QImage sent through QDataStream

[dfink]

Hi,

I'm trying to send images over a network connection the easiest way possible. Its not a Video stream and its only local network, so the bandwidth usage is not that important.

I've tried two approaches till now:

First using the overloaded << operator "QDataStream << QImage"

Client Side:


// QImage image is already instantiated
_socket->connectToHost("127.0.0.1", 11111, QIODevice::ReadWrite);
QDataStream stream(_socket);
stream << image;
_socket->waitForBytesWritten(-1);


Server Side:


QAbstractSocket *socket = _server.nextPendingConnection();
QDataStream stream(socket, QIODevice::ReadWrite);
socket->waitForReadyRead(-1);
stream >> inQImage;
if (inQImage.isNull()){
qDebug("Received Image is empty");
}

Result:
libpng error: Read Error
Received Image is empty

Second Approach using the QImageWriter and QImageReader:
Client Side:


QImageWriter writer(socket, "png");
writer.write(image);
socket->waitForBytesWritten(-1);


Server Side:


_socket->waitForReadyRead(-1);
QImageReader imReader(_socket,"png");
imReader.read(_image);
_image.save("received.png","png");


The Result is also an libpng error: Read Error but the Image is partially transferred. When using a sufficiently small image it works.
It also works when the IODevice is not a QTcpSocket but a QFile

In both cases is seams that not all data is sent, is there a possibility that waitForBytesWritten() is not working as expected?

thanks in advance
Daniel

[wysota]

readyRead is emitted whenever there is any data available, not whenever there is all data available. Your image simply didn't fully arive just yet.

[dfink]

Thanks for the quick reply.

Shouldn't both the ImageReader and the >> operator check how big the picture is, and repeatedly read from the IODevice.
If not, has anyone a solution?

[wysota]

Shouldn't both the ImageReader and the >> operator check how big the picture is, and repeatedly read from the IODevice.
No they shouldn't. How should they know that no more data would come?


If not, has anyone a solution?
Send the size of the image before sending the image itself. Then continue waiting until there is enough data ready. Only then read the data from the socket.

[dfink]

Thanks for the Hints, it finally works.
I used repeated Calls to readRawData(char* , int len) of QDataStream which did the trick.
What I find weird though is that using QDataStream & QDataStream::readBytes ( char *& s, uint & l ) didn't work at all.

[wysota]

readBytes() and readRawBytes() do two completely different things so you can't exchange one for the other.

[hkaraoguz84]

Hi,

This thread is a bit old but now I have the same problem.

I have connected the readyread signal to my slot but i cannot receive the whole image when readyread is emitted. I only receive 8192 bytes of. How can i make the program wait for whole to come? Is there any example code?

Thanks...