Linear filtering when scaling QNanoImages from fbos

[daljit97]

I am trying to use QNanoImage to perform some zooming operations on a QOpenGLFramebufferObject. I can see that when I load an image from a file and scale the image (using painter->scale), linear filtering is used by default and the image is scaled "smoothly" (similar to the QML standard smooth: true). Now when I try to do the same thing using an fbo, it seems that nearest filtering is always used: the image appears pixelated. The code I am using is the following:

            QNanoImage cacheImage = QNanoImage::fromFrameBuffer(m_cacheBuffer.get(),
                                                                QNanoImage::FLIPY |
                                                                QNanoImage::GENERATE_MIPMAPS);
            painter->scale(m_contentScale);
            painter->drawImage(cacheImage,0,0);

So I decided to blit the fbo directly using:

            QOpenGLFramebufferObject::blitFramebuffer(fbo,QRect(0,0,1000,1000),
                                                      m_cacheBuffer.get(), QRect(0,0,width()/m_contentScale, height()/m_contentScale),
                                                      GL_COLOR_BUFFER_BIT,
                                                      GL_LINEAR);

which actually uses linear filtering and the image appears "smoothly" scaled (it works only if I explicity set GL_COLOR_BUFFER_BIT as the GLbitfield). I don't have too much experience with OpenGL, so I am not sure why would that be the case. Could this be due to format that my fbo is created with?