The classic situation that this problem can occur is when one wants to create a QPixmap with a transparent background and then draws on only some of the available pixels. This process is akin to creating a transparent GIF. Well there is a wrong way and a right way. I discovered this because I was using the wrong way and was getting away with it as long as I was writing to every pixel. Then there was a case when I was not writing to all pixels and the QPixmap unwritten to pixels (i.e., the background) was not transparent as I expected. Here is the wrong way.

The Wrong Way

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QPixmap *pix = new QPixmap(Size());
 
QPainter painter;
painter.begin(pix);
painter.fillRect(0,0,pix->width(),pix->height(),QBrush(QColor(0,0,0,0)));
painter.drawPixmap(pix->rect(),*m_image);
painter.end();

The problem is that line 4 does not fill the QPixmap with anything as the alpha is set to 0. Think of fill with alpha as a merge operation and not a replace. I was thinking of it as a replace and therefore I was replacing every pixel with a fully transparent black. However, because fillRect is really a merge I was merging a transparent pixel with the background effectively doing nothing.

The correct way is as follows:

The Right Way

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QPixmap *pix = new QPixmap(Size());
pix->fill(Qt::transparent);
 
QPainter painter;
painter.begin(pix);
painter.drawPixmap(pix->;rect(),*m_baseSymbol);
painter.end();

Notice that line 2 is new and this fill command is a replace fill.

Summary

In summary QPixmap fill(…) command is a replace fill; whereas, QPainter fillRect(…) is a merge fill. I hope this help clarity the nature of these two operations.

Related posts:

1. Grabbing all QGraphicItems current viewable in a QGraphicView