Create a loupe for one aplication Artoolkit

I am creating a magnifying glass in an application Artoolkit. But I am faced with some problems.
A magnifying glass should move in a vertical orientation from top to bottom.
So that when a model associated with a brand is rendered a magnifying glass will follow this model leaving a path from top to bottom. :frowning:

the code:

glRasterPos2i (recenter_x, recenter_y);
glPixelZoom ( 2, 2 );
glCopyPixels (recenter_x, recenter_y, 100, 100, GL_COLOR);

glReadBuffer(GL_BACK); //selecção do buffer para a leitura
glDrawBuffer(GL_FRONT); //selecção do buffer para a escrita

glDrawPixels(100, 100, GL_RGB, GL_UNSIGNED_BYTE, checkImage);


Can you help me please?