OSG render scene into image - opengl

Question

I trying to render an OSG scene into a image in my Qt program. Refer to the example of SnapImageDrawCallback(https://www.mail-archive.com/osg-users#lists.openscenegraph.org/msg45360.html).
class SnapImageDrawCallback : public osg::CameraNode::DrawCallback {
public:
SnapImageDrawCallback()
{
_snapImageOnNextFrame = false;
}
void setFileName(const std::string& filename) { _filename = filename; }
const std::string& getFileName() const { return _filename; }
void setSnapImageOnNextFrame(bool flag) { _snapImageOnNextFrame = flag;}
bool getSnapImageOnNextFrame() const { return _snapImageOnNextFrame; }
virtual void operator () (const osg::CameraNode& camera) const
{
if (!_snapImageOnNextFrame) return;
int x,y,width,height;
x = camera.getViewport()->x();
y = camera.getViewport()->y();
width = camera.getViewport()->width();
height = camera.getViewport()->height();
osg::ref_ptr<osg::Image> image = new osg::Image;
image->readPixels(x,y,width,height,GL_RGB,GL_UNSIGNED_BYTE);
if (osgDB::writeImageFile(*image,_filename))
{
std::cout << "Saved screen image to `"<<_filename
<<"`"<< std::endl;
}
_snapImageOnNextFrame = false;
}
protected:
std::string _filename;
mutable bool _snapImageOnNextFrame;
};
I set this as a the osg::Viewer's camera's FinalDrawCallback, but I failed with a blank image, and get this warning "Warning: detected OpenGL error 'invalid operation' at start of State::apply()" when invoke image->readPixels, My osgViewer::Viewer in embedded in QQuickFramebufferObject. Can any one give some suggestions?

Answer 1

Not sure to give you the right pointer, you should provide more details about your setup and what you're after.
As a general note, if you're trying to render with OSG into a QtQuick widget the best approach is to have osg to render to an FBO in a separate shared GL context, and copy the FBO contents back the qtquick widget.
I had tested this approach some times ago, see code here:
https://github.com/rickyviking/qmlosg
Another similar project here: https://github.com/podsvirov/osgqtquick

Answer 2

you can use pbo
ext->glGenBuffers(1, &pbo);
ext->glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, pbo);
ext->glBufferData(GL_PIXEL_PACK_BUFFER_ARB, _width*_height*4, 0, GL_STREAM_READ);
glReadPixels(0, 0, _width, _height, _pixelFormat, _type, 0);
GLubyte* src = (GLubyte*)ext->glMapBuffer(GL_PIXEL_PACK_BUFFER_ARB,
GL_READ_ONLY_ARB);
if(src)
{
memcpy(image->data(), src, _width*_height*4);
ext->glUnmapBuffer(GL_PIXEL_PACK_BUFFER_ARB);
}
ext->glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, 0);