OpenGL : reading color buffer

Question

I attached 4 color buffers to a framebuffer and render in each of them. Each color buffer has the size of the window. I'm trying to read the color of the pixels of one of these color buffers using the coordinates of the mouse pointer.

mouse move event handler

void mouseMoveEvent(QMouseEvent *event)
{
    int x = event->pos().x();
    int y = event->pos().y();

    makeCurrent();
    glBindFramebuffer(GL_READ_FRAMEBUFFER, FBOIndex::GEOMETRY);
    {
        // I save the values I'm interested in in the attachment GL_COLOR_ATTACHMENT3
        // but I always get 0 from any other attachment I try
        glReadBuffer(GL_COLOR_ATTACHMENT3);

        QVector<GLubyte> pixel(3);
        glReadPixels(x, geometry().height() - y, 1, 1, GL_RGB, GL_UNSIGNED_BYTE, &(pixel[0]));

        QString PixelColor = QColor(pixel[0], pixel[1], pixel[2]).name();    
        qDebug() << PixelColor; // => always 0
    }
    glBindFramebuffer(GL_FRAMEBUFFER, defaultFramebufferObject());
    doneCurrent();
}

But for every color buffer I always read the value 0.

The color buffers are written correctly during the rendering phase, I tested each of them by displaying the texture to which they are attached. I also tested the pixel-reading selected by the mouse pointer to the default framebuffer and it works correctly.

Where am I wrong? Thanks!

EDIT

The seemingly strange thing is that, if I use a "dedicated" framebuffer, I can correctly read the values stored in the texture.

void mouseMoveEvent(QMouseEvent *event)
{
    int x = event->pos().x();
    int y = event->pos().y();

    GLuint fbo;

    makeCurrent();
    glGenFramebuffers(1, &fbo);
    glBindFramebuffer(GL_FRAMEBUFFER, fbo);
    {
        GLuint texture = textures[TextureIndex::COLOUR];
        glBindTexture(GL_TEXTURE_2D, texture);
        glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture, 0);

        QVector<GLubyte> pixel(3);
        glReadPixels(x, geometry().height() - y, 1, 1, GL_RGB, GL_UNSIGNED_BYTE, &(pixel[0]));

        QString PixelColor = QColor(pixel[0], pixel[1], pixel[2]).name();
        qDebug() << PixelColor; // => correct value
    }
    glBindFramebuffer(GL_FRAMEBUFFER, defaultFramebufferObject());
    glDeleteFramebuffers(1, &fbo);
    doneCurrent();
}

But clearly it seems useless to use another framebuffer when I already have one with exactly the information I need.

I also tried to read directly the values of the texture (as suggested by @Spektre), but also in this case I always get 0.

void mouseMoveEvent(QMouseEvent *event)
{
    int x = event->pos().x();
    int y = event->pos().y();

    makeCurrent();
    {
        GLuint texture = textures[TextureIndex::COLOUR];
        glBindTexture(GL_TEXTURE_2D, texture);
        glTexSubImage2D(GL_TEXTURE_2D, 0, x, geometry().height() - y, 1, 1, GL_RGB, GL_UNSIGNED_BYTE, &(pixel[0]));

        QString PixelColor = QColor(pixel[0], pixel[1], pixel[2]).name();
        qDebug() << PixelColor; // => always 0
    }
    doneCurrent();
}

Answer 1

My approach is correct, but I was not binding to the correct framebuffer.

FBOIndex::GEOMETRY is an enum value that I use to index a FBOs array, where I store all the framebuffer object names, so in general it is not a correct framebuffer object name.

I have defined a method addFBO(index) that creates a framebuffer and stores it at the position index in the FBOs array. The method returns the framebuffer object name of the generated framebuffer. If a framebuffer already exists at the position index, then the method simply returns the associated framebuffer object name.

So, by changing the code in the following way, I finally get the desired result.

void mouseMoveEvent(QMouseEvent *event)
{
    int x = event->pos().x();
    int y = event->pos().y();

    makeCurrent();
    glBindFramebuffer(GL_READ_FRAMEBUFFER, addFBO(FBOIndex::GEOMETRY));
    {
        glReadBuffer(GL_COLOR_ATTACHMENT3);

        QVector<GLubyte> pixel(3);
        glReadPixels(x, geometry().height() - y, 1, 1, GL_RGB, GL_UNSIGNED_BYTE, &(pixel[0]));

        QString PixelColor = QColor(pixel[0], pixel[1], pixel[2]).name();    
        qDebug() << PixelColor; // => correct value
    }
    glBindFramebuffer(GL_FRAMEBUFFER, defaultFramebufferObject());
    doneCurrent();
}