QOpenGLWidget::grabFrameBuffer doesn't work with mutli-sampling

belgiansteve

Hi,

Unless I'm doing something wrong, the QOpenGLWidget::grabFrameBuffer doesn't work when multi-sampling is used. This example produces the issue. The image that is saved should be all red, but it's not.

#include <QApplication>
#include <QOpenglWidget>

class MyOpenGLWidget : public QOpenGLWidget
{
public:
	MyOpenGLWidget()
	{
		QSurfaceFormat f = format();
		f.setSamples(4);
		setFormat(f);
	}

	void initializeGL()
	{
		glClearColor(1.f, 0.f, 0.f, 1.f);
	}

	void resizeGL(int w, int h)
	{
		glViewport(0, 0, w, h);
	}

	void paintGL()
	{
		glClear(GL_COLOR_BUFFER_BIT);
	}

	void mouseReleaseEvent(QMouseEvent* ev)
	{
		QImage im = grabFramebuffer();
		im.save("test.bmp", "BMP");
	}
};

int main(int argc, char **argv)
{
    QApplication app(argc, argv);

	MyOpenGLWidget wnd;
	wnd.resize(400, 300);
	wnd.show();

    return app.exec();
}

If I don't call setSamples(4) it works fine. Any ideas on what might be going on here?

Thanks,

Steve

Chris Kawa

As stated in the docs this operation uses glReadPixels. It does not support reading from a multisample FBO.
You must first blit it to another, non-multisample FBO and read from there. See glBlitFramebuffer.

belgiansteve

Ok, thanks for the reply!

Steve