in porting Qt3 to Qt4, problem with bitBlt

[munna]

I think it should be something like

DrawingFunctions::paintEvent( QPaintEvent* event)
{
       QPainter painter(this);
       painter.drawLine( xs, ys, xe, ye );
}

DrawingFunctions::paintEvent( QPaintEvent* event)
{
       QPainter painter(this);
       painter.drawLine( xs, ys, xe, ye );
}

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The points are calculated somewhere else and on every piantEvent the line is drawn. If you are not able to see the line then check if the co-ordiantes are negative or beyond the widget's size.

I think the buffer part i.e.

QPainter p( drawings );         /// set drawing pixmap
QPainter b( buffer ) ;

QPainter p( drawings );         /// set drawing pixmap
QPainter b( buffer ) ;

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is not needed since from Qt 4.0, QWidget automatically double-buffers its painting, so there's no need to write double-buffering code in paintEvent() to avoid flicker.

I think you can first try out painting examples that come with Qt to get a better picture of the whole thing.