How to make white part of the image transparent by using Android opencv

I can't link url more than 2 so I post my pictures to this blog. please see my problems here. http://blog.naver.com/mail1001/220650041897

I want to know how to make white part of the image, which is white paper with writing on it, transparent by using Android opencv.

I have studied through url(I wrote it on blog) which makes the black background transparent, and I think "Alpha Channel" has something to do with it.

I think it will work as I make Alpha Channel by making the part I want to make transparent black and the other part white, and merge this Alpha Channel to the original RGB Channel.

So I made two experiments.

1) I made the papaer part black and the writing part white to make the Alpha Channel. And merged it to RGB Channel.

(Please see blog. Experiment 1's alpha channel picture)

I thought the writing should be the same and the background should be transparent, but the background only turned to white and little transparent.

(Please see blog. Experiment 1's result picture)

2) This time, the paper part is white and the writing part is black. But this time only writing turned to transparent.

(Please see blog. Experiment 2's alpha channel picture and result picture)

In the second experiment, what I meant to make transparent turned transparent but it did not work the same in the first experiment.

Which part am I doing wrong? Is there any concept I understand wrongly?

This is source what I tested.

Bitmap test(Bitmap image) {
// convert image to matrix
Mat src = new Mat(image.getHeight(), image.getWidth(), CvType.CV_8UC4);
Utils.bitmapToMat(image, src);

// init new matrices
Mat dst = new Mat(image.getHeight(), image.getWidth(), CvType.CV_8UC4);
Mat tmp = new Mat(image.getHeight(), image.getWidth(), CvType.CV_8UC4);
Mat alpha = new Mat(image.getHeight(), image.getWidth(), CvType.CV_8UC4);

// convert image to grayscale
Imgproc.cvtColor(src, tmp, Imgproc.COLOR_BGR2GRAY);

// threshold the image to create alpha channel with complete transparency in black background region and zero transparency in foreground object region.
Imgproc.threshold(tmp, alpha, 100, 255, Imgproc.THRESH_BINARY_INV);
//Imgproc.threshold(tmp, alpha, 100, 255, Imgproc.THRESH_BINARY);

// split the original image into three single channel.
List<Mat> rgb = new ArrayList<Mat>(3);
Core.split(src, rgb);

// Create the final result by merging three single channel and alpha(BGRA order)
List<Mat> rgba = new ArrayList<Mat>(4);
rgba.add(rgb.get(0));
rgba.add(rgb.get(1));
rgba.add(rgb.get(2));
rgba.add(alpha);
Core.merge(rgba, dst);

// convert matrix to output bitmap
Bitmap output = Bitmap.createBitmap(image.getWidth(), image.getHeight(), Bitmap.Config.ARGB_8888);
Utils.matToBitmap(dst, output);
//Utils.matToBitmap(alpha, output);
return output;
}

Thank you for your kindness answer.

I tried it but it was same Experiment 1's result picture. T_T

Edit code

Bitmap makeBackgroundWhite(Bitmap image) {
    // convert image to matrix
    Mat src = new Mat(image.getHeight(), image.getWidth(), CvType.CV_8UC4);
    Utils.bitmapToMat(image, src);

    // init new matrices
    Mat dst = new Mat(image.getHeight(), image.getWidth(), CvType.CV_8UC4);
    Mat tmp = new Mat(image.getHeight(), image.getWidth(), CvType.CV_8U);
    Mat alpha = new Mat(image.getHeight(), image.getWidth(), CvType.CV_8U);

    // convert image to grayscale
    Imgproc.cvtColor(src, tmp, Imgproc.COLOR_BGR2GRAY);

    // threshold the image to create alpha channel with complete transparency in black background region and zero transparency in foreground object region.
    Imgproc.threshold(tmp, alpha, 100, 255, Imgproc.THRESH_BINARY_INV);

    // split the original image into three single channel.
    List<Mat> bgra = new ArrayList<Mat>(4);
    Core.split(src, bgra);

    // Create the final result by merging three single channel and alpha(BGRA order)
    bgra.remove(3);
    bgra.add(alpha);
    Core.merge(bgra, dst);

    // convert matrix to output bitmap
    Bitmap output = Bitmap.createBitmap(image.getWidth(), image.getHeight(), Bitmap.Config.ARGB_8888);
    Utils.matToBitmap(dst, output);

    return output;
}