Question

Ok. This is a bit odd. Long story short. I am fething raw BGR images from a camera, compressing them to JPG with OpenCV and sending with UDP protocol to a PC. This is how I compress images:

// memblock contains raw image
IplImage* fIplImageHeader;
fIplImageHeader = cvCreateImageHeader(cvSize(160, 120), 8, 3);
fIplImageHeader->imageData = (char*) memblock;

// compress to JPG
vector<int> p;
p.push_back(CV_IMWRITE_JPEG_QUALITY);
p.push_back(75);
vector<unsigned char> buf;
cv::imencode(".jpg", fIplImageHeader, buf, p);

This is how I send them with UDP:

n_sent = sendto(sk,&*buf.begin(),(int)size,0,(struct sockaddr*) &server,sizeof(server));

This is how I receive them in a PC:

int iRcvdBytes=recvfrom(iSockFd,buff,bufferSize,0,
(struct sockaddr*)&cliAddr,(socklen_t*)&cliAddrLen);
// print how many bytes we have received
cout<<"Received "<<iRcvdBytes<<" bytes from the client"<<endl;

I am getting this output:

    Received 57600 bytes from the client
    Received 57600 bytes from the client
    ...

If I remove the JPG compression at the program fetching images from camera, the output is the same:

    Received 57600 bytes from the client
    Received 57600 bytes from the client
    ...

However, when I save the received image on a disk, it’s size is around 7.8KB while uncompressed raw image saved to disk takes about 57KB space.

What’s going on here?

Answer 1

The “size” you pass to send is the size of the compressed buffer, right? It’s not obvious from your code snippets where “size” comes from (as ypnos suggests, I would have expected buf.size() ).