::warpAffine and cv::getRectSubPix to crop the rotated image. The relevant lines in my application are:

// This is the RotatedRect, I got it from a contour for example...
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;
// thanks to http://felix.abecassis.me/2011/10/opencv-rotation-deskewing/
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 on your image in src,
// the result is the rotated image in rotated. I am doing
// cubic interpolation here
warpAffine(src, rotated, M, src.size(), INTER_CUBIC);
// crop the resulting image, which is then given in cropped
getRectSubPix(rotated, rect_size, rect.center, cropped);

Solution 2

There are lots of useful posts around, I'm sure you can do a better search.

Crop:

• cropping IplImage most effectively

Rotate:

• OpenCV: how to rotate IplImage?
• Rotating or Resizing an Image in OpenCV

Compute angle:

• OpenCV - Bounding Box & Skew Angle