QPixmap with transparent background

[Perdrix]

I even tried moving the pixmap from being a class instance variable to a local variable
in drawOnPixmap, omitting the eraseRect() call and adding the calls

imageView->setOverlayPixmap(pixmap);
imageView->update();

to the end of mf.

but that didn't work either :(

[mrjj]

Hi
what type is imageView. ?
Some widgets like QStacked and QScrollArea have a background of their own.

[Perdrix]

@mrjj It is a direct sub-class of QWidget. In the paint mf of ImageView, I'm just overlaying the pixmap that was set by setOverlayPixmap ...

I tried without the use of the overlay pixmap - instead using the paint mf of the overlaying widget to draw over the underlying image but had the same problem :(
David

[mrjj]

@Perdrix

What platform ?
I tried fast on Windows and it does draw it transparent.

Also have you tested your image with a QLabel that it does show up transparent ?

[Perdrix]

I'm not building the pixmap from a png, it's all done with QPainter. - that may make all the difference.

I am doing this on Windows

D.

[mrjj]

@Perdrix
well it might
do you do
pix.fill(Qt::transparent);
to have it transparent from the beginning. ?

[Perdrix]

@Perdrix said in QPixmap with transparent background:

painter.fillRect(rcClient, brush);

No - see code posted above - I used painter.fillRect using a transparent brush

[candy76041820]

@Perdrix Perhaps filling a rect with transparent brush means painting a transparent rect onto the canvas, which actually is a no-op.

[Chris Kawa]

The default mode for painter is to blend the painted shape with existing pixels using the alpha channel, so as @candy76041820 said - filling a rectangle with transparency does nothing.

To replace the contents of that rectangle instead of blending you have to change composition mode to QPainter::CompositionMode_Source, or, simpler, as @mrjj suggested use fill() method of QPixmap.

[Perdrix]

Using pixmap.fill(Qt::transparent); solved the problem

Thanks a lot