Best way to store a Mat object in Android

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Although I have not run timings, I suspect that going through formats such as XML or JSON would require more space and computing resources than using primitives.

Below is some code that can be used to save and retrieve MatOfKeyPoint to SQLite database in an Android environment. This is using OpenCV 2.4.11 so SIFT is available.

What you see when you run this application is your test image (which you need to supply and put in the drawable folder) with added keypoints.

The siftTest() method starts by computing keyPoints which is MatOfKeyPoint type. The code saves the underlying data of that object in the database, then reads that data out and creates a new object keyPointsFromDb, the contents of which are applied to the original image and the result is displayed.

public class MainActivity extends AppCompatActivity {
    static {
        System.loadLibrary("native-lib");
        System.loadLibrary("opencv_java");
        System.loadLibrary("nonfree");

    }
    private ImageView imageView;
    private Bitmap inputImage; // make bitmap from image resource
    private FeatureDetector detector = FeatureDetector.create(FeatureDetector.SIFT);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        inputImage = BitmapFactory.decodeResource(getResources(), R.drawable.test);
        imageView = (ImageView) this.findViewById(R.id.imageView);
        siftTest();
    }

    public void siftTest() {
        Mat rgba = new Mat();
        Utils.bitmapToMat(inputImage, rgba);
        MatOfKeyPoint keyPoints = new MatOfKeyPoint();
        Imgproc.cvtColor(rgba, rgba, Imgproc.COLOR_RGBA2GRAY);
        detector.detect(rgba, keyPoints);

        // Save to database
        MatchingDatabaseAdapter.addKeypoints(keyPoints, getApplicationContext());

        // Opens database cursor
        MatchingDatabaseAdapter.getAllRecordsCursor(getApplicationContext()); 

        // Gets the first item in the database (as an example... you could loop through many/all)
        MatOfKeyPoint keyPointsFromDb = MatchingDatabaseAdapter.getKeypointsFromNextCursorPosition();

        // Closes database
        MatchingDatabaseAdapter.closeDb(); 

        Features2d.drawKeypoints(rgba, keyPointsFromDb, rgba);
        Utils.matToBitmap(rgba, inputImage);
        imageView.setImageBitmap(inputImage);

    }
}

Here is the database code which contains some details associated with converting to the byte array required for the database. I didn't include everything associated with using a database, since that's really a different topic.

public class MatchingDatabaseAdapter {
    ...
    ...
    ...

    public static void addKeypoints(MatOfKeyPoint keyPoints, Context context) {
        float[] data = new float[(int)keyPoints.total() * keyPoints.channels()]; // make a spot to save the data
        keyPoints.get(0,0,data); // load the data;
        ByteBuffer buffer = ByteBuffer.allocate(data.length * 4);
        for (int i = 0; i < data.length; i++){
            buffer.putFloat(data[i]);
        }
        byte[] byteArray = buffer.array();
        addBlob(byteArray, keyPoints.rows(), keyPoints.cols(), keyPoints.type(), context);
    }

    public static void addBlob(byte[] blob, int rows, int columns, int mattype, Context context) throws SQLException {
        if (mDb == null) mDb = openDb(context);
        ContentValues contentValues = new ContentValues();
        contentValues.put(DatabaseHelper.BLOB_FIELD_NAME, blob);
        contentValues.put(DatabaseHelper.ROWS_FIELD_NAME, rows);
        contentValues.put(DatabaseHelper.COLUMNS_FIELD_NAME, columns);
        contentValues.put(DatabaseHelper.MATTYPE_FIELD_NAME, mattype);
        long x = mDb.insert(DatabaseHelper.TABLE_NAME, null, contentValues);
        Log.v(TAG, "insert said " + x + " and blob was " + blob.length + " long.");
        closeDb();
    }

    public static Cursor getAllRecordsCursor(Context context) {
        if (mDb == null || !mDb.isOpen()) mDb = openDb(context);
        mCursor = mDb.query(DatabaseHelper.TABLE_NAME, null, null, null, null,null, null);
        boolean hasRecords = mCursor.moveToFirst();
        Log.v(TAG, "MatchingDatabaseAdapter.getAllRecordsCursor() cursor created. " + mCursor + " and " + (hasRecords?"has records":"has NO RECORDS"));
        return mCursor;
    }

     public static MatOfKeyPoint getKeypointsFromNextCursorPosition() {
        MatOfKeyPoint keyPoints = null;
        if (mCursor != null) {
            Log.v(TAG, "mCursor has ...

I use my own Mat <-> xml read/write.

