Convert Mat to Blob and then back to Mat

Question

Basically I am trying facial recognition using OpenCV Android. I need to convert a Mat image that is received during face detection via inputFrame.gray(); in CvCameraViewFrame into a blob that is a byte[] to save into an SQLite database. Then, during recognition, convert this byte[] back to a Mat file that can be used in .cpp files in the jni folder, as recognition code is native.

Answer 1

[edit]

it turned out to be quite easy with androids onboard methods:

import org.opencv.core.Mat;

import android.content.Context;
import android.content.ContentValues;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteDatabase.CursorFactory;
import android.database.sqlite.SQLiteOpenHelper;

class SqTable extends SQLiteOpenHelper {
    String table = "mydb";

    public SqlTable(Context context, String name, CursorFactory factory, int version) {
        super(context, name, factory, version);     
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("create table "+table+" (name TEXT UNIQUE, t INTEGER, w INTEGER, h INTEGER, pix BLOB);");
    }

    @Override
    public void onUpgrade(SQLiteDatabase arg0, int arg1, int arg2) {
    }
    
    public void dbput(String name, Mat m) {
        long nbytes = m.total() * m.elemSize();
        byte[] bytes = new byte[ (int)nbytes ];
        m.get(0, 0,bytes);
        
        dbput(name, m.type(), m.cols(), m.rows(), bytes); 
    }

    public void dbput(String name, int t, int w, int h, byte[] bytes) {
        Log.d("dbput", name + " " + t + " " + w + "x" + h);
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues values = new ContentValues();
        values.put("name", name);
        values.put("t", t); 
        values.put("w", w); 
        values.put("h", h); 
        values.put("pix", bytes);
        db.insert(table, null, values);
        db.close();
    }
    
     public Mat dbget(String name) {
        SQLiteDatabase db = this.getReadableDatabase();
        String [] columns = {"t","w","h","pix"};
        Cursor cursor = db.query(table,columns," name = ?", 
                new String[] { name }, // d. selections args
                null, // e. group by
                null, // f. having
                null, // g. order by
                null); // h. limit
     
        if (cursor != null)
            cursor.moveToFirst();
     
        int t = cursor.getInt(0);
        int w = cursor.getInt(1);
        int h = cursor.getInt(2);
        byte[] p = cursor.getBlob(3);
        Mat m = new Mat(h,w,t);
        m.put(0,0,p);
        Log.d("dbget("+name+")", m.toString());
        return m;
    }
};

// later use in your Activity:
SqlTable sql = new SqlTable(this,"imgs",null,1);
Mat m = new Mat(200,400, CvType.CV_8UC3,new Scalar(0,100,0));
Core.putText(m, "world (~)", new Point(30,80), Core.FONT_HERSHEY_SCRIPT_SIMPLEX, 2.2, new          Scalar(200,200,200));
sql.dbput("hello",m);
// Mat m = sql.dbget("hello");