how to clean data in qimage with transparent?

[rolandsun]

I want to draw a real time scan radar image on widget with background terrian data, so I want to clean previous data to no color with transparent to show background correctly , I try to use codes as following

QColor qclr=QColor(Qt::tansparent);
painter->setPen(qclr);
painter->setBrush(qclr);
painter->drawConvexPolygon(pg);

QColor qclr=QColor(Qt::tansparent);
painter->setPen(qclr);
painter->setBrush(qclr);
painter->drawConvexPolygon(pg);

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but it does not works, previous data are not cleared. if I replace color with Qt::gray ,it works but I can not see the background .

I have read the post http://www.qtcentre.org/threads/3310...nsparent-color , it seem that there was no solution at that time ,how about now ?

thanks

[Ginsengelf]

Hi, probably the data is not "cleared" because you paint above it with a transparent brush, so you can see through the newly painted pixels and see the old data.

Have you tried wysota's solution from the other thread (set QPainter::QPainter::CompositionMode_Clear)?

Ginsengelf

[wysota]

What is the painter object working on? Is it a pixmap, a widget or what?

[rolandsun]

painter works on QImage.

[wysota]

Why don't you just create an empty QImage and draw on that instead of clearing the one you have?

[rolandsun]

In my case, I have to update the image every serveral milliseconds, only part of the image should be erased and repainted. so I think create a new QImage is infeasible.

[wysota]

In that case you should indeed use Clear composition mode.

[rolandsun]

Finally QPainter::CompositionMode_Source works ,thanks.