Scenes such as this where the light is reflecting on the water are accomplished through a process called "backlighting", "underlighting" or "bipack glow". To create the effect a matte is cut and light is shone through the holes.

The above clip uses the second Ranma film and the fourth Urusei Yatsura film as examples. This was a common technique in the late 1980s and early 1990s. We can see it used for the sunlight in a Ranma OVA as well as the light from a lantern in Ranma movie 2 and more water effects.

The fun thing was that cel paint only had to be so thick on a normal shot but for backlighting you needed the color opaque cause the back was painted black as well or the light would absolutely show through. Such an involved process

When I was growing up I used to think animation was some kind of magic, and to be honest I still kind of do.

I'VE ALWAYS WANTED TO KNOW! THANK YOU! <3

@C_4_D_3 I alllllllways wondered how this was done! So cool!! Thanks!!

こういう制作の裏側もっと見たい アナログ時代の神業・・・

Wish this technique was still used water has never been more striking to look at again

Technology went backwards after this.

@rattibha unroll

I really miss seeing that type of lighting. Nothing new even comes close.