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How to set color effect on document(image) using OpenCV(java)

currently, I'm working on a document scanner project who captures and filter document with different effects and color but I'm a beginner to work with OpenCV.
but, now I understand how medianBlurFilter , gaussianBlurFilter , cannyFilter and bilateralFilter works.
just I am starting to do this type of filter with the use of OpenCV but I can't understand how to achieve this.
Input :
How to achieve this?:

//bitmap is a normal document image
Bitmap newB = Bitmap.createBitmap(bitmap.getWidth(), bitmap.getHeight(), Bitmap.Config.ARGB_8888);
Canvas canvas = new Canvas(newB);
canvas.drawColor(Color.argb(sp,255, 0, 0));
Mat src = new Mat();
Utils.bitmapToMat(bitmap, src);
Mat dst = new Mat();
Utils.bitmapToMat(newB, dst);
Core.addWeighted(src, 1f, dst, 0.5f, 0.5, dst);
//bitmapNew is a filtered document image
Bitmap bitmapNew = Bitmap.createBitmap(bitmap.getWidth(), bitmap.getHeight(), Bitmap.Config.ARGB_8888);
Utils.matToBitmap(dst, bitmapNew);

OpenCV: how to change color of all non transparent pixels to black?

I use OpenCV (Java) and I want to change color of all the non transparent pixels in my Mat to black (colors defined in ARGB_8888 but could be another format)
How can I do that ?
Mat mat = new Mat();
bmp32 = bitmap.copy(Bitmap.Config.ARGB_8888, true);
Utils.bitmapToMat(bmp32, mat);
// and then ????
Thanks !

// allocate space as mixChannels requires so
Mat bgr = new Mat(mat.size(), CvType.CV_8UC3);
Mat alpha = new Mat(mat.size(), CvType.CV_8U);
List<Mat> src = new ArrayList<Mat>();
src.add(mat);
List<Mat> dst = new ArrayList<Mat>();
dst.add(alpha);
dst.add(bgr);
// A->alpha, RGB->bgr in reverse order
MatOfInt fromTo = new MatOfInt(0,0, 1,3, 2,2, 3,1);
mixChannels(src, dst, fromTo);
// now, make non-transparent parts black
Mat mask;
compare​(alpha, Scalar(0), mask, opencv_core.CMP_EQ);
Mat res;
Core.bitwise_and(bgr, bgr, res, mask);

How to convert YUV to JPG in Java

There are arrays of bytes YUV NV21
How to do the conversion and save it to jpg file?
I use next code
byte[] mYUVData = YUV_420_888toNV21(planeY, planeU, planeV);
Mat mYuv = new Mat(1080 + 1080/2, 1040, CvType.CV_8UC1);
mYuv.put(0, 0, mYUVData);
Mat mRgba = new Mat();
Imgproc.cvtColor( mYuv, mRgba, Imgproc.COLOR_YUV420sp2RGBA );
// Read image as before
MatOfByte mob=new MatOfByte();
Imgcodecs.imencode(".jpg", mYuv, mob);
byte ba[]=mob.toArray();
BufferedImage bi= ImageIO.read(new ByteArrayInputStream(ba));
ImageIO.write(bi, "jpg", new File("C://Users/it/Downloads/image.jpg"));
But i have wrong jpg image

How to crop rectangles in opencv?

