How to make PerspectiveTransform work?

Question

I just want to reproduce the result as posted here.

I rewrite the source to EmguCV form.

    Image<Bgr, byte> image = new Image<Bgr, byte>(@"B:\perspective.png");

    CvInvoke.cvShowImage("Hello World!", image);

    float[,] scrp = { { 43, 18 }, { 280,40}, {19,223 }, { 304,200} };
    float[,] dstp = { { 0,0}, { 320,0}, { 0,240 }, { 320,240 } };
    float[,] homog = new float[3, 3];


    Matrix<float> c1 = new Matrix<float>(scrp);
    Matrix<float> c2 = new Matrix<float>(dstp);
    Matrix<float> homogm = new Matrix<float>(homog);


    CvInvoke.cvFindHomography(c1.Ptr, c2.Ptr, homogm.Ptr, Emgu.CV.CvEnum.HOMOGRAPHY_METHOD.DEFAULT, 0, IntPtr.Zero);
    CvInvoke.cvGetPerspectiveTransform(c1, c2, homogm);

    Image<Bgr, byte> newImage = image.WarpPerspective(homogm, Emgu.CV.CvEnum.INTER.CV_INTER_CUBIC, Emgu.CV.CvEnum.WARP.CV_WARP_DEFAULT, new Bgr(0, 0, 0));

    CvInvoke.cvShowImage("newImage", newImage);

This is the testing image.

The newImage is always a blank image.

Can anyone help me make my source code work???

Answer 1

Hopefully, I found the answer by myself.

I rewrote the source to the wrong form. I should use Point[] and there is CameraCalibration.GetPerspectiveTransform function to use.

    PointF[] srcs = new PointF[4];
    srcs[0] = new PointF(253, 211);
    srcs[1] = new PointF(563, 211);
    srcs[2] = new PointF(563, 519);
    srcs[3] = new PointF(253, 519);


    PointF[] dsts = new PointF[4];
    dsts[0] = new PointF(234, 197);
    dsts[1] = new PointF(520, 169);
    dsts[2] = new PointF(715, 483);
    dsts[3] = new PointF(81, 472);

    HomographyMatrix mywarpmat = CameraCalibration.GetPerspectiveTransform(srcs, dsts);
    Image<Bgr, byte> newImage = image.WarpPerspective(mywarpmat, Emgu.CV.CvEnum.INTER.CV_INTER_NN, Emgu.CV.CvEnum.WARP.CV_WARP_FILL_OUTLIERS, new Bgr(0, 0, 0));

Answer 2

Here is an extension method for EmguCV version 3 (which have many functions deprecated):

public static Image<TColor, TDepth> GetAxisAlignedImagePart<TColor, TDepth>(
    this Image<TColor, TDepth> input,
    Quadrilateral rectSrc,
    Quadrilateral rectDst,
    Size targetImageSize)
    where TColor : struct, IColor
    where TDepth : new()
{
    var src = new[] { rectSrc.P0, rectSrc.P1, rectSrc.P2, rectSrc.P3 };
    var dst = new[] { rectDst.P0, rectDst.P1, rectDst.P2, rectDst.P3 };

    using (var matrix = CvInvoke.GetPerspectiveTransform(src, dst))
    {
        using (var cutImagePortion = new Mat())
        {
            CvInvoke.WarpPerspective(input, cutImagePortion, matrix, targetImageSize, Inter.Cubic);
            return cutImagePortion.ToImage<TColor, TDepth>();
        }
    }
}