Qt image resource is null

Kelenyche

Hi folks
It seems like this has been posted and asked a lot, but I've done just about every step I can find, and nothing is working, so I need help!
I need to set an image as a qt resource, and load it in as the background of a Qchart.

So here's what I've done:
Saved the image into a sub-directory in my program.
created a qrc file named resources.qrc. Here's the contents of that file:

<RCC>
    <qresource>
    <file>images/scribble.jpg</file>
    </qresource>
</RCC>

Modified the .pro file with the following contents:

RESOURCES += extra/resources.qrc

Attempted to load the file with the following code, and done some null checking:

QImage my_image(":/images/scribble.jpg");
if(my_image.isNull())
{
    outputTextBox->append("image is null");
}

I've also tried:

    QImage my_image;
    my_image.load(":/images/scribble.jpg");

I've completely removed my build folder on the build system and re-initialized the repo and rebuilt the code, but that isn't changing anything either.

I've confirmed it's finding the resources.qrc by changing both the name and filepath of the qrc file, which throws errors when I run qmake ---.pro
I've also changed the filepath in the code, i.e. QImage my_image(":/images/scrible.jpg"); or QImage my_image(":/imag/scribble.jpg"); which results in Qt throwing an error on the command line that says the image is not found at runtime. (note the intentional spelling errors)

So it seems to me that it's finding the image and qrc file, but despite that, is not able to load it in for some reason.
What am I doing wrong??

Christian Ehrlicher

Do you actually add the resource to your application?

SGaist

Hi,

Can you show the structure of your project ?

Did you try to load the file from the disk just to check that it loads properly ?

Kelenyche

@Christian-Ehrlicher
Yes, it's in a folder, available to the application. Like I mentioned, if I change the filepath or name in the code, it throws an error saying it can't find it - so it does seem to be finding it but isn't reading it somehow?

Kelenyche

@SGaist

My project structure is:

project
-headers
*.h
-sources
-main.cpp
*.cpp (file that calls the image is in here)
-extra
-resources.qrc
-images
-scribble.jpg

I think that's all the relevant info on my structure. What's a way to load the file from disk to see that my program sees it?

Kelenyche

@Kelenyche said in Qt image resource is null:

@SGaist

My project structure is:

project
-headers
*.h
-sources
-main.cpp
*.cpp (file that calls the image is in here)
-extra
-resources.qrc
-images
-scribble.jpg

I think that's all the relevant info on my structure. What's a way to load the file from disk to see that my program sees it?

Oh none of my nice spacing came through lol.

SGaist

Try editing it with coding tags.

For the test, just use the full path to the file on the disk.

Kelenyche

 project
     headers
         *.h
     sources
         main.cpp
         *.cpp (file that calls the image is in here)
     extra
         resources.qrc
         images
             scribble.jpg

I'll try the full disk path.

Kelenyche

Full disk path is also coming up with a null image.

Fwiw, I'm doing this on a raspberry pi, is it possible there's some permission problems?

SGaist

Next step: start your application with the QT_DEBUG_PLUGINS environment variable set to 1 to see what's going on with the image plugins.

Kelenyche

@SGaist
Where should this be set? I'm also doing all this off the command line, no qt creator. Would this be in the .pro file, or an argument called with qmake program.pro?

SGaist

On the command line directly

Kelenyche

Ah, ok so I ran:
QT_DEBUG_PLUGINS=1 ./program
and it dumped a bunch of info. I'm not seeing any problems or failures in here...?

Got keys from plugin meta data ("ico", "cur")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/aarch64-linux-gnu/qt5/plugins/imageformats/libqjpeg.so"
Found metadata in lib /usr/lib/aarch64-linux-gnu/qt5/plugins/imageformats/libqjpeg.so, metadata=
{
    "IID": "org.qt-project.Qt.QImageIOHandlerFactoryInterface",
    "MetaData": {
        "Keys": [
            "jpg",
            "jpeg"
        ],
        "MimeTypes": [
            "image/jpeg",
            "image/jpeg"
        ]
    },
    "className": "QJpegPlugin",
    "debug": false,
    "version": 330499
}

This seems like the most relevant portion, but again it seems to have succeeded in recognizing jpgs, so I don't see a problem. Let me know if there's anything else I should be looking for. I'm not seeing parts that are specific to my code, it just looks like Qt's background stuff for loading certain plugins.

