QGraphicsView + native opengl painting -> rendering to image

[Stef]

I have a QGraphicsView and on this view I place several subclassed QGraphicsItems which implement their own paint method. Now I want to render the graphicsview/scene to an image and save this to disk. I use the following method to render the scene to an image:

QPainter *pngPainter = new QPainter();
QImage *image = new QImage(QSize(width,height), QImage::Format_ARGB32_Premultiplied);
 
pngPainter->begin(image);
  pngPainter->setRenderHint(QPainter::Antialiasing);
  _ui->graphicsView->scene()->render(pngPainter);
pngPainter->end();
image->save(fileName);
 
delete pngPainter;
delete image;

QPainter *pngPainter = new QPainter();
QImage *image = new QImage(QSize(width,height), QImage::Format_ARGB32_Premultiplied);

pngPainter->begin(image);
  pngPainter->setRenderHint(QPainter::Antialiasing);
  _ui->graphicsView->scene()->render(pngPainter);
pngPainter->end();
image->save(fileName);

delete pngPainter;
delete image;

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This works perfectly when I use QPainter commands in the paint procedure of the QGraphicsItems, like so (block is a QRectF):

void CNodeGraphicsItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
   // color setup
   painter->setPen(Qt::red);
   painter->setBrush(Qt::red);
 
   painter->drawRect(block);
}

void CNodeGraphicsItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
   // color setup
   painter->setPen(Qt::red);
   painter->setBrush(Qt::red);
  
   painter->drawRect(block);
}

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However, If (for performance reasons and in order to use shaders) I use native opengl commands, like so:

void CNodeGraphicsItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
   painter->beginNativePainting();
   glBegin(GL_QUADS);
     setGLColor(Qt::red); // Own function to set glColor3f;
     glTexCoord2f (0.0, 0.0);glVertex2d(block.left(),block.top());
     glTexCoord2f (1.0, 0.0);glVertex2d(block.right(),block.top());
     glTexCoord2f (1.0, 1.0);glVertex2d(block.right(),block.bottom());
     glTexCoord2f (0.0, 1.0);glVertex2d(block.left(),block.bottom());
  glEnd();
  painter->endNativePainting();
}

void CNodeGraphicsItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
   painter->beginNativePainting();
   glBegin(GL_QUADS);
     setGLColor(Qt::red); // Own function to set glColor3f;
     glTexCoord2f (0.0, 0.0);glVertex2d(block.left(),block.top());
     glTexCoord2f (1.0, 0.0);glVertex2d(block.right(),block.top());
     glTexCoord2f (1.0, 1.0);glVertex2d(block.right(),block.bottom());
     glTexCoord2f (0.0, 1.0);glVertex2d(block.left(),block.bottom());
  glEnd();
  painter->endNativePainting();
}

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It no longer works and the output is an entire black image. The rendering on the QGraphicsView gives the expected result though (a red rectangle). Any ideas on how to solve this / make it work with native opengl commands?

I know I can grab the framebuffer:

QImage img = ((QGLWidget*)_ui->graphicsView->viewport())->grabFrameBuffer(true);
  img.save(fileName, "PNG", 100);

QImage img = ((QGLWidget*)_ui->graphicsView->viewport())->grabFrameBuffer(true);
  img.save(fileName, "PNG", 100);

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This works, however the output image is only the size of the graphicsView, I want to be able
to output high-res images.

[wysota]

Try rendering to a gl pixel (or frame) buffer and then converting the result to a QImage. By the way, never create QImage instances on the heap, it makes no sense.

[Stef]

Thank you for pointing me into the right direction. I now render to an offscreen framebuffer and convert that to QImage. For completeness here is my solution:

QGLFramebufferObjectFormat format;
 format.setSamples(16);
 
 QGLFramebufferObject renderFbo(4096, 4096);
 
 QPainter fboPainter(&renderFbo);
 
 fboPainter.setRenderHint(QPainter::Antialiasing);
 fboPainter.setRenderHint(QPainter::HighQualityAntialiasing);
 _ui->graphicsView->scene()->render(&fboPainter);
 fboPainter.end();
 
 renderFbo.toImage().save(fileName, "PNG", 100);

QGLFramebufferObjectFormat format;
 format.setSamples(16);

 QGLFramebufferObject renderFbo(4096, 4096);

 QPainter fboPainter(&renderFbo);

 fboPainter.setRenderHint(QPainter::Antialiasing);
 fboPainter.setRenderHint(QPainter::HighQualityAntialiasing);
 _ui->graphicsView->scene()->render(&fboPainter);
 fboPainter.end();

 renderFbo.toImage().save(fileName, "PNG", 100);

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