Sending image in blocks via datastream does not work for other block sizes??

[ganapathi]

Hi

I am sending an image in bl via network as shown below:

******sender.cpp file**********

Sender::Sender(int no)
{
    socket = new QUdpSocket( this );
    a = 0;
    img_block = 0;
    block_size = 20;
    num = no;
 
 
    if(num == 1)
    image = new QImage("C:/Users/Public/Pictures/Sample Pictures/Desert.jpg" );
if( image->isNull() )
    qDebug() << "Failed to open test.jpg" ;
    QTimer *timer = new QTimer( this );
 
    timer->isSingleShot();
      QTimer::singleShot(1000, this, SLOT(broadcastLine()) );
      connect( this, SIGNAL(done()), this, SLOT(broadcastLine()) );
 
 
}
 
 
void Sender::broadcastLine()
{
 
  QByteArray buffer( block_size+3*image->width(), 0 );
  QDataStream stream( &buffer, QIODevice::WriteOnly );
  stream.setVersion( QDataStream::Qt_5_5 );
 
  stream << (quint16)image->width() << (quint16)image->height();
 
stream << img_block;
 
      for( int x=0; x<image->width(); ++x )
      {
                for(int y=block_size*a; y <block_size+(block_size*a);y++)
          {
 
              if( y < image->height())
              {
               QRgb rgb = image->pixel( x, y );
               stream << (quint8)qRed( rgb ) << (quint8)qGreen( rgb ) << (quint8)qBlue( rgb );
              }
              else
              {
                  img_block = y = a = 0;
                  img_block = -block_size;
              }
          }
      }
 
 
    if(num==1)
     socket->writeDatagram( buffer, QHostAddress::Broadcast,9988 );
img_block = img_block + block_size;
a++;
 
     emit done();
 
}

Sender::Sender(int no)
{
    socket = new QUdpSocket( this );
    a = 0;
    img_block = 0;
    block_size = 20;
    num = no;


    if(num == 1)
    image = new QImage("C:/Users/Public/Pictures/Sample Pictures/Desert.jpg" );
if( image->isNull() )
    qDebug() << "Failed to open test.jpg" ;
    QTimer *timer = new QTimer( this );

    timer->isSingleShot();
      QTimer::singleShot(1000, this, SLOT(broadcastLine()) );
      connect( this, SIGNAL(done()), this, SLOT(broadcastLine()) );


}


void Sender::broadcastLine()
{

  QByteArray buffer( block_size+3*image->width(), 0 );
  QDataStream stream( &buffer, QIODevice::WriteOnly );
  stream.setVersion( QDataStream::Qt_5_5 );

  stream << (quint16)image->width() << (quint16)image->height();
 
stream << img_block;

      for( int x=0; x<image->width(); ++x )
      {
                for(int y=block_size*a; y <block_size+(block_size*a);y++)
          {

              if( y < image->height())
              {
               QRgb rgb = image->pixel( x, y );
               stream << (quint8)qRed( rgb ) << (quint8)qGreen( rgb ) << (quint8)qBlue( rgb );
              }
              else
              {
                  img_block = y = a = 0;
                  img_block = -block_size;
              }
          }
      }


    if(num==1)
     socket->writeDatagram( buffer, QHostAddress::Broadcast,9988 );
img_block = img_block + block_size;
a++;
    
     emit done();
   
}

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The above program works for block_size< 20 but not for 32 which I am interested. I dont have even the slightest idea of whats happening and even how to debug this?

Regards

[yeye_olive]

The logic of your program is very hard to follow, with lots of side effects on global variables (by global, I mean non-local to a function; your code snippet does not tell us whether they are global or instance members of the Sender class). The two lines

img_block = y = a = 0;
img_block = -block_size;

img_block = y = a = 0;
img_block = -block_size;

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are most likely buggy:

• they overwrite the global variable a, which is used in the condition of the inner for loop;
• they overwrite the index y of the inner for loop:
• they overwrite (twice) the global variable img_block, which is never read in the remainder of the function; the effect could be observed if broadcastLine() were called again, but from your snippet we have no idea when/if that happens.

Could you explain exactly what you are trying to achieve? On a side note, please wrap your code in CODE tags in your posts to make it much more readable.

[ganapathi]

The logic of your program is very hard to follow, with lots of side effects on global variables (by global, I mean non-local to a function; your code snippet does not tell us whether they are global or instance members of the Sender class). The two lines

img_block = y = a = 0;
img_block = -block_size;

img_block = y = a = 0;
img_block = -block_size;

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are most likely buggy:

• they overwrite the global variable a, which is used in the condition of the inner for loop;
• they overwrite the index y of the inner for loop:
• they overwrite (twice) the global variable img_block, which is never read in the remainder of the function; the effect could be observed if broadcastLine() were called again, but from your snippet we have no idea when/if that happens.

Could you explain exactly what you are trying to achieve? On a side note, please wrap your code in CODE tags in your posts to make it much more readable.

I am trying to send the same image indefinitely which is ensured by one-shot timer to run Sender::broadcastLine() ; after this I emit the done() signal which has Sender::broadcastLine() as slot and thus indefinite sending of image.


Here is my sender.h file:

[CODE]
#ifndef SENDER_H
#define SENDER_H
#include <QObject>
class QUdpSocket;
class QImage;
 
class Sender : public QObject
{
  Q_OBJECT
public:
  Sender(int no);
 
 signals:
  void done();
 
private slots:
void broadcastLine();
 
private:
  QUdpSocket *socket;
  QImage *image;
  quint16 a;
  quint16 img_block;
  quint8 block_size;
 
 //  QUdpSocket *socket1;
  int num;
};
 
#endif // SENDER_H

[CODE]
#ifndef SENDER_H
#define SENDER_H
#include <QObject>
class QUdpSocket;
class QImage;

class Sender : public QObject
{
  Q_OBJECT
public:
  Sender(int no);

 signals:
  void done();

private slots:
void broadcastLine();

private:
  QUdpSocket *socket;
  QImage *image;
  quint16 a;
  quint16 img_block;
  quint8 block_size;

 //  QUdpSocket *socket1;
  int num;
};

#endif // SENDER_H

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[/CODE]

Yes, img_block ,y,a and block_size are not global but instance variables.

img_block = y = a = 0;
    img_block = -block_size;

img_block = y = a = 0;
    img_block = -block_size;

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The above lines ensure to start the image sending from first again( because i send the image indefinitely). Yaa it can be modified like

y=a=0;
img_block = -block_size;

y=a=0;
img_block = -block_size;

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Does this help?

why does it work for block_size = 20 but not 32?



Regards