OpenCV Mat to JavaCV Mat conversion

Question

I am in the process of writing an Android application that uses JavaCV for some facial recognition. I have come across a slight problem where I need to convert from an org.opencv.core.Mat that the onCameraFrame(CvCameraViewFrame inputFrame) function returns to a org.bytedeco.javacpp.opencv_core.Mat that the org.bytedeco.javacpp.opencv_contrib.FaceRecognizer requires.

I have found similar questions here and here but neither got a working solution.

Answer 1

There's an easier, more efficient way documented at https://github.com/bytedeco/javacpp/issues/38#issuecomment-140728812, in short:

To JavaCPP/JavaCV:

Mat mat2 = new Mat((Pointer)null) { { address = mat.getNativeObjAddr(); } };

To official Java API of OpenCV:

Mat mat = new Mat(mat2.address());

EDIT: OpenCVFrameConverter now provides an easier and safer way to do this, for example:

OpenCVFrameConverter.ToMat converter1 = new OpenCVFrameConverter.ToMat();
OpenCVFrameConverter.ToOrgOpenCvCoreMat converter2 = new OpenCVFrameConverter.ToOrgOpenCvCoreMat();
Mat mat = ...;
org.opencv.core.Mat cvmat = converter2.convert(converter1.convert(mat));
Mat mat2 = converter2.convert(converter1.convert(cvmat));

Answer 2

You can use java.awt.image.BufferedImage as interface.

Just convert your org.opencv.core.Mat object to java.awt.image.BufferedImage and then take the result object to convert it to org.bytedeco.javacpp.opencv_core.Mat.

Now, these are the functions that you will need:

1) Convert org.opencv.core.Mat to java.awt.image.BufferedImage:

public BufferedImage matToBufferedImage(Mat frame) {       
        int type = 0;
        if (frame.channels() == 1) {
            type = BufferedImage.TYPE_BYTE_GRAY;
        } else if (frame.channels() == 3) {
            type = BufferedImage.TYPE_3BYTE_BGR;
        }
        BufferedImage image = new BufferedImage(frame.width() ,frame.height(), type);
        WritableRaster raster = image.getRaster();
        DataBufferByte dataBuffer = (DataBufferByte) raster.getDataBuffer();
        byte[] data = dataBuffer.getData();
        frame.get(0, 0, data);
        return image;
    }

2) Convert java.awt.image.BufferedImage to org.bytedeco.javacpp.opencv_core.Mat:

public Mat bufferedImageToMat(BufferedImage bi) {
        OpenCVFrameConverter.ToMat cv = new OpenCVFrameConverter.ToMat();
        return cv.convertToMat(new Java2DFrameConverter().convert(bi)); 
    }

Make sure to have all the necessary jars and imports.

You could go deeper into JNI stuff, but for test use cases, this should be enough.