Underlighting (aka "Bipack Glow") is an animation technique that caught on in the 1980's, although it dates back much further.
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.
There were, however, two big drawbacks to this technique. First of all, 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; as a result, underlighting would typically be relegated to static background elements if needed for a prolonged portion of the animation, with underlit moving objects typically being on the smaller side to reduce the workload for the animators. And second, adding texture to glowing objects was much more difficult compared to hand-painted glow effects, though not outright impossible; typically texture was added by placing a translucent foil or a glass tube filled with a mix of soap/shampoo, water, and glitter between the background and the backlight. This second drawback meant the technique was used primarily for amorphous objects such as "energy beams" or lightning.
Underlighting eventually faded out as digital ink and paint software became more accessible to animation studios in the late 1990's and early 2000's, with these computer programs allowing animators to more easily produce glow effects and give them levels of texture and flexibility not previously possible with cel animation. Among live-action producers, the rise of CGI also rendered underlighting obsolete for similar reasons in regards to producing energy effects and whatnot, though with the added benefit of CGI allowing these same kinds of effects to more believably mesh with the live-action footage. However, in the context of hand-drawn animation, creating the exact kinds of glow effects underlighting provided through digital technology is excruciatingly difficult, due to the unexpectedly complex nature of real-world light requiring a large amount of legwork in realistically replicating tapering fringes on a frame-by-frame basis (to say nothing of how light refracts against foils and glitter fluid). Consequently, underlighting-esque effects have largely died off in this field, rendering it a de-facto Forgotten Trope.
- The 1984 anime adaptation of Fist of the North Star frequently made use of this, both textured and untextured, for a wide number of purposes, ranging from censoring blood to providing extra "oomph" for battle auras and energy effects. The 1986 recap film further ups the ante with it, using backlit foil for battle auras in addition to making greater use of it for ambient lighting and effects.
- The 1988 film adaptation of AKIRA uses underlighting to depict sunlight shining through a window in the correctional school Kaneda attends, replicating the effect of the camera focusing on the objects in front of and around the window rather than on the view through the glass. Underlighting is also frequently used throughout the film for a large variety of other lighting effects, from lights on bikes and neon signs to the bright light that envelops Neo-Tokyo when the esper children reconnect with Akira's remains.
- 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.
- 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 Tinker Bell 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...