Convert OpenCV Mat object to BufferedImage

ImageIO.read(...) (and the javax.imageio package in general) is for reading/writing images from/to file formats. What you have is an array containing "raw" pixels. It's impossible for ImageIO to determine file format from this byte array. Because of this, it will return null.

Instead, you should create a BufferedImage from the bytes directly. I don't know OpenCV that well, but I'm assuming that the result of Imgproc.cvtColor(rgba, rgba, Imgproc.COLOR_RGB2GRAY, 0) will be an image in grayscale (8 bits/sample, 1 sample/pixel). This is the same format as BufferedImage.TYPE_BYTE_GRAY. If this assumption is correct, you should be able to do:

// Read image to Mat as before
Mat rgba = ...;
Imgproc.cvtColor(rgba, rgba, Imgproc.COLOR_RGB2GRAY, 0);

// Create an empty image in matching format
BufferedImage gray = new BufferedImage(rgba.width(), rgba.height(), BufferedImage.TYPE_BYTE_GRAY);

// Get the BufferedImage's backing array and copy the pixels directly into it
byte[] data = ((DataBufferByte) gray.getRaster().getDataBuffer()).getData();
rgba.get(0, 0, data);

Doing it this way, saves you one large byte array allocation and one byte array copy as a bonus. :-)

I used this kind of code to convert Mat object to Buffered Image.

static BufferedImage Mat2BufferedImage(Mat matrix)throws Exception {        
    MatOfByte mob=new MatOfByte();
    Imgcodecs.imencode(".jpg", matrix, mob);
    byte ba[]=mob.toArray();

    BufferedImage bi=ImageIO.read(new ByteArrayInputStream(ba));
    return bi;
}