[SOLVED] QDeclarativeImageProvider and memory leaks with QImage

mbarclaygmail.com

Hello,

I have a 3rd party library that is generating pixel buffers. I create a QImage with the buffer and return it to the GUI using QDeclarativeImageProvider. I am using QImage so the buffers can be generated asynchronously. The problem is: QImage doesn't take ownership of the buffer. But I'm returning a QImage to the GUI, and there's no telling how many times it will be copied as its passed around the QDeclarative library. How do I know when QImage is done referencing my buffer so I can delete it? Or, is it possible to create a QImage that owns its buffer and will release it automatically?

Thanks,
Matt

task_struct

Hello,

QImage uses "implicit data sharing":http://qt-project.org/doc/qt-4.8/implicit-sharing.html#implicit-data-sharing so you will only one copy of data. When last QImage that uses this data is destroyed data buffer will be released.

lgeyer

[quote author="MattB" date="1332203247"]Or, is it possible to create a QImage that owns its buffer and will release it automatically?[/quote]
Subclass QImage and reimplement its destructor, which will delete the user-supplied buffer.

[quote author="task_struct" date="1332246203"]Hello,
QImage uses "implicit data sharing":http://qt-project.org/doc/qt-4.8/implicit-sharing.html#implicit-data-sharing so you will only one copy of data. When last QImage that uses this data is destroyed data buffer will be released.[/quote]
This is not true for QImage using a user-supplied buffer.

mbarclaygmail.com

[quote author="Lukas Geyer" date="1332247257"][quote author="MattB" date="1332203247"]Or, is it possible to create a QImage that owns its buffer and will release it automatically?[/quote]
Subclass QImage and reimplement its destructor, which will delete the user-supplied buffer.[/quote]

Won't this lead to the buffer being lost/deleted prematurely?

@
MyQImage MyProvider::requestImage(QString id, QSize *size, QSize requestedSize) {
...
...

// Let's pretend this creates an 800x600 Image buffer
uchar* data = generateSomeImageData();

MyQImage image(data, 800, 600, QImage::Format_ARGB32);

return image;

}
@

Qt is expecting a QImage back from this method. If Qt does:

@QImage image = provider->requestImage(id, &size, requestedSize);@

Won't my custom data in MyQImage get sliced?

Here's the line that's causing me problems:

http://qt.gitorious.org/qt/qt/blobs/4.7/src/gui/image/qimage.cpp#line901

I wonder if I can just do:

@image->data_ptr()->own_data = true;@

That should cause QImageData to delete the buffer when it destructs:

http://qt.gitorious.org/qt/qt/blobs/4.7/src/gui/image/qimage.cpp#line263

And that should happen when all QImage's that share the data have destructed.

mbarclaygmail.com

[quote author="MattB" date="1332256874"]
I wonder if I can just do:

@image->data_ptr()->own_data = true;@
[/quote]

Heh, no I can't. That's not part of the Qt API (defined in qimage_p.h). Better drink some more coffee... :)

mbarclaygmail.com

OK, looks like I found a solution:

@MyQImage MyProvider::requestImage(QString id, QSize *size, QSize requestedSize) {
...
...

// Let's pretend this creates an 800x600 Image buffer
uchar* data = generateSomeImageData();

QImage *source = new QImage(data, 800, 600, QImage::Format_ARGB32);
QImage image(800, 600, QImage::Format_ARGB32);
QPainter painter(&image);
painter.drawImage(QPoint(0,0), *source);

delete source;
delete data;

return image;

}
@

My memory profile looks correct using this solution.

lgeyer

Well, that's cheap ;-) I thought that's exactly the behaviour you want to prevent - having an additional buffer - and you want to re-use the already existing buffer.

If you just want to construct a QImage from some binary data use QImage::fromData() instead.

[quote author="MattB" date="1332256874"]And that should happen when all QImage's that share the data have destructed.
[/quote]
Use QImage::isDetached() when deconstructing a QImage to find out if its buffer is still shared with another instance or if it is safe to delete the buffer.

mbarclaygmail.com

Ideally, yes, I would avoid the buffer copy, but QImage::fromData() wants a buffer in one of the formats specified by QImageReader::supportedImageFormats() . None of those formats worked with my data, so I'm stuck with the QPainter approach.