Underlighting (aka "Bipack Glow") is an animation technique that caught on in the 1980s, although it dates back much further. CGI animation eventually rendered it obsolete.
Some parts of an animated scene may need to glow. Traditionally, a glow was painstakingly drawn by an animator, who had to pay careful attention to get the fringe colors just right to make it convincing. Even in the best cases, a hand-drawn glow still didn't look very "glowy."
The solution was to cut a hole in the background in the shape you wanted the glowing object to have, and then project a bright colored light from underneath the scene. The result was a rather awesome-looking glowing object, with tapering fringes that looked exactly as you'd expect a brightly glowing object to look.
The two drawbacks to this technique were: (1) It was somewhat labor-intensive, as the hole in the animation background would have to move to track where the glow was supposed to be frame by frame; and (2) glowing objects could have no surface texture, they would be just a brightly glowing area. This second drawback meant the technique was used primarily for amorphous objects such as "energy beams" or lightning.
- Transformers: Generation 1 used underlighting frequently for the yellow Autobot gunshots, the purple Decepticon gunshots, and sometimes the red and blue glowing eyes.
- Space Ace used underlighting extensively for laser shots, glowing instrument panels, molten lava, the lightning effects streaming off the quarterstaves in the final scene, the glint on the surface of the mirror, et cetera.
- On The Secret Of NIMH underlighting was used for the glowing eyes on Nicodemus and the Great Owl.
- Arguably, the lightsabers in the live action Star Wars movies could be said to use a kind of underlighting; the boundary between pre-CGI special effect scene composition and underlit animation can be blurry.
- Also used for the Tron Lines in the original TRON, since having lights on the costumes was impractical at the time; ironically, this method ended up being so impractical that it was never repeated for another feature film. The writers referred to it as "backlit animation". The sequel drops it in favor of having light strips on the suits, which by then were much cheaper and more feasible to use.
- Underlighing was used on Peter Pan to make Tinkerbell glow.
- For The Powerpuff Girls, underlighting was used for the streaks the girls left behind as they fly away, and for when they use their heat vision.
- Underlighting was used in a very subtle manner in The Great Mouse Detective. It provided the glow for the coals when a character was smoking a pipe.
- The second season of Spider-Man and His Amazing Friends used underlighting for Iceman and Firestar's transformation sequences, and for Video Man's eyes.
- The Thief and the Cobbler: Just one of the very many animation special effects used to no end. Things made to glow this way include the golden balls, all fire, beams of light, Zigzag's flashy smoke bombs, brightly lit areas, and many more...