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

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  1. POHow to draw QGLFrameBufferObject onto the painter from within QGraphicsItem::paint()
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    <h1>Small version of my question</h1> <p>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)</p> <pre><code>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? } </code></pre> <p>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?</p> <h1>Big version of my question</h1> <h2>Situation sketch</h2> <p>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.</p> <p>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.</p> <p>Simplified example: (I don't call sourcepixmap in my draw() method as it is not needed to demonstrate my problem)</p> <pre><code>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-&gt;drawImage(QPoint(0,0), pbuffer-&gt;toImage(),rect); } </code></pre> <h2>Actual problem</h2> <p>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()?</p> <p>I know I also can copy the pbuffer to a texture by using :</p> <pre><code>GLuint dynamicTexture = pbuffer.generateDynamicTexture(); pbuffer.updateDynamicTexture(dynamicTexture); </code></pre> <p>but I have no idea on how to get this texture onto the "painter". </p>
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    1. COI have the same problem here. I don't think that QPainter can draw OpenGL textures, since QPainter abstracts the paint device to draw on, which doesn't have to use OpenGL. Therefore, I don't think that there is a pure QPainter based solution which doesn't need to copy the image data from the graphics card into your process (software). I also don't think (but I'm not very sure) that converting to QPixmap directly solves this problem. But maybe I'm wrong? Thus the bounty. :)
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