How to change black color to transparent from a bitmap Mat?

Question

Can the mat or umat have null values in the middle? I need it to behave like .png format. If not, how can I achieve this purpose? Mat and Umat act like a rectangle image to me.

I have tried rectangle copy method, grabcut method, but none of the method works right.

I started the code as below:

Bitmap bmpUImage = new Bitmap(plotBox.Image);
Image<Bgr, Byte> uimage = new Image<Bgr, byte>(bmpUImage);
UMat input = uimage.ToUMat();

A mat or umat act like a .png, having some cutout/null values in the middle part of the image.

So, you want transparency for certrain image parts!? All you need is to add an alpha channel to your image and modify the desired part accordingly. Please, have a look at [this SO question, and the answers](https://stackoverflow.com/questions/40527769). — HansHirse, Apr 03 '19 at 04:31
Thanks HansHirse to provide me an idea on the alpha channel! Since I'm using emgu, I found a better link for my propose here that might benefits others: https://stackoverflow.com/questions/45660427/emgu-c-sharp-opencv-make-color-black-transparent — Jemt tinhwa, Apr 03 '19 at 10:02

score 0 · Accepted Answer · answered Apr 03 '19 at 10:04

///////////////// add black as transparency /////////////////
        Bitmap bmpUImage = new Bitmap(plotBox_Method.Image);

        Mat output = new Mat();
        Mat Shading_Image = new Image<Bgr, byte>(bmpUImage).Mat;

        // puting a black rectangle for transparent area later
        Shading_Image.CopyTo(output);
        CvInvoke.Rectangle(output, mask_rect, new MCvScalar(0,0,0), -1);

        Mat imageMat = output;
        Mat finalMat = new Mat(imageMat.Rows, imageMat.Cols, Emgu.CV.CvEnum.DepthType.Cv8U, 4);
        Mat tmp = new Mat(imageMat.Rows, imageMat.Cols, Emgu.CV.CvEnum.DepthType.Cv8U, 1);
        Mat alpha = new Mat(imageMat.Rows, imageMat.Cols, Emgu.CV.CvEnum.DepthType.Cv8U, 1);

        CvInvoke.CvtColor(imageMat, tmp, Emgu.CV.CvEnum.ColorConversion.Bgr2Gray);
        CvInvoke.Threshold(tmp, alpha, 0, 255, Emgu.CV.CvEnum.ThresholdType.Binary);

        CvInvoke.Imshow("alpha", alpha);

        VectorOfMat rgb = new VectorOfMat(3);

        CvInvoke.Split(imageMat, rgb);

        Mat[] rgba = { rgb[0], rgb[1], rgb[2], alpha };

        VectorOfMat vector = new VectorOfMat(rgba);

        CvInvoke.Merge(vector, finalMat);

        imgPlot.Image = finalMat.ToImage<Bgra, Byte>().ToBitmap(); 
        ///////////////// end of add black as transparency /////////////////

How to change black color to transparent from a bitmap Mat?

1 Answers1