OpenGL frame buffer object problem

Hi, everyone I'm now learning the PPT - GPGPU Programming which is written by Prof. Shih-hsuan Hsu of NTU. In the second FBO demo, the program first render the image into a texture and then map the texture to a quad to display it. The problem I met is: I change the glut resize callback function as followed to fit the window size: void resize_callback(int width, int height) { glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluOrtho2D(0, WIN_WIDTH, 0, WIN_HEIGHT ); glViewport(0, 0, width, height);// origin is WIN_WIDTH & WIN_HEIGHT } But when I resize the window the final image does not fit well. When I magnify the window the rendered image is only part of the original image. When I minify the window the rendered image is much less than the window. I hope the image is just the same width and height with the window. But it does not work. On the other side, when I use a texture that is read from disk the code works fine. Is there anything else to set the texture? Am I clear? Could anyone help me? Thank you very much.