Kelenyche

@Kelenyche said in Qt image resource is null:

Ah, ok so I ran:
QT_DEBUG_PLUGINS=1 ./program
and it dumped a bunch of info. I'm not seeing any problems or failures in here...?
Got keys from plugin meta data ("ico", "cur")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/aarch64-linux-gnu/qt5/plugins/imageformats/libqjpeg.so"
Found metadata in lib /usr/lib/aarch64-linux-gnu/qt5/plugins/imageformats/libqjpeg.so, metadata=
{
    "IID": "org.qt-project.Qt.QImageIOHandlerFactoryInterface",
    "MetaData": {
        "Keys": [
            "jpg",
            "jpeg"
        ],
        "MimeTypes": [
            "image/jpeg",
            "image/jpeg"
        ]
    },
    "className": "QJpegPlugin",
    "debug": false,
    "version": 330499
}
This seems like the most relevant portion, but again it seems to have succeeded in recognizing jpgs, so I don't see a problem. Let me know if there's anything else I should be looking for. I'm not seeing parts that are specific to my code, it just looks like Qt's background stuff for loading certain plugins.

This chunk also seems like it recognized that it needs jpgs:

Got keys from plugin meta data ("svg", "svgz")
QFactoryLoader::QFactoryLoader() checking directory path "/home/pi/Desktop/program/imageformats" ...
loaded library "/usr/lib/aarch64-linux-gnu/qt5/plugins/imageformats/libqjpeg.so"
QFactoryLoader::QFactoryLoader() checking directory path "/usr/lib/aarch64-linux-gnu/qt5/plugins/accessible" ...
QFactoryLoader::QFactoryLoader() checking directory path "/home/pi/Desktop/program/accessible" ...
QFactoryLoader::QFactoryLoader() checking directory path "/usr/lib/aarch64-linux-gnu/qt5/plugins/accessiblebridge" ...
QFactoryLoader::QFactoryLoader() checking directory path "/home/pi/Desktop/program/accessiblebridge" ...
QFactoryLoader::QFactoryLoader() checking directory path "/usr/lib/aarch64-linux-gnu/qt5/plugins/iconengines" ...
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/aarch64-linux-gnu/qt5/plugins/iconengines/libqsvgicon.so"
Found metadata in lib /usr/lib/aarch64-linux-gnu/qt5/plugins/iconengines/libqsvgicon.so, metadata=
{
    "IID": "org.qt-project.Qt.QIconEngineFactoryInterface",
    "MetaData": {
        "Keys": [
            "svg",
            "svgz",
            "svg.gz"
        ]
    },
    "className": "QSvgIconPlugin",
    "debug": false,
    "version": 330499
}

SGaist

That part looks good.
And I am guessing that it also says somewhere that the plugin was successfully loaded ?

The next step: did you check that the file is a valid jpeg ?

Kelenyche

Yea I can't find any problems in that debug info, no failures or anything.

I actually created the jpg in paint, it's literally just some scribbles. I have a .png I've been trying to use but no luck, so I wanted to try a different file type, with a differently formatted name to see if anything would change. I don't see how it wouldn't be a valid jpg, having just saved it from paint?

SGaist

Getting really strange...

No special characters in the path ? Not that it should play a role but who knows...

If you use QFile to open it, does it work ?

Kelenyche

Nope, the full path is: "/home/pi/Desktop/program/extra/images/scribble.jpg"

Alright I'm not familiar with QFile, but I used:

QFile testfile("/home/pi/Desktop/program/extra/images/scribble.jpg");

if(testfile.exists())
{
    outputTextBox->append("opened the file");
}

and it seems to have succeeded.
will testfile.exists() do the same test as isNull in QImage?

SGaist

No, it just confirms the existence of the file not whether it is valid.

Use the QImageReader class. It's lower level but may give you more information about what is going wrong.

Kelenyche

Ok so I have:

QImage testimage(64,64,QImage::Format_RGB32);
QImageReader reader("/home/pi/Desktop/program/extra/images/scribble.jpg");

if(reader.read(&testimage))
{
    outputTextBox->append("opened the file");
}

It's telling me the file is good now...

SGaist

Within the same application, do you have the same issue if you load a different QImage variable using the usual way ?

Kelenyche

I think so - I initially tried with the png, then switched to the jpg, and neither one is doing what I expect, so I think they're both coming in null with QImage.

Kelenyche

Ooh, I think I'm onto something. So the way my code was implemented, I read the image, did some resizing and stuff, then was checking if it was null after that, since my textbox wasn't added to the gui yet. I switched to QDebug since that's more handy, and moved that code up - it looks like my resizing stuff was causing the image to become null? I now have a not-resized image that's painting onto the background of the graph!

I'll post my full fix once I figure this out. :)

Kelenyche

Yep, this chunk is causing it to become null:

    int chart_width = static_cast<int>(splineChart->plotArea().width());
    int chart_height = static_cast<int>(splineChart->plotArea().height());
    int view_width = static_cast<int>(splineChartView->width());
    int view_height = static_cast<int>(splineChartView->height());
    scribble = scribble.scaled(QSize(chart_width, chart_height));

So in QImage::scaled, if width or height are zero, it returns a null QImage. For some reason, my width and height are returning 0, so that makes sense. Sorry for the wild goose chase, and thank you for your help!

Important: Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

Qt image resource is null