bool invert = false;

        QMatrix4x4 viewMatrix       = m_camera->viewMatrix();
        QMatrix4x4 modelViewMatrix  = viewMatrix * m_modelMatrix;
        QMatrix4x4 modelViewProject = m_camera->projectionMatrix() * modelViewMatrix;
        QMatrix4x4 inverted         = m_viewportMatrix * modelViewProject;

        inverted = inverted.inverted(&invert);

        float posZ;
        int   posY = (int)m_viewportSize.y() - (int)mouse_position.y() - 1;

        m_funcs->glReadPixels((int)mouse_position.x(), posY, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &posZ);

        QVector4D clickedPointOnScreen(mouse_position.x(), posY, posZ, 1.0f);
        QVector4D clickedPointIn3DOrgn = inverted * clickedPointOnScreen;

        clickedPointIn3DOrgn = clickedPointIn3DOrgn / clickedPointIn3DOrgn.w();

        terrain_pos = clickedPointIn3DOrgn.toVector3DAffine();