How to serialize/deserialize OpenCv Mat in Java?

Question

I want to serialize OpenCv Mat(https://docs.opencv.org/3.4.3/javadoc/index.html) to save in Preference(https://developer.android.com/reference/android/preference/Preference) and then deserialize it.

Heres how I serialize it.

        Mat cameraMatrix = {put some Mat value here};
        Gson gson = new Gson();
        String json;

        // serialize
        json = gson.toJson(Double.toString(cameraMatrix));

        // deserialize          
        cameraMatrix = gson.fromJson(json, Mat.class);

Unfortunately, it doesnt work as Java Mat stores nativeObject instead of real Mat values.

How do I serialize and deserialize Mat in Java?

What is `Mat`? Maybe at least put a link to that in your question. And: what makes you think that this Mat class supports such operations? — GhostCat, Nov 16 '18 at 08:07
Could you store the key information and reconstruct it at runtime? — MeetTitan, Nov 16 '18 at 08:09
Similar with this: [How to read image from database(sqlite Django) not local file?](https://stackoverflow.com/questions/53124921/how-to-read-image-from-databasesqlite-django-not-local-file/53138304#53138304) — Kinght 金, Nov 16 '18 at 08:38

score 2 · Answer 1 · answered Nov 16 '18 at 08:40

I got it working for my use case. Here are the serialization/deserialization class. I use the table here for reference. http://ninghang.blogspot.com/2012/11/list-of-mat-type-in-opencv.html

import com.google.gson.Gson;

import org.opencv.core.Mat;

class CommonUtility
{
//  Serialization/deserialization utility
    public static String SerializeFromMat(Mat mat)
    {
        SerializedMat serializedMat = new SerializedMat();
        serializedMat.setType(mat.type());
        serializedMat.setRows(mat.rows());
        serializedMat.setCols(mat.cols());

        if (serializedMat.getType()==0||
                serializedMat.getType()==8||
                serializedMat.getType()==16||
                serializedMat.getType()==24)
        {
            serializedMat.setBytes(new byte[(int)(mat.total()*mat.elemSize())]);
            mat.get(0,0,serializedMat.bytes);
        }
        else if (serializedMat.getType()==1||
            serializedMat.getType()==9||
            serializedMat.getType()==17||
            serializedMat.getType()==25)
        {
            serializedMat.setBytes(new byte[(int)(mat.total()*mat.elemSize())]);
            mat.get(0,0,serializedMat.bytes);
        }
        else if (serializedMat.getType()==2||
                serializedMat.getType()==10||
                serializedMat.getType()==18||
                serializedMat.getType()==26)
        {
            serializedMat.setShorts(new short[(int)(mat.total()*mat.elemSize())]);
            mat.get(0,0,serializedMat.shorts);
        }
        else if (serializedMat.getType()==3||
                serializedMat.getType()==11||
                serializedMat.getType()==19||
                serializedMat.getType()==27)
        {
            serializedMat.setShorts(new short[(int)(mat.total()*mat.elemSize())]);
            mat.get(0,0,serializedMat.shorts);
        }
        else if (serializedMat.getType()==4||
                serializedMat.getType()==12||
                serializedMat.getType()==20||
                serializedMat.getType()==28)
        {
            serializedMat.setInts(new int[(int)(mat.total()*mat.elemSize())]);
            mat.get(0,0,serializedMat.ints);
        }
        else if (serializedMat.getType()==5||
                serializedMat.getType()==13||
                serializedMat.getType()==21||
                serializedMat.getType()==29)
        {
            serializedMat.setFloats(new float[(int)(mat.total()*mat.elemSize())]);
            mat.get(0,0,serializedMat.floats);
        }
        else if (serializedMat.getType()==6||
                serializedMat.getType()==14||
                serializedMat.getType()==22||
                serializedMat.getType()==30)
        {
            serializedMat.setDoubles(new double[(int)(mat.total()*mat.elemSize())]);
            mat.get(0,0,serializedMat.doubles);
        }

        Gson gson = new Gson();
        return gson.toJson(serializedMat);
    }

    public static Mat DeserializeToMat(String json)
    {
        Gson gson = new Gson();
        SerializedMat serializedMat = gson.fromJson(json, SerializedMat.class);
        Mat mat = new Mat(serializedMat.getRows(),serializedMat.getCols(),serializedMat.getType());

        if (serializedMat.getType()==0||
                serializedMat.getType()==8||
                serializedMat.getType()==16||
                serializedMat.getType()==24)
        {
            mat.put(0,0,serializedMat.getBytes());
        }
        else if (serializedMat.getType()==1||
                serializedMat.getType()==9||
                serializedMat.getType()==17||
                serializedMat.getType()==25)
        {
            mat.put(0,0,serializedMat.getBytes());
        }
        else if (serializedMat.getType()==2||
                serializedMat.getType()==10||
                serializedMat.getType()==18||
                serializedMat.getType()==26)
        {
            mat.put(0,0,serializedMat.getShorts());
        }
        else if (serializedMat.getType()==3||
                serializedMat.getType()==11||
                serializedMat.getType()==19||
                serializedMat.getType()==27)
        {
            mat.put(0,0,serializedMat.getShorts());
        }
        else if (serializedMat.getType()==4||
                serializedMat.getType()==12||
                serializedMat.getType()==20||
                serializedMat.getType()==28)
        {
            mat.put(0,0,serializedMat.getInts());
        }
        else if (serializedMat.getType()==5||
                serializedMat.getType()==13||
                serializedMat.getType()==21||
                serializedMat.getType()==29)
        {
            mat.put(0,0,serializedMat.getFloats());
        }
        else if (serializedMat.getType()==6||
                serializedMat.getType()==14||
                serializedMat.getType()==22||
                serializedMat.getType()==30)
        {
            mat.put(0,0,serializedMat.getDoubles());
        }

        return mat;
    }

    private static class SerializedMat
    {
        byte[] bytes;
        short[] shorts;
        int[] ints;
        float[] floats;
        double[] doubles;

        int type;
        int rows;
        int cols;

        byte[] getBytes()
        {
            return bytes;
        }

        void setBytes(byte[] bytes)
        {
            this.bytes = bytes;
        }

        short[] getShorts()
        {
            return shorts;
        }

        void setShorts(short[] shorts)
        {
            this.shorts = shorts;
        }

        int[] getInts()
        {
            return ints;
        }

        void setInts(int[] ints)
        {
            this.ints = ints;
        }

        float[] getFloats()
        {
            return floats;
        }

        void setFloats(float[] floats)
        {
            this.floats = floats;
        }

        double[] getDoubles()
        {
            return doubles;
        }

        void setDoubles(double[] doubles)
        {
            this.doubles = doubles;
        }

        int getType()
        {
            return type;
        }

        void setType(int type)
        {
            this.type = type;
        }

        int getRows()
        {
            return rows;
        }

        void setRows(int rows)
        {
            this.rows = rows;
        }

        int getCols()
        {
            return cols;
        }

        void setCols(int cols)
        {
            this.cols = cols;
        }

        SerializedMat()
        {
        }
    }
}

score 1 · Answer 2 · answered Nov 16 '18 at 08:17

Mat cannot directly be serialized as the pixels are stored natively. Here's what you can do:

Convert Mat to Bitmap.
Get pixels array from bitmap or compress the bitmap to byte array.
Serialize pixels array or byte array to JSON or Base64 as required.

Do the reverse for deserialization.

How to serialize/deserialize OpenCv Mat in Java?

2 Answers2

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