OpenGL + QT: render to texture and display it back

Question

After some trouble I've managed to correctly render to texture inside a Frame Buffer Object in a Qt 4.8 application: I can open an OpenGL context with a QGLWidget, render to a FBO, and use this one as a texture.

Now I need to display the texture rendered in a QPixmap and show it in some other widget in the gui. But.. nothing is shown.

Those are some pieces of code:

// generate texture, FBO, RBO in the initializeGL
glGenTextures(1, &textureId);
glBindTexture(GL_TEXTURE_2D, textureId);
glGenFramebuffers(1, &fboId);
glBindFramebuffer(GL_FRAMEBUFFER, fboId);
glGenRenderbuffers(1, &rboId);
glBindRenderbuffer(GL_RENDERBUFFER, rboId);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, TEXTURE_WIDTH, TEXTURE_HEIGHT);
glBindRenderbuffer(GL_RENDERBUFFER, 0);

// now in paintGL
glBindFramebuffer(GL_FRAMEBUFFER, fboId);
// .... render into texture code ....
if(showTextureInWidget==false) {
    showTextureInWidget = true;
    char *pixels;
    pixels = new  char[TEXTURE_WIDTH * TEXTURE_HEIGHT * 4];
    glReadPixels(0, 0, TEXTURE_WIDTH, TEXTURE_HEIGHT, GL_RGB, GL_UNSIGNED_BYTE, pixels);
    QPixmap qp = QPixmap(pixels);
    QLabel *l = new QLabel();
    // /* TEST */ l->setText(QString::fromStdString("dudee"));
    l->setPixmap(qp);
    QWidget *d = new QWidget;
    l->setParent(d);
    d->show();
}
glBindFramebuffer(GL_FRAMEBUFFER, 0); // unbind
// now draw the scene with the rendered texture

I see the Widget opened but.. there is nothing inside it. If I decomment the test line.. I see the "dudee" string so I know that there is a qlabel but.. no image from the QPixmap.

I know that the original data are ´unsigned char´ and I'm using ´char´ and I've tried with some different color parameters (´GL_RGBA´, ´GL_RGB´ etc) but I don't think this is the point.. the point is that I don't see anything..

Any advice? If I have to post more code I will do it!

Edit:

I haven't posted all the code, but the fact I'd like to be clear is that the texture is correctly rendered as a texture inside a cube. I'm just not able to put it back in the cpu from gpu

Edit 2:

Thanks to the peppe answer I found out the problem: I needed a Qt object that accept as a constructor some raw pixels data. Here is the complete snippet:

    uchar *pixels;
    pixels = new uchar[TEXTURE_WIDTH * TEXTURE_HEIGHT * 4];
    for(int i=0; i < (TEXTURE_WIDTH * TEXTURE_HEIGHT * 4) ; i++ ) {
        pixels[i] = 0;
    }

    glBindFramebuffer(GL_FRAMEBUFFER, fboId);
    glReadPixels( 0,0,  TEXTURE_WIDTH, TEXTURE_HEIGHT, GL_RGBA, GL_UNSIGNED_BYTE, pixels);
    glBindFramebuffer(GL_FRAMEBUFFER, 0);

    qi = QImage(pixels, TEXTURE_WIDTH, TEXTURE_HEIGHT, QImage::Format_ARGB32);
    qi = qi.rgbSwapped();

    QLabel *l = new QLabel();
    l->setPixmap(QPixmap::fromImage(qi));
    QWidget *d = new QWidget;
    l->setParent(d);
    d->show();

Answer 1

Given that that's not all of your code and -- as you say -- the texture is correctly filled, then there's a little mistake going on here:

glReadPixels(0, 0, TEXTURE_WIDTH, TEXTURE_HEIGHT, GL_RGB, GL_UNSIGNED_BYTE, pixels);
QPixmap qp = QPixmap(pixels);

The QPixmap(const char *) ctor wants a XPM image, not raw pixels. You need to use one of the QImage ctors to create a valid QImage. (You can also pass ownership to the QImage, solving the fact that you're currently leaking pixels...)

Once you do that, you'll figure out that

1. the image is flipped vertically, as OpenGL has the origin in the bottom left corner, growing upwards/rightwards, while Qt assumes origin in the top left, growing to downwards/rightwards;

2. the channels might be swapped -- i.e. OpenGL is returning data with the wrong endianess. I don't remember in this case if using glPixelStorei(GL_PACK_SWAP_BYTES) or GL_UNSIGNED_INT_8_8_8_8 as the type may help, eventually you need to resort to a CPU-side loop to fix your pixel data :)