QImage and Qt::transparent

[babu198649]

hi
i want to paint on an image.How to make the image background as transparent.

the following code can change the background color of the image .

QColormap colormap=QColormap::instance();
image.fill(colormap.pixel(Qt::green));

QColormap colormap=QColormap::instance();
image.fill(colormap.pixel(Qt::green));

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but it fails to make the background transparent even when the code is changed to

QColormap colormap=QColormap::instance();
image.fill(colormap.pixel(Qt::transparent));

QColormap colormap=QColormap::instance();
image.fill(colormap.pixel(Qt::transparent));

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i wnt to paint on aQImage rather than QPixmap is because, i am painting in a multithreaded program.

how to make the background of a QImage as transparent.

[marcel]

try:

image.fill(qRgba(0, 0, 0, 255));

image.fill(qRgba(0, 0, 0, 255));

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and make sure the QImage depth is 32 bits.

[babu198649]

The image depth is 32 but had not worked.

image = QImage(1000,1000, QImage::Format_RGB32);
qDebug()<<"depth "<<image.depth(); //prints 32
image.fill(qRgba(0, 0, 0, 255));

image = QImage(1000,1000, QImage::Format_RGB32);
qDebug()<<"depth "<<image.depth(); //prints 32
image.fill(qRgba(0, 0, 0, 255));

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[marcel]

how do you test if the image is transparent or not?

[babu198649]

after the above code

//code in worker thread

QPainter painter(&image);
painter.save();
//painting is performed
painter.restore();

QPainter painter(&image);
painter.save();
//painting is performed
painter.restore();

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//code in main gui thread.

pixmap=QPixmap::fromImage(worker_thread.image);//image is converted to pixmap
update();//inside the  paintevent the pixmap is painted on to widget.

pixmap=QPixmap::fromImage(worker_thread.image);//image is converted to pixmap
update();//inside the  paintevent the pixmap is painted on to widget.

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i get black as the background color.

But if any color is given(instead of Qt::transparent) such as green then green color is the background color

image.fill(colormap.pixel(Qt::green));

image.fill(colormap.pixel(Qt::green));

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[aamer4yu]

what are u trying to achieve ? I am still not clear abt it .

Are u making an application by which user can draw over a area, with an image in the bcground ??
Sort of image editing application ? am i getting u right ?

[babu198649]

Sort of image editing application ?

no

A image is constructed(made) by painting on a QImage object with QPainter .Before painting on the image the image background should be set to transparent.

[aamer4yu]

ohh oh !!!
I guess i caught the mistake...

image.fill(qRgba(0, 0, 0, 255)); should be
image.fill(qRgba(0, 0, 0, 0));

255 specifies NO OPACITY !!

Hope this will solve the prob

[babu198649]

thanks for reply

but it did not work

[wysota]

QImage::Format_RGB32 doesn't support transparency. Use QImage::Format_ARGB32

[babu198649]

thanks wysota
constructing the image with QImage::Format_ARGB32 and then using image.fill(qRgba(0,0,0,0));
makes the image transparent

[Cupidvogel]

Hi, sorry to bump this thread, but is it possible to make a QIcon out of an image where some parts are transparent, and then fill the opaque parts of the icon with some background color?