Perspective correction of UIImage from Points

Question

I\'m working on a app where I\'ll let the user take a picture e.g of a business card or photograph.

The user will then mark the four corners of the object (which the

Answer 1

From iOS8+ there is Filter for Core Image called CIPerspectiveCorrection. All you need to do is pass the image and four points.

Also there is one more filter supporting iOS6+ called CIPerspectiveTransform which can be used in similar way (skewing image).

Answer 2

If this image were loaded in as a texture, it'd be extremely simple to skew it using OpenGL. You'd literally just draw a full-screen quad and use the yellow correction points as the UV coordinate at each point.

Answer 3

I'm not sure if you've tried the Opencv library yet, but it has a very nice way to deskew an image. I've got here a small snippet that takes an array of corners, your four corners for example, and a final size to map it into.

You can read the man page for warpPerspective on the OpenCV site.

cv::Mat deskew(cv::Mat& capturedFrame, cv::Point2f source_points[], cv::Size finalSize)
{
    cv::Point2f dest_points[4];

    // Output of deskew operation has same color space as source frame, but
    // is proportional to the area the document occupied; this is to reduce
    // blur effects from a scaling component.
    cv::Mat deskewedMat = cv::Mat(finalSize, capturedFrame.type());

    cv::Size s = capturedFrame.size();

    // Deskew to full output image corners
    dest_points[0] = cv::Point2f(0,s.height); // lower left
    dest_points[1] = cv::Point2f(0,0);        // upper left
    dest_points[2] = cv::Point2f(s.width,0);  // upper right
    dest_points[3] = cv::Point2f(s.width,s.height);  // lower right

    // Build quandrangle "de-skew" transform matrix values
    cv::Mat transform = cv::getPerspectiveTransform( source_points, dest_points  );
    // Apply the deskew transform
    cv::warpPerspective( capturedFrame, deskewedMat, transform, s, cv::INTER_CUBIC );

    return deskewedMat;
}

Answer 4

I don't know exact solution of your case, but there is approach for trapezoid: http://www.comp.nus.edu.sg/~tants/tsm/TSM_recipe.html - the idea is to continuously build transformation matrix. Theoretically you can add transformation that converts your shape into trapecy.

And there are many questions like this: https://math.stackexchange.com/questions/13404/mapping-irregular-quadrilateral-to-a-rectangle , but I didn't check solutions.