QVideoFrame::map fails on Ubuntu Touch

s710

I am working on an app for Ubuntu Touch, which shall be able to read QR codes. It relies on a custom video filter subclass, however I am unable to extract single images out of the QVideoFrame instances. The QVideoFrame::map will fail. My research indicates that instead of direct access I must use an OpenGL handle to read a 2D texture, similar to those cases:

So I am trying to implement a QVideoFrame::map replacement using OpenGL functions.

However, although the approach seems to be clear from the above examples, I cannot seem to get a valid QImage ouf of the QVideoFrame. I have a number of uncertainties; I have never worked with OpenGL, and I am also not too familiar with image formats. In fact, I am not working on my own implementation, but instead trying to make the qzxing library work in my app. I have it working under SailfishOS already (where QVideoFrame::map just works), and I am porting my app to Ubuntu Touch.

Currently I have this:

QImage img(frame.width(), frame.height(), QImage::Format_RGBA8888);

GLuint textureId = frame.handle().toUInt();
QOpenGLContext *ctx = QOpenGLContext::currentContext();
QOpenGLFunctions *f = ctx->functions();
GLuint fbo;
f->glGenFramebuffers(1, &fbo);
GLuint prevFbo;
f->glGetIntegerv(GL_FRAMEBUFFER_BINDING, (GLint *) &prevFbo);
f->glBindFramebuffer(GL_FRAMEBUFFER, fbo);
f->glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, textureId, 0);
f->glReadPixels(0, 0, frame.width(), frame.height(), GL_RGBA, GL_UNSIGNED_BYTE, img.bits());

const QString path = QStandardPaths::writableLocation(QStandardPaths::PicturesLocation) + "/qrtest/test.png";
qDebug() << "Saving image  at:" << path << img.save(path) << " size: " << img.size() << "/" << img.sizeInBytes();

When I try to save the image to disk (in order to check how it looks, if it even contains anything at all), QImage::save will return false and will print:

Saving image at: "/home/phablet/Pictures/qrtest/test.png" false  size:  QSize(1920, 1088) / 8355840

The input video stream seems to be of the format QVideFrame::Format_RGB32, at least this is what frame.pixelFormat() reports.
Does this even matter, or will GL_RGBA work anyway, and I just have to make sure img is of the correct format, so the internal buffer is large enough? Would therefore be QImage::Format_RGBA8888 correct?

Why is the resulting image seemingly invalid? How can I fix this?