/// <summary>
/// Extracts shape inside the given RotatedRect and un-warps it
/// </summary>
/// <param name="input">Input image</param>
/// <param name="points">Sorted corners of the shape</param>
/// <param name="corners">Downscaling option, biggest size with be re-scaled to this value. 0 means no downscaling (default value)</param>
/// <returns>New OpenCV Mat with un-warped object</returns>
public static Mat UnwrapShape(this Mat img, Point2f[] corners, int maxSize = 0)
{
if (corners.Length != 4)
{
throw new OpenCvSharpException("argument 'points' must be of length = 4");
}
// grab bounds corners, sort them in correct order and
// get width/height of the shape
float width = (float)Point2f.Distance(corners[0], corners[1]); // lt -> rt
float height = (float)Point2f.Distance(corners[0], corners[3]); // lt -> lb
// downscaling
if (maxSize > 0 && (width > maxSize || height > maxSize))
{
if (width > height)
{
var s = maxSize / width;
width = maxSize;
height = height * s;
}
else
{
var s = maxSize / height;
height = maxSize;
width = width * s;
}
}
// compute transform
Point2f[] destination = new Point2f[]
{
new Point2f(0, 0),
new Point2f(width, 0),
new Point2f(width, height),
new Point2f(0, height)
};
var transform = Cv2.GetPerspectiveTransform(corners, destination);
// un-warp
return(img.WarpPerspective(transform, new Size(width, height), InterpolationFlags.Cubic));
}
private static Mat FourPointTransform(Mat image, Mat pts)
{
//obtain a consistent order of the points and unpack them
//individually
Tuple <Point2f, Point2f, Point2f, Point2f> orderedPoints = OrderPoints(pts);
Point2f tl = orderedPoints.Item1, tr = orderedPoints.Item2, br = orderedPoints.Item3, bl = orderedPoints.Item4;
//compute the width of the new image, which will be the
//maximum distance between bottom-right and bottom-left
//x-coordiates or the top-right and top-left x-coordinates
double widthA = Point2f.Distance(bl, br);
double widthB = Point2f.Distance(tl, tr);
int maxWidth = Math.Max((int)widthA, (int)widthB);
//compute the height of the new image, which will be the
//maximum distance between the top-right and bottom-right
//y-coordinates or the top-left and bottom-left y-coordinates
double heightA = Point2f.Distance(tr, br);
double heightB = Point2f.Distance(tl, bl);
int maxHeight = Math.Max((int)heightA, (int)heightB);
//now that we have the dimensions of the new image, construct
//the set of destination points to obtain a "birds eye view",
//(i.e. top-down view) of the image, again specifying points
//in the top-left, top-right, bottom-right, and bottom-left
//order
var dst = new[]
{
new Point2f(0, 0),
new Point2f(maxWidth - 1, 0),
new Point2f(maxWidth - 1, maxHeight - 1),
new Point2f(0, maxHeight - 1),
};
//compute the perspective transform matrix and then apply it
using (Mat m = Cv2.GetPerspectiveTransform(new[] { tl, tr, br, bl }, dst))
{
Mat warped = image.WarpPerspective(m, new Size(maxWidth, maxHeight));
return(warped);
}
}