QPixmap with transparent background

Perdrix

I have a widget whose ctor contains:

setAttribute(Qt::WA_TransparentForMouseEvents);
setAttribute(Qt::WA_NoSystemBackground);
setAttribute(Qt::WA_WState_ExplicitShowHide);

it has a null paintEvent handler

void EditStars::paintEvent([[maybe_unused]] QPaintEvent* event)
{
    event.ignore();
}

and the mouseMoveEvent and a few other places do this:

drawOnPixmap();

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

The drawOnPixmap() code does:

QRect rcClient{ rect() };
size_t	width = rcClient.width(), height = rcClient.height();

QPainter painter(&pixmap);
//QPalette palette{ QGuiApplication::palette() };
QBrush brush{ Qt::transparent };
painter.setBrush(brush);
painter.setRenderHint(QPainter::Antialiasing);
painter.setRenderHint(QPainter::SmoothPixmapTransform);
painter.eraseRect(rcClient);
painter.fillRect(rcClient, brush);

the paintEvent handler of imageView does most of its other works then does:

painter.setBackgroundMode(Qt::TransparentMode);
painter.drawPixmap(0, 0, *pOverlayPixmap);

but the overlay pixmap isn't displayed with a transparent background so I can't see through it to the underlying widget :(

How to get this to work?

Thanks
D.

mrjj

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

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.

alt text

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