/*
 * Only implement XML <-> opencv Mat read and write.
 */

public class TaFileStorage {
    // static
    public static final int READ = 0;
    public static final int WRITE = 1;

    // varaible
    private File file;
    private boolean isWrite;
    private Document doc;
    private Element rootElement;

    public TaFileStorage() {
        file = null;
        isWrite = false;
        doc = null;
        rootElement = null;
    }


    // read or write
    public void open(String filePath, int flags ) {
        try {
            if( flags == READ ) {
                open(filePath);
            }
            else {
                create(filePath);
            }
        } catch(Exception e) {
            e.printStackTrace();
        }

    }


    // read only
    public void open(String filePath) {
        try {
            file = new File(filePath);
            if( file == null || file.isFile() == false ) {
                System.err.println("Can not open file: " + filePath );
            }
            else {
                isWrite = false;
                doc = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(file);
                doc.getDocumentElement().normalize();
            }
        } catch(Exception e) {
            e.printStackTrace();
        }

    }

    // write only
    public void create(String filePath) {
        try {
            file = new File(filePath);
            if( file == null ) {
                System.err.println("Can not wrtie file: " + filePath );
            }
            else {
                isWrite = true;
                doc = DocumentBuilderFactory.newInstance().newDocumentBuilder().newDocument();

                rootElement = doc.createElement("opencv_storage");
                doc.appendChild(rootElement);
            }
        } catch(Exception e) {
            e.printStackTrace();
        }
    }

    public Mat readMat(String tag) {
        if( isWrite ) {
            System.err.println("Try read from file with write flags");
            return null;
        }

        NodeList nodelist = doc.getElementsByTagName(tag);
        Mat readMat = null;

        for( int i = 0 ; i<nodelist.getLength() ; i++ ) {
            Node node = nodelist.item(i);

            if( node.getNodeType() == Node.ELEMENT_NODE ) {
                Element element = (Element)node;

                String type_id = element.getAttribute("type_id");
                if( "opencv-matrix".equals(type_id) == false) {
                    System.out.println("Fault type_id ");
                }

                String rowsStr = element.getElementsByTagName("rows").item(0).getTextContent();
                String colsStr = element.getElementsByTagName("cols").item(0).getTextContent();
                String dtStr = element.getElementsByTagName("dt").item(0).getTextContent();
                String dataStr = element.getElementsByTagName("data").item(0).getTextContent();

                int rows = Integer.parseInt(rowsStr);
                int cols = Integer.parseInt(colsStr);
                int type = CvType.CV_8U;

                Scanner s = new Scanner(dataStr);

                if( "f".equals(dtStr) ) {
                    type = CvType.CV_32F;
                    readMat = new Mat( rows, cols, type );
                    float fs[] = new float[1];
                    for( int r=0 ; r<rows ; r++ ) {
                        for( int c=0 ; c<cols ; c++ ) {
                            if( s.hasNextFloat() ) {
                                fs[0] = s.nextFloat();
                            }
                            else {
                                fs[0] = 0;
                                System.err.println("Unmatched number of float value at rows="+r + " cols="+c);
                            }
                            readMat.put(r, c, fs);
                        }
                    }
                }
                else if( "i".equals(dtStr) ) {
                    type = CvType.CV_32S;
                    readMat = new Mat( rows, cols, type );
                    int is[] = new int[1];
                    for( int r=0 ; r<rows ; r++ ) {
                        for( int c=0 ; c<cols ; c++ ) {
                            if( s.hasNextInt() ) {
                                is[0] = s.nextInt();
                            }
                            else {
                                is[0] = 0;
                                System.err.println("Unmatched number of int value at rows="+r + " cols="+c);
                            }
                            readMat.put(r, c, is);
                        }
                    }
                }
                else if( "s".equals(dtStr) ) {
                    type = CvType.CV_16S;
                    readMat = new Mat( rows, cols, type );
                    short ss[] = new short[1];
                    for( int r=0 ; r<rows ; r++ ) {
                        for( int c=0 ; c<cols ; c++ ) {
                            if( s.hasNextShort() ) {
                                ss[0] = s.nextShort();
                            }
                            else {
                                ss[0] = 0;
                                System.err.println("Unmatched number of int value at rows="+r + " cols="+c);
                            }
                            readMat.put(r, c, ss);
                        }
                    }
                }
                else if( "b".equals(dtStr) ) {
                    readMat = new Mat( rows, cols, type );
                    byte bs[] = new byte[1];
                    for( int r=0 ; r<rows ; r++ ) {
                        for( int c=0 ; c<cols ; c++ ) {
                            if( s.hasNextByte() ) {
                                bs[0 ...