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How can i send image using UDP in Matlab code ?

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I want to create UDP communication between Java and Matlab. Matlab is us a server and while Java is the client. The client asks to the server to image. How can I write a code in Matlab to send the image to the client?

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Accepted Answer

Sebastian Castro
Sebastian Castro on 4 Nov 2018
If you have Instrument Control Toolbox, you can use this functionality:
- Sebastian

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Walter Roberson
Walter Roberson on 5 Nov 2018
I understand that. I am not at my desk so I cannot test udp at this time.
However, I can scan your code to see if there are other problems as well as the udp problem. When I do that I see a problem with the way the Java code would decode the array it receives (once we get to the point where it is able to receive the array.) I think you will need to send the jpg file content instead of the byte array.
Walter Roberson
Walter Roberson on 5 Nov 2018
Your Java side is expecting to read a single datagram of maximum length 1024. Your image content is likely to be larger than that. On the MATLAB side you are not creating any output buffer so you will get the default which is 512. The maximum output buffer size for udp objects is 4096.
Are you expecting to send very small images? If not then you need to create a protocol on top of udp to send all of the bytes in multiple packets, taking into account that the packets could arrive out of order. https://www.mathworks.com/help/instrument/outputbuffersize.html

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More Answers (1)

Walter Roberson
Walter Roberson on 5 Nov 2018
runudp.m :
%this is my code to send the image in Matlab
Sender = 'localhost'; % LocalHost for testing on the same computer
portSender = 33781;
ipReceiver = 'localhost'; % LocalHost for testing on the same computer
portReceiver = 33780;
BS = 1024;
udpReceiver = udp(Sender,portSender, 'LocalPort', portReceiver, 'OutputBufferSize', BS);
fopen(udpReceiver);
image = imread('peppers.png');
fwrite(udpReceiver , image(1:BS), 'uint8')
fclose(udpReceiver);
delete(udpReceiver);
xferudp.java :
import java.io.IOException;
import java.net.InetAddress;
import java.net.DatagramSocket;
import java.net.DatagramPacket;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JLabel;
import java.awt.image.BufferedImage;
import java.awt.FlowLayout;
import java.io.ByteArrayInputStream;
import javax.swing.ImageIcon;
class xferudp {
//this java class code to receive the image and display
public static void main(String args[]) throws IOException {
InetAddress IPAddress = InetAddress.getByName("localhost"); // LocalHost for testing on the same computer
DatagramSocket clientSocket = new DatagramSocket(33781, IPAddress);
byte[] imageBuffer = new byte[1024 ] ;
DatagramPacket p=new
DatagramPacket(imageBuffer,imageBuffer.length);
System.out.println("UDP S: Receiving...");
clientSocket.receive(p);
byte[] buffer = p.getData();
BufferedImage img =ImageIO.read(new ByteArrayInputStream(buffer));
ImageIcon icon=new ImageIcon(img);
JFrame frame=new JFrame();
frame.setLayout(new FlowLayout());
frame.setSize(200,300);
JLabel lbl=new JLabel();
lbl.setIcon(icon);
frame.add(lbl);
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
}
To use,
!javac -d . -classpath . xferudp.java
!sudo java xferudp
This gets as far as
Exception in thread "main" java.lang.NullPointerException
at javax.swing.ImageIcon.<init>(ImageIcon.java:204)
at xferudp.main(xferudp.java:35)
which might have something to do with the fact that we did not a valid image over to be decoded.
The write error was definitely tied to writing too much for the socket. Remember, there is no automatic segmentation for UDP, and no automatic reconstruction by order -- every UDP packet is independent of each other.

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Walter Roberson
Walter Roberson on 6 Nov 2018
Have MATLAB transmit a complete image file, not the result of imread. The complete file must fit within BS bytes. BS must not be set beyond 4096, and it is possible that the largest you can actually use is 1492 or so (total of 1506 bytes in the udp packet including headers.)
If the file to send might be more than fits into one packet then you have a lot more work to do.
Walter Roberson
Walter Roberson on 6 Nov 2018
I got it to work for a png file (but not for all image files.) Code is attached. Rename the xferudp.java.txt to xferudp.java and invoke the java compiler, and then (in a window different than the MATLAB window) sudo to run the code as shown above. Then in MATLAB, execute runudp . It will send over the coins.png sample image and the java side will display it.
If you need to send images that are more than 65535 bytes for the file, then you will need to send as multiple packets, and you will need to deal with reassembly taking into account that packets might not get delivered.

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