Solved 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