How to draw QGLFrameBufferObject onto the painter from within QGraphicsItem::paint()

Question

Small version of my question

In a QGraphicsItem::paint() function I have a QGLFrameBufferObject. How do I get it on the paintdevice of the painter that is passed as an argument? (provided that the QGraphicsItem is in a scene that is being rendered by a QGraphicsView that has a QGLWidget as viewport => painter is using opengl engine)

QGraphicsItem::paint(QPainter* painter, ...)
{
    QGLFramebufferObject fbo;
    QPainter p(fbo);
    ... // Some painting code on the fbo
    p.end();

    // What now? How to get the fbo content drawn on the painter?
}

I have looked at the framebufferobject and pbuffer examples provided with Qt. There the fbo/pbuffer is drawn in a QGLWidget using custom opengl code. Is it possible to do the same thing within a paint() method of a QGraphicsItem and take the position of the QGraphisItem in the scene/view into account?

Big version of my question

Situation sketch

I have a QGraphicsScene. In it is an item that has a QGraphicsEffect (own implementation by overriding draw() from QGraphicsEffect). The scene is rendered by a QGraphicsView that has a QGLWidget as viewport.

In the QGraphicsEffect::draw(QPainter*) I have to generate some pixmap which I then want to draw using the painter provided (the painter has the QGLWidget as paintdevice). Constructing the pixmap is a combination of some draw calls and I want these to be done in hardware.

Simplified example: (I don't call sourcepixmap in my draw() method as it is not needed to demonstrate my problem)

class OwnGraphicsEffect: public QGraphicsEffect
{
     virtual void draw(QPainter* painter);
}

void OwnGraphicsEffect::draw(QPainter* painter)
{
    QRect rect(0,0,100,100);
    QGLPixelBuffer pbuffer(rect.size(), QGLFormat(QGL::Rgba));
    QPainter p(pbuffer);
    p.fillRect(rect, Qt::transparent);
    p.end();

    painter->drawImage(QPoint(0,0), pbuffer->toImage(),rect);
}

Actual problem

My concerns are with the last line of my code: pbuffer->toImage(). I don't want to use this. I don't want to have a QImage conversion because of performance reasons. Is there a way to get a pixmap from my glpixelbuffer and then use painter->drawpixmap()?

I know I also can copy the pbuffer to a texture by using :

GLuint dynamicTexture = pbuffer.generateDynamicTexture();
pbuffer.updateDynamicTexture(dynamicTexture);

but I have no idea on how to get this texture onto the "painter".

Answer 1

Extending leemes' answer, here is a solution which can also handle multisample framebuffer objects.

First, if you want to draw on a QGLWidget, you can simply use the OpenGL commands leemes suggested in his answer. Note that there is a ready-to-use drawTexture() command available, which simplifies this code to the following:

void Widget::drawFBO(QPainter &painter, QGLFramebufferObject &fbo, QRect target)
{
    painter.beginNativePainting();
    drawTexture(target, fbo.texture());
    painter.endNativePainting();
}

To draw multisample FBOs, you can convert them into non-multisample ones using QGLFramebufferObject::blitFramebuffer (Note that not every hardware / driver combination supports this feature!):

if(fbo.format().samples() > 1)
{
    QGLFramebufferObject texture(fbo.size()); // the non-multisampled fbo
    QGLFramebufferObject::blitFramebuffer(
            &texture, QRect(0, 0, fbo.width(), fbo.height()),
            &fbo, QRect(0, 0, fbo.width(), fbo.height()));
    drawTexture(targetRect, texture.texture());
}
else
    drawTexture(targetRect, fbo.texture());

However, as far as I know, you can't draw using OpenGL commands on a non-OpenGL context. For this, you first need to convert the framebuffer to a (software) image, like a QImage using fbo.toImage() and draw this using your QPainter instead of the fbo directly.

Answer 2

I think I figured it out. I use the QPainter::beginNativePainting() to mix OpenGL commands in a paintEvent:

void Widget::drawFBO(QPainter &painter, QGLFramebufferObject &fbo, QRect target)
{
    painter.beginNativePainting();

    glEnable(GL_TEXTURE_2D);
    glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
    glBindTexture(GL_TEXTURE_2D, fbo.texture());
    glBegin(GL_QUADS);
    glTexCoord2d(0.0,1.0); glVertex2d(target.left(),      target.top());
    glTexCoord2d(1.0,1.0); glVertex2d(target.right() + 1, target.top());
    glTexCoord2d(1.0,0.0); glVertex2d(target.right() + 1, target.bottom() + 1);
    glTexCoord2d(0.0,0.0); glVertex2d(target.left(),      target.bottom() + 1);
    glEnd();

    painter.endNativePainting();
}

I hope this will also work in the paintEvent of a QGraphicsItem (where the QGraphicsView uses a QGLWidget as the viewport), since I only tested it in QGLWidget::paintEvent directly.

There is, however, still the following problem: I don't know how to paint a multisample framebuffer. The documentation of QGLFramebufferObject::texture() says:

If a multisample framebuffer object is used then the value returned from this function will be invalid.