Opencv send mat from java to python with socket

Question

I have found many examples in opencv of sending a mat through socket from java to java or c++, but I can't get it to work on python.

The server code:

MatOfByte bytemat = new MatOfByte();
    Highgui.imencode(".jpg", out, bytemat);
    byte[] bytes = bytemat.toArray();
    r.write(String.valueOf(bytes.length));
    Log.d(TAG, String.valueOf(bytes.length));
    r.write(bytes);

The python code:

def recvall(sock, count):
buf = b''
while count:
    newbuf = sock.recv(count)
    if not newbuf: return None
    buf += newbuf
    count -= len(newbuf)
return buf

length = recvall(camera_socket, 5)
if not length:
    continue
print length
data = recvall(camera_socket, int(length))
if not data:
    continue

nparr = np.fromstring(data, np.uint8)
frame = cv2.imdecode(nparr, cv2.CV_LOAD_IMAGE_UNCHANGED)

window = cv2.namedWindow('frame', cv2.WINDOW_NORMAL)
cv2.imshow('frame', frame)

The weird part is that imdecode returns None always. I just can't get it to work. PS: the java client works using ObjectInputStream

----EDIT---- Thanks all for advices, I've replaced the byte stream with predefined bytes and discovered that Java was sending some headers when sending bytes because it was using ObjectOutputStream.

Now the java code for writing to socket is:

    DataOutputStream oos = null;
    try {
        oos = new DataOutputStream(os);
        oos.write(byteImage);
    } catch (Exception e) {
        Log.e(TAG, "Error while writing to OutputStream", e);
        cancel();
        setState(STATE_NONE, this.type);
    }

What is your error message? Have you compared file sizes prior and after sending, so are you sure that the transmission has been successful and complete? — tfv
– tfv, Commented May 6, 2016 at 20:06
There is no error message, the number of bytes transmitted is the same. I really don't know how to debug this thing — andunhill
– andunhill, Commented May 7, 2016 at 12:22
can you write the result of imdecode into a file and share the file with us? imdecode in java works with that file? — tfv
– tfv, Commented May 7, 2016 at 12:35
The imdecode returns None in python. The best I can try is to send back the bytes and use imdecode in java but I suspect it would work. The number of transmited bytes is the same — andunhill
– andunhill, Commented May 7, 2016 at 12:37
You could pickle "data" and share it with us: pickle.dump(data, open('data_file.p', 'w')) . Sending the data back is also a good diea, if it works, transmission is intact. What hardware and operation systems are you using on both machines? — tfv
– tfv, Commented May 7, 2016 at 12:58

tfv · Accepted Answer · 2016-05-07 15:44:13Z

1

Try using np.uint8(nparr) for conversion as in:

frame = np.uint8(nparr)

This example works:

import numpy as np
import cv2

nparr = np.zeros((512, 512))
nparr[200:300, 400:450]=255

cv2.imshow("Result", np.uint8(nparr))
cv2.waitKey()

[EDIT] In case of a colour image please keep in mind that OpenCV2 images are BGR instaed of RGB, so you may vae to use

rgb = cv2.cvtColor(frame_in_bgr, cv2.COLOR_BGR2RGB)

edited May 7, 2016 at 15:44

answered May 7, 2016 at 13:13

tfv

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2 Comments

andunhill Over a year ago

I have tried with GRAYSCALE, UNCHANGED and COLOR and it still does not work.

tfv Over a year ago

I have modified my solutions, it is simpler, just use np.uint8(nparr)

andunhill · Accepted Answer · 2016-05-07 14:21:38Z

0

Thanks all for advices, I've replaced the byte stream with predefined bytes and discovered that Java was sending some headers when sending bytes because it was using ObjectOutputStream.

Now the java code for writing to socket is:

DataOutputStream oos = null;
        try {
            oos = new DataOutputStream(os);
            oos.write(byteImage);
        } catch (Exception e) {
            Log.e(TAG, "Error while writing to OutputStream", e);
            cancel();
            setState(STATE_NONE, this.type);
        }

This now works. The only problem left is that the colors are inverted. Any tip on how to invert them again?

answered May 7, 2016 at 14:21

andunhill

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2 Comments

tfv Over a year ago

Please do not add answers but change your original posting.

tfv Over a year ago

For inverted colours, you'll have to convert from BRGB to BGR, see my edited answer above.

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