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

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.