This is the collection of Math that I find useful as an animator that sometimes has to deal with geometry time to time.

It's Photoshop technique to retouch the image. This will smooth the uneven skin color but still preserve the texture. Here is the replication of the technique in After Effects and my explanation how this works.

By knowing how they work, you can choose the right one or when something doesn't look right you know how to fix. Also, It prevents you from unnecessarily stacking all bunch of layers, gradient + color + levels + curves + again + again + + +. From my professional experience, I've seen these so many time.