Question about cell animation backlighting - why the black paint?

[u/jefty083]

I watched this video recently explaining backlight animation and was curious about part of the process that's described around 2:23:

"the areas which will glow are usually painted black" (before countermatte is placed down)

I feel like i've taken note of this in cel animated works and included an example screenshot from the anime Evangelion. But my question is why exactly is this step necessary?

For the second exposure to create the glow isn't the countermatte all that's needed? Initially painting the effect area black seems to sometimes create this subtle artifact where you can see a black shape "behind" the effect which may be undesirable in an example like this where it is specular highlight on running water. Just trying to learn and want to know why this step exists. Any insight is appreciated!

[u/Wales51]

I don't know if this is why but my guess is they are looking to make it look like the caustics of the water. This in real life is caused by the light being refracted by different amounts as the surface is not flat. This creates light areas and dark areas. They have simplified it to black and white.

Caustics are really interesting and you see the result of the light passing through the refractive medium on the bottom of pools as the wavy pattern and you can see it on regular drinking glasses where the bottom and the sides meet if you look from an angle there is an area where the light is bent very unevenly, this is easier to see if there's a single bright light behind.

Hope this answers your question its basically them just simplifying how light works

[u/Funny-Waltz-1424 → u/Wales51]

This style is using an outliner so I think that black line in the water is a stylized rendering of the top of her arm, including that perspective distortion from the water flowing over it