Generate image checksum

[bunjee]

What is the maximum size of such pixmap?

No maximum size.

Is the checksum valid only during a single session or you want to keep it after user closes the application?

Valid during a given session.

Is it important for you that two exactly the same pixmaps that come from two different sources have the same checksum?

Yes it is, it speeds up the display that I don't regenerate the pixmap.

[patrik08]

I use md5 to check image stream or to stop connection param if the same as last..

static inline QByteArray fastmd5( const QString xml )
{
     QCryptographicHash formats( QCryptographicHash::Md5 );
     formats.addData(xml.toUtf8());
     return formats.result();
}
 
static inline QByteArray fastmd5( const QByteArray xml )
{
     QCryptographicHash formats( QCryptographicHash::Md5 );
     formats.addData(xml);
     return formats.result();
}

static inline QByteArray fastmd5( const QString xml )
{
     QCryptographicHash formats( QCryptographicHash::Md5 );
     formats.addData(xml.toUtf8());
     return formats.result();
}

static inline QByteArray fastmd5( const QByteArray xml )
{
     QCryptographicHash formats( QCryptographicHash::Md5 );
     formats.addData(xml);
     return formats.result();
}

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/* fill params */
/* take qmenu from other layer edit or same as last? */
void PageWindow::PlayOn()
{
	TextLayer *iter = CurrentView->LayerModel();
	const int LayID = iter->data(ObjectNameEditor).toInt();
	const QString editable = LayID == 0 ? "no" : "yes";
	bool canedit = LayID == 0 ? false : true;
	const QString Pmyhash = QString("%1-%2-%3-%4-%5").arg(editable) /* can edit area */
	                                          .arg(Clang)  /* language */
	                                          .arg(wdoc->ID()) /* document nr .*/
	                                          .arg(LayID)  /* layer nummer*/
	                                          .arg(iter->textCursor().position());  /* cursor position */
	emit menupage(canedit,fastmd5(Pmyhash));
}
 
 
void Gui_Main::editor( bool e , const QByteArray hashpage )
{
   if (LastHash == hashpage) {
   /* not make job ist the same edit status as last! */
   return;
   } else {
   LastHash = hashpage;
   }
  /* continue ok*/

/* fill params */
/* take qmenu from other layer edit or same as last? */
void PageWindow::PlayOn()
{
	TextLayer *iter = CurrentView->LayerModel();
	const int LayID = iter->data(ObjectNameEditor).toInt();
	const QString editable = LayID == 0 ? "no" : "yes";
	bool canedit = LayID == 0 ? false : true;
	const QString Pmyhash = QString("%1-%2-%3-%4-%5").arg(editable) /* can edit area */
	                                          .arg(Clang)  /* language */
	                                          .arg(wdoc->ID()) /* document nr .*/
	                                          .arg(LayID)  /* layer nummer*/
	                                          .arg(iter->textCursor().position());  /* cursor position */
	emit menupage(canedit,fastmd5(Pmyhash));
}


void Gui_Main::editor( bool e , const QByteArray hashpage )
{
   if (LastHash == hashpage) {
   /* not make job ist the same edit status as last! */
   return;
   } else {
   LastHash = hashpage;
   }
  /* continue ok*/

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My QImage cache work in this way to compose Multiple Animated Portable Network Graphics APNG... and all frame is unique...

http://www.qt-apps.org/content/show....?content=82221

class StreamFile
{
  public:
  StreamFile() 
  :d(new QBuffer())
  {
  d->open(QIODevice::ReadWrite);
  start();
  }
  ~StreamFile()
  {
   d->close();
  }
  bool clear()
  {
    d->write(QByteArray()); 
    return d->bytesAvailable() == 0 ? true : false;
  }
  void start() { 
    d->seek(0);
  }
  bool LoadFile( const QString file ) { 
    if (clear()) {
     QFile f(file); 
        if (f.exists()) {
          if (f.open(QFile::ReadOnly)) {
           d->write(f.readAll());
           f.close();
           start();
           return true;
          }
        }
    }
    return false;
  }
  bool PutOnFile( const QString file ) { 
      QFile f(file);
      if (f.open(QFile::WriteOnly)) {
        uint bi = f.write(d->readAll());
        start();
        return bi > 0 ? true : false;
      }
   return false; 
  }
  QBuffer *device() { return d; }
  bool isValid() { return img.loadFromData(stream()); }
  QByteArray stream() { return d->data(); }
  QImage img;
  QBuffer *d;
};

class StreamFile
{
  public:
  StreamFile() 
  :d(new QBuffer())
  {
  d->open(QIODevice::ReadWrite);
  start();
  }
  ~StreamFile()
  {
   d->close();
  }
  bool clear()
  {
    d->write(QByteArray()); 
    return d->bytesAvailable() == 0 ? true : false;
  }
  void start() { 
    d->seek(0);
  }
  bool LoadFile( const QString file ) { 
    if (clear()) {
     QFile f(file); 
        if (f.exists()) {
          if (f.open(QFile::ReadOnly)) {
           d->write(f.readAll());
           f.close();
           start();
           return true;
          }
        }
    }
    return false;
  }
  bool PutOnFile( const QString file ) { 
      QFile f(file);
      if (f.open(QFile::WriteOnly)) {
        uint bi = f.write(d->readAll());
        start();
        return bi > 0 ? true : false;
      }
   return false; 
  }
  QBuffer *device() { return d; }
  bool isValid() { return img.loadFromData(stream()); }
  QByteArray stream() { return d->data(); }
  QImage img;
  QBuffer *d;
};

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[jacek]

Valid during a given session.

Ok, so this reduces the chance of a collision.

No maximum size.
[...]
Yes it is, it speeds up the display that I don't regenerate the pixmap.

But unfortunately these two make things worse.

MD5 hashes have 128 bits, so a chance for a collision is quite low and they're of better quality than simple CRCs. The only problem is that they take more time to compute, but I don't think that it will be noticeable for the user if you compute them once per image.