Errors when reading a QImage sent through a socket

[wysota]

You're overcomplicating things. If you are using QDataStream then just stream the image directly into it.

QImage img;
QDataStream str(&socket);
str << img;

QImage img;
QDataStream str(&socket);
str << img;

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QImage img;
QDataStream str(&socket);
str >> img;

QImage img;
QDataStream str(&socket);
str >> img;

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Currently you are misusing the data stream, it is not a general purpose binary stream but a serialization mechanism - it adds additional information to data it is being passed to, so you can't write a byte array directly to a socket and on the other end stream it out using QDataStream as the data lacks that additional information QDataStream requires.

[volatile_ah]

Thanks very much wysota.

I changed my client code to the following:

// Block until data is available for reading
if(!video_socket.waitForReadyRead())
{
	qWarning()<<"ERROR: could not read image: "<<video_socket.errorString();
	return;
}
 
// Read the image from the socket via the data stream
in>>received_image;
 
qDebug()<<"Image received";

// Block until data is available for reading
if(!video_socket.waitForReadyRead())
{
	qWarning()<<"ERROR: could not read image: "<<video_socket.errorString();
	return;
}
		
// Read the image from the socket via the data stream
in>>received_image;
		
qDebug()<<"Image received";

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And server to:

// Write the image to the socket via the data stream
out<<captured_image.getImage();
 
// Block until the data stream has been written to the socket
if(!video_socket.waitForBytesWritten())
	qWarning()<<"ERROR: Unable to send image: "<<video_socket.errorString();	
 
qDebug()<<"Image sent";

// Write the image to the socket via the data stream
out<<captured_image.getImage();
			
// Block until the data stream has been written to the socket
if(!video_socket.waitForBytesWritten())
	qWarning()<<"ERROR: Unable to send image: "<<video_socket.errorString();	
				
qDebug()<<"Image sent";

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And for some images it works perfectly.

However, for other images (seems to be the larger the image the more likely it is to happen), I get the following message:

libpng error: Read Error

A bit of googling shows this may be a Linux problem rather than a problem with what I am doing in Qt. However, I havent managed to find a reason for it yet. I've tried different QDataStream versions (Qt_4_0 to Qt_4_5) but the error stil occurs. My libpng (Ubuntu distro) seems to be all up to date (at least from the repository anyway). Hmmm....

[joergwl]

Hi,

I got the same problem also on Ubuntu (januty)

If anyone has an idea about how to overcome this issue, it would be most appreciated.

Ciao
joergwl

[joergwl]

SOLVED

I used the fortune samples for a start and the block length is defined as quint16, which is a bit to small to send bigger images.

After changing the block length to qint64 all works fine

joergwl

[wysota]

The same problem meaning what?