Fast plotting widget

Hello,

I am writing a real time plotting widget.
I need to trace 12 signals as fast as possible.The height space of the widget is subdivided in 12 pieces each of them hosts a signal.

At the moment It seems like the performances with qt3 are much better than thoose with Qt4. In Qt3 code I trace on a Pixmap and then I bitBlt to the visible area.

Some pieces of the Qt4 code follows:

Inside constructor i use a timer to call the method processSomeThing "as fast as possible":

Code:

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QTimer *timer = new QTimer(this);
	connect(timer, SIGNAL(timeout()), this, SLOT(processSomeThing()));
        timer->start(0);

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I use a bidimensional array to store data:

Code:

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int points[12][640];

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Inside processSomeThing for each call I add 12 points and update current_x position (it wraps after the 640th point). After adding 12 points I call

Code:

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repaint(current_x-1,0,1,400);

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This calls paintEvent to retrace a 1x400 rectangle.

Inside paintEvent I trace 12 lines each of them starts from the previous value to the current values

Code:

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for (i=0; i<ngraphs; i++)
        {
          prev_y = ((i+1)*y_space)-ampliAndClipValue(points[i][current_x-1]);
          new_y =  ((i+1)*y_space)-ampliAndClipValue(points[i][current_x]);
 
          painter.drawLine(current_x-1,prev_y,current_x,new_y);
        }

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More in general I am asking what are the best optimisations for realtime plotting widgets (For examples polylines or so on) and why Qt3 code is faster then Qt4 one.

Thanks in advance