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. enter image description here

The newImage is always a blank image.

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

score 5 · Accepted Answer · answered Jan 16 '13 at 04:30

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));

cool,tnx! Do you have an idea how can I get the ratio of the edges if I have some image with rectangular object ? — Michael, May 06 '13 at 14:50
What do you mean by "the ratio of the edges"? If you mean the values such as 253, 211, you have to measure the images manually in Paint program or other similar programs. — Gqqnbig, May 07 '13 at 08:15

score 2 · Answer 2 · answered Apr 27 '16 at 09:32

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>();
        }
    }
}

How to make PerspectiveTransform work?

2 Answers2