these questions all have some overlap, so i’ll be answering them together!
First off, these are all questions that ultimately come down to understanding light.
With light theres a few things to understand - i explain light in a little more depth here: http://helpfulharrie.tumblr.com/post/102124105196/when-coloring-a-figurefaceobject-how-do-you
so im just going to quickly refresh!
point 1: light travels in a straight line, until it hits something and bounces off
this is how we get shadows, as shadows are the absence of light.
Light bouncing into our eyes is how we see!
point 2: light will bounce indefinitely - but with diminishing return.
every time light hits something, some of it is absorbed by the object. Therefore, bounced light is always weaker than the source.
Think about how when dropping a ball, each bounce is smaller than the one before
Because it’s weaker than the source, you can’t see it where the source light is shining - as the source light overpowers it.
However, if this light bounces into the shadows where the source light can’t reach, you can see it!
Also, the further light travels, the weaker it will get. this tutorial explains this neatly [http://helpfulharrie.tumblr.com/post/38673324830/]!
Now then to address the first question:
The core shadow is simply the true shadow where the bounced light hasnt reached,
for a real life example, here’s just a living room photo from ikea lmao [http://www.ikea.com/gb/en/catalog/categories/departments/living_room/]
and here are the core shadows on that sofa
can you see how they’re in places the light can’t reach?
now then for the next question!
what you are asking about is the bounce light I discussed earlier - that lighter edge before the dark linework is actually bounced light!
I’d recommend reading this tutorial to see how they do it: [http://helpfulharrie.tumblr.com/post/55506603265]
If you just add light before every dark line without thinking about a light source, it’ll look very flat. But if you determine a lightsource first, that should help!
lastly - with cel shading, with simple cel shading you’re just focused on that core shading without any bounce light
As long as you get that single block shadow down, it’ll be fine. From there, you can add more layers of shading [http://helpfulharrie.tumblr.com/post/55502345448/]
and you can use gradients [http://helpfulharrie.tumblr.com/post/38673324830/] [http://tamarar.deviantart.com/art/Gradient-Shading-Tutorial-76002002]
But as long as you get a grasp of how light works, your celshading should look much better.
For further reading, I recommend this tutorial: [http://androidarts.com/art_tut.htm]
I hope that helps! And if you’d like, you can leave me a tip here [https://ko-fi.com/A5544ZX] so i can keep running the blog!