I manage to draw two rectangles from an image I captured. The question is, how to crop them out and save them individually so that I can perform my OCR in each of it? I have tried searching but I can't find what I wanted...
What I have done so far ..
Mat imageMat = new Mat();
Mat resizeImg = new Mat();
Mat hierarchy = new Mat();
//Size(width,height)
Size imageSize = new Size(1000,800);
Utils.bitmapToMat(image, imageMat);
//Resize Image
Imgproc.resize(imageMat,resizeImg,imageSize);
//convert the resized image to bitmap
image = Bitmap.createBitmap(resizeImg.cols(), resizeImg.rows(),
Bitmap.Config.ARGB_8888);
//convert back to Mat
Utils.bitmapToMat(image, imageMat);
//Create rectangles
Imgproc.rectangle(imageMat,new Point(0,150), new Point(500,250), new Scalar(255, 0, 255),1);
Imgproc.rectangle(imageMat,new Point(0,500), new Point(650,650), new Scalar(255, 0, 255),1);
//What I have tried doing
Rect rectCrop = new Rect(0,150,500,250);
Mat image_output= imageMat.submat(rectCrop);
Bitmap Cropimage = Bitmap.createBitmap(imageMat.cols(), imageMat.rows(), Bitmap.Config.ARGB_8888);
Utils.bitmapToMat(Cropimage, image_output);
//TODO: save only the content in the rectangles to MAT
//testing purpose
Utils.matToBitmap(imageMat,image);
imageView.setImageBitmap(image);

This is how to get croped image with rectangular shape.
Rect rect = new Rect(x1, y1, x2, y2);
Mat cropedMat = new Mat(mat, rect);
if you want you can convert croped mat to bitmap
Bitmap Cropedimage = Bitmap.createBitmap(cropedMat.cols(), cropedMat.rows(), Bitmap.Config.ARGB_8888);
Utils.matToBitmap(cropedMat, Cropedimage);

OpenCV on Android converting to grayscale not working

I am trying to convert some OpenCV Mat to grayscale for Contours detection algorithms. For some reason the image after convertion is all black. Mine code (b is Android Bitmap):
Mat tmp = new Mat (b.getWidth(), b.getHeight(), CvType.CV_8UC1);
Utils.bitmapToMat(b, tmp);
Imgproc.cvtColor(tmp, tmp, Imgproc.COLOR_BGR2GRAY);
//there could be some processing
Imgproc.cvtColor(tmp, tmp, Imgproc.COLOR_GRAY2BGRA, 4);
Utils.matToBitmap(tmp, b);
And now I am drawing this bitmap and it is all black. When I applied contour detection to this bitmap (in place of comment) there were no match, so I assume problem is with convertion. After removing convertion (simply call bitmapToMat and matToBitmap) then bitmap is not all black so convertion to Mat are also working. Bitmap is in ARGB_8888 and there are no errors, just the output bitmap is all black.
Edit: Just to be sure I tried to save a bitmap with ocv imwrite - it's still all black so the problem is for 100% at cvtColor...

If the bitmap b is from the android device, then try using COLOR_RGB2GRAY instead of COLOR_BGR2GRAY because BGR is the default pixel format for OpenCV images, not all images.
Try this code:
Mat tmp = new Mat (b.getWidth(), b.getHeight(), CvType.CV_8UC1);
Utils.bitmapToMat(b, tmp);
Imgproc.cvtColor(tmp, tmp, Imgproc.COLOR_RGB2GRAY);
//there could be some processing
Imgproc.cvtColor(tmp, tmp, Imgproc.COLOR_GRAY2RGB, 4);
Utils.matToBitmap(tmp, b);

I tried this method and it works perfectly, first you must convert it into grey scale, then to Canny, and finally to Bitmap.
this function returns a black and white image.
public static Bitmap edgesim(Mat img1) {
Bitmap image1;
//mat gray img1 holder
Mat imageGray1 = new Mat();
//mat canny image
Mat imageCny1 = new Mat();
//mat canny image
Mat imageCny2 = new Mat();
/////////////////////////////////////////////////////////////////
//Convert img1 into gray image
Imgproc.cvtColor(img1, imageGray1, Imgproc.COLOR_BGR2GRAY);
//Canny Edge Detection
Imgproc.Canny(imageGray1, imageCny1, 10, 100, 3, true);
///////////////////////////////////////////////////////////////////
//////////////////Transform Canny to Bitmap/////////////////////////////////////////
image1= Bitmap.createBitmap(imageCny1.cols(), imageCny1.rows(), Bitmap.Config.ARGB_8888);
Utils.matToBitmap(imageCny1, image1);
return image1;
}