How to clear or clean up a QIMage
Following method of mine get a const reference to a QIMage.
MyMethod(const QImage & img) {
// save it to a file
img.save("/path/to/save/the/qimage");
// now I want to clan up img from memory. How should I do it?
}
Question:
How should I clean up the QImage object from memory after use?
Note:
Note that it is a const & QImage. So, answer would involve casting the QImage into non-const?
Also, I am looking at trying to get a QImageData pointer to the data & delete it. Not sure if that is the correct approach here. Do suggest.
You need a non-const reference or a pointer. With a pointer the answer is obvious. With a reference you just assign a default-constructed QImage to it.
MyMethod(QImage & img) {
img.save("/path/to/save/the/qimage");
img = QImage();
}
However, this may still not clean up the memory occupied by the image, if there are additional QImage instances referencing that same image. To overcome this hurdle you need to avoid multiple QImage instances referencing the same image. A Qimage instance is like a shared pointer in this regard.
A const-cast would be considered to reveal a design flaw in your case. I'd recommend against it.
Related
We are migrating from Old OpenGl to Modern OpenGL. I am trying to port two functions which uses QT/OpenGL and want to convert to Modern OpenGL. QImage content should be converted to OpenGL Format. Then I want to read the pixels of QImage and render in OpenGL. How to do this in Modern OpenGL. I know glcopypixels() / glDrawPixels() is deprecated. Any pointers? I have the following code but it is in old OpenGL. Basically the whole idea is writing to back buffer and restoring the back buffer and render pixels to avoid redraw. I am using QOpenglWidget Class given by QT Framework (QT 5.1). I have tried many things converting to OpenGL format from QImage. But it did not work. Need your help. Thanks in Advance.
QImage _savedBackBuffer;
void SaveBackBuffer()
{
glReadBuffer(GL_BACK);
QImage buf = this->grabFramebuffer();
_savedBackBuffer = convertToGLFormat(buf); // convertToGLFormat is not available in
QOpenGLWidget class
}
void restoreBackBuffer()
{
glDrawBuffer(GL_BACK);
**glDrawPixels**( _savedBackBuffer.width(), _savedBackBuffer.height(),
GL_RGBA, GL_UNSIGNED_BYTE, _savedBackBuffer.bits() ); ---> glDrawPixels is Deprecated. How to handle this call.
}
flush () {
glReadBuffer (GL_BACK);
glDrawBuffer(GL_FRONT);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
**glCopyPixels**(0, 0, _scrWidth, _scrHeight, GL_COLOR); // glCopyPixels Deprecated
...
glflush();
}
I have added below code to grab the Framebuffer. But still I am getting an Empty QImage. Anything wrong with my code.
saveBackBuffer()
{
_bSavingBackBuffer = true;
QString fileName("C:\\Users\\ey617e\\Desktop\\yourFile.png");
QFile file(fileName);
file.open(QIODevice::WriteOnly);
glReadBuffer(GL_BACK);
makeCurrent();
QOpenGLFramebufferObjectFormat format;
format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
QOpenGLFramebufferObject * fbo = new
QOpenGLFramebufferObject(_scrWidth, _scrHeight, format);
fbo->bind();
paintGL();
_savedBackBuffer = fbo->toImage();
_savedBackBuffer.save(file.fileName(), "PNG");
fbo->release();
}
paintGL()
{
QOpenGLPaintDevice fboPaintDev(_scrWidth, _scrHeight);
QPainter painter(&fboPaintDev);
painter.setRenderHints(QPainter::Antialiasing | QPainter::TextAntialiasing);
painter.beginNativePainting();
drawDisplayLists(_underIllusDisplayLists);
drawDisplayLists(_illusDisplayLists);
painter.endNativePainting();
painter.drawText(20, 40, "Foo");
painter.end();
}
You can create a QOpenGLTexture object directly from a QImage: https://doc.qt.io/qt-5/qopengltexture.html#QOpenGLTexture-1
You can then use that texture directly for any image related OpenGL operations.
Basically the whole idea is writing to back buffer and restoring the back buffer and render pixels to avoid redraw.
Don't do that! It will actually impair performance, since drawing on top of previously rendered content introduces implicit synchronization points, thereby eliminating options to render new contents in parallel to advancing the presentation swap chain.
As "counterintuitive" as it may sound, just redraw the whole thing, each and every frame. If your codebase is that old, then the complexity of what you're drawing very likely is going to be so low, that you could easily render thousands of frames per second.
