Shooting through glass

I photographed this clouded leopard in the Nashville, Tennessee zoo, and the cat's enclosure is behind glass.  There is no other option in taking this picture but to shoot through the large pane of glass, but as you can see there is no degradation of image quality.  Shooting through glass isn't a problem as long as (1) the glass is clean, and (2) the lens axis is close to perpendicular to the plane of the glass.  If the lens is oblique to the glass, image quality can suffer.  I used flash for this shot, and that meant that the flash had to be used off-camera to prevent unwanted reflections.