Errors when reading a QImage sent through a socket

[volatile_ah]

Hey guys,

First up I'm new to this whole Qt-programming, but it's been a fun challenge thus far.

What I'm trying to do is send a QImage across the network using a QTcpSocket. Let's for a second pretend I don't care about bandwidth, I just want to try and get this working.

Below is my server code (snippet):

QBuffer image_buffer;
QImageWriter writer(&image_buffer, "PNG");
if(!writer.write(captured_image.getImage()))
	qWarning()<<"ERROR: Unable to write image to buffer: "<<writer.errorString();
 
QByteArray data;
QDataStream out(&data, QIODevice::WriteOnly);
out.setVersion(QDataStream::Qt_4_0);
out<<(quint32)image_buffer.data().size();
 
qDebug()<<"Image size = "<<image_buffer.data().size();
 
out<<image_buffer.data();
 
video_socket.write(data);
if(!video_socket.waitForBytesWritten())
	qWarning()<<"ERROR: Unable to send image: "<<video_socket.errorString();

QBuffer image_buffer;
QImageWriter writer(&image_buffer, "PNG");
if(!writer.write(captured_image.getImage()))
	qWarning()<<"ERROR: Unable to write image to buffer: "<<writer.errorString();
				
QByteArray data;
QDataStream out(&data, QIODevice::WriteOnly);
out.setVersion(QDataStream::Qt_4_0);
out<<(quint32)image_buffer.data().size();
			
qDebug()<<"Image size = "<<image_buffer.data().size();
			
out<<image_buffer.data();
			
video_socket.write(data);
if(!video_socket.waitForBytesWritten())
	qWarning()<<"ERROR: Unable to send image: "<<video_socket.errorString();

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My client code (snippet) is as follows:

QDataStream in(&video_socket);
in.setVersion(QDataStream::Qt_4_0);
 
while(video_socket.bytesAvailable() < sizeof(quint32))
{
	qDebug()<<"Waiting for image size...";
	if(!video_socket.waitForReadyRead())
	{
		qWarning()<<"ERROR: Failed to read image size: "<<video_socket.errorString();
		return;
	}
}
 
quint32 image_size;
in>>image_size;
 
qDebug()<<"Image Size = "<<image_size;
 
while(video_socket.bytesAvailable() < image_size)
{
	qDebug()<<"Waiting for image...";
	if(!video_socket.waitForReadyRead())
	{
		qWarning()<<"ERROR: Failed to read image: "<<video_socket.errorString();
		return;
	}
}
 
QByteArray data;
in>>data;
 
qDebug()<<"Data size = "<<data.size();
 
QBuffer image_buffer(&data);
QImageReader reader(&image_buffer, "PNG");	
QImage received_image;
 
if(!reader.read(&received_image))
{
	qWarning()<<"ERROR: Unable to read received image from buffer: "<<reader.errorString();
	return;
}

QDataStream in(&video_socket);
in.setVersion(QDataStream::Qt_4_0);
		
while(video_socket.bytesAvailable() < sizeof(quint32))
{
	qDebug()<<"Waiting for image size...";
	if(!video_socket.waitForReadyRead())
	{
		qWarning()<<"ERROR: Failed to read image size: "<<video_socket.errorString();
		return;
	}
}
		
quint32 image_size;
in>>image_size;
		
qDebug()<<"Image Size = "<<image_size;
		
while(video_socket.bytesAvailable() < image_size)
{
	qDebug()<<"Waiting for image...";
	if(!video_socket.waitForReadyRead())
	{
		qWarning()<<"ERROR: Failed to read image: "<<video_socket.errorString();
		return;
	}
}
		
QByteArray data;
in>>data;
		
qDebug()<<"Data size = "<<data.size();
		
QBuffer image_buffer(&data);
QImageReader reader(&image_buffer, "PNG");	
QImage received_image;
		
if(!reader.read(&received_image))
{
	qWarning()<<"ERROR: Unable to read received image from buffer: "<<reader.errorString();
	return;
}

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However, the client only seems to be able to read the size of the image, then dies.

Below is the output from the qDebug() statements in the server:

Image size = 384387
Data written = 384395

And below is my output from qDebug() statements in the client program:

Waiting for image size...
Image Size = 384387
Waiting for image...
Waiting for image...
Waiting for image...
ERROR: Failed to read image: "The remote host closed the connection"

So I guess my question is, can anyone see why it is doing this? I'm at a loss...

Any help would be terrific!!!

Regards,
Adrian

[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?