On the other hand retaining the contents of the backbuffer constitutes a cache and thus introduces the complexity of deciding upon cache invalidation.
I bet, that just redrawing using modern methods (geometry in buffer objects, index buffers, untangling of sync points) and simplifying the rendering code path by mere elimination the code that's responsible for determining when to actually redraw portions of the picture will actually vastly outperform anything what you had before.
I have a requirement to read pixel values from the picture displayed on the GraphicScene layout. How can I display image using QImage without using pixmap in Qt so that I am able to read the pixel values?
On most platforms, a QPixmap is a thin wrapper around a QImage. The conversions between the two are cheap - especially the pixmap-to-image conversion. Thus, you can use the QGraphicsPixmapItem and use item->pixmap().toImage() without much worry. To confirm that QPixmap is indeed a wrapper, the following check will do:
bool isPixmapThin(const QPixmap &pix) {
auto const a = pix.toImage();
auto const b = pix.toImage();
return a.bits() == b.bits();
}
In all cases, ensure that the image you take from the pixmap won't detach, i.e. always make it const (as in the code example above).
I'd like to display some QImage through QGraphicsScene, my code's very straightforward:
mainwindow.h
QImage *sourceImage;
QGraphicsView *imageView;
QGraphicsScene *imageScene;
mainwindow.cpp
imageScene = new QGraphicsScene;
imageView = new QGraphicsView;
imageView->setScene(imageScene);
sourceImage = new QImage;
sourceImage.load(":/targetimage.png");
imageScene.addPixmap(QPixmap::fromImage(sourceImage));
And then the complier points out exactly what I did wrong: QGraphicsScene::addPixmap accepts only const QPixmap as argument, and I was trying to convert QImage to const QPixmap, which is not allowed because QPixmap::fromImage within only accept const QImage, like a const hell.
The official documentation on this method doesn't make much sense to me either, if I'd like to make for example, an image viewer, and during runtime I'd sure load different images into QImage sourceImage, and how can I accomplish that using a const QImage?
This problem has been agonizing, thanks for any advice. Moreover could you light me a bit if there's any vision on the philosophical reason why guys in Qt make these methods const?
Try
imageScene.addPixmap(QPixmap::fromImage(*sourceImage));
Some advice:
there is no need to allocate the QImage on the heap (using new).
Use:
QImage sourceImage;
Then you do not need to dereference the pointer when calling QPixmap::fromImage
Just to clarify: the constness has nothing to do with the error.
is QImage based on QSharedData ? Do Qimage follow pimpl or copy on write ?
e.g. would copying(through copy con or assignment) an Qimage make a deep copy of pixels ?
QImage is copy on write. It will not copy pixels until you start modifying it.
For what it's worth when passing it into functions, I always use const QImage & anyway not to freak anyone out.
Hopefully this isn't too stupid but I want to make sure I'm doing this right.
Some Qt functions return Qt objects as values, but we may want to store them in a pointer somewhere. For example, in QDomDocument, the function documentElement returns a QDomElement, not a pointer to it. Now, as a member of my class I have:
QDomElement *listRootElement;
In a function that sets things up I am using this:
listRootElement = new QDomElement;
*listRootElement = mainIndex->documentElement();
(mainIndex is a QDomDocument.)
This seems to work, but I just want to make sure I'm doing it right and that nothing will come back to bite me.
It would be very similar for some of the image functions where a QPixmap might be returned, and I want to maintain pointers to QPixMap's.
Thanks for any comments!
Assuming that you want to store a pointer to a QDomElement for some reason, and assuming that you aware of the potential pitfalls with pointers (like, two pointers might point to the same object):
The only thing to keep in mind is that the popular 'parent takes care of deleting children' system which Qt uses is only available for QObject (sub-)classes. So when new'ing a QString or a QDomElement or something like that, keep in mind that you do have to delete it yourself, too.
I'm guessing, but I think this:
listRootElement = new QDomElement(mainIndex->documentElement());
...may allow the compiler to optimise better (see this question for some reasoning).
You're overwriting the initially allocated object:
QDomElement *listRootElement; // undefined ptr value, I'd prefer null or new right away
listRootElement = new QDomElement;
*listRootElement = mainIndex->documentElement();
You're essentially doing:
int *x = new int(42);
*x = 47;
This works because both QDomElement and int implements the assignment operator (=).
Note that there's no need to delete anything, as the returned temporary is copied into your newly allocated object.