Basics of Shading
This tutorial is aimed for the basics of shading, unfortunately it wont make you a master right away, but I'll attempt to point out the important factors you need to focus on in order to get better. The same factors are present in all methods of shading, wheneven you're doing realistic shading, soft shading or cellshade. In the tutorial I'll use a cellshade technique and highlight a few handy program specific features for cellshading as well.
Shading is an important part of drawing to make it look three dimensional. Sketching and linearting also include shading - aside from defining the edges of two materials, your lines are there to define the darkest areas of your drawing as well = shading.
That being said, shading is very heavily tied the ability to sketch. If you don't have a good grasp of anatomy or picturing three dimensional spaces you wont be able to start shading either. Good shading on wonky anatomy will make mistakes stand out even more. Although it can help your sketching skills a lot if you practise shading on lines that are correct with anatomy: trace photos and try shading them for example, or see if there's any free linearts from experienced artists. But remember to always check the licence of the material you use, unless it's yours.
Here is the picture I'll be shading in the tutorial:
1. The first thing you always do is decide your main lightsource(s) (you can also imagine it, as long as you memorize the position). Notice: When you draw your lightsource on a two dimensional drawing, you can't actually see it's z-axis (whenever it's more behind your object or more infront of it). You'll actually just have to memorize this.
Tip: Ligthsource affects the mood of the picture
Think: a normal lighsource is always from above (the sun is up on the sky, lamps are usually in the ceiling), so using a normal lightsource will give an easy / neutral feeling for a picture, even without a background. In the other hand, using a lightsource that comes directly from the viewer will give the picture a more photograph / profile picture feeling, it's unnatural and purposely set up for the specific pose and event.
Tip 2: Find simple shapes
Imagine simple shapes for different parts of the body, such as a ball for the head and a cylinder for muzzle. It makes it easier to picture where the shades will go.
I can't stress it enough, but understanding the three dimensional shape of your object or character is essential for being able to figure out where shades will go. Sometimes when I start shading I'll notice some anatomy mistakes that need fixing before I can continue to shade.
Tip: Inspect shades. Make yourself some objects that are basic shapes: box, ball, cylinder... (fold paper, get clay, find suitable objects from your household). Then take a flashlight and inspect! Or if you know how to use blender that's effective too, and doesn't waste paper.
After inspecting how shades work on various shapes, you'll notice that a smooth curve will have soft shade and corner will produce a sharp edge. But since I'm only using cellshade for this tutorial all edges will be sharp instead, this is one of the challenging bits of cellshading: figuring out where the edge of shade will look good.
Tip 3: Enchance your lines with shading.
When you do a cellshade or softshade technique, don't define all corners and muscles with hard black lines, that can look very ugly. Instead further define them with shading.
Tip 4: Don't be afraid to shade large areas.
A problem I've noticed with a lot of starter artists is being afraid to shade larger chunks of a drawing. If you have a lightsource that would cast shade on a large portion of your character, don't be afraid to shade it all!
Krita has a nice feature to help shading if you have a solid flatcolour layer(s) below. First create a group (1), then put the flats below (3) and create a new layer for shading above them and tick on the alpha lock on the shade layer (2). This cuts off any overflowing strokes and makes shading faster to do!
Here's an actual example of how I enchacesed the shapes of the character with shading:
Tip: Multiply shades.
Instead of shading each coloured area with a different shade, pick one colour to shade the overall picture with and use "Multiply" layer mode. Multiply adds the colours like paint making the results darker, it equally darkens the colours so you don't have to think and choose so much!
We're not done yet! Why?
If we were to somehow get in a space with only a single lightsource and ignore all reflections except the ones bouncing to our eyes: All shade would be of equal darkness, aside from the transistion/edge areas. That means you'd use only one layer to lay all shading on. This is not possible. All materials reflect different amounts of light, otherwise we couldn't see them (they'd be perfect black). That means there's always more darker areas (also lighter areas, but I'll get to that in the next shading tutorial, so you don't get overwhelmed.)
So adding another layer of shades to define some details and areas that are much more in shade.
Moving to more cellshade / softshade specific: Whenever you should add clear highlights depends on several things. Firstly, if you used neutral light for flatcolours you should add some highlights, but if you coloured assuming the colours how they'd be in properly lighted room you don't need to add highlights. Secondly, it also depends on your lightsource, how strong is your light? If it's the sun, you should add highlights. Thirdly, how reflective is the material you're shading? That also defines how you draw the highlights, a matte surface has very soft highlights in which case you should use airbrush for them. A lizard skin, or wet skin will make sharper highlights, like in my example:
I used "Overlay" layer mode for highlights, it boosts the lightness and saturation of the colour below, tinting to the direction of the colour you used when drawing the highlights:
That looks a bit too rough, so I softened the highlights with using an airbrush as eraser. And now we are done with the basic shading. Next it'll get more crazy.