Learning how to draw shading should come from a good understanding of 3 point lighting. Once this is understood, for digital paintings and illustrations, a good shading technique that focuses on drawing light instead of shadows would be the best way to go about shading and highlighting.
It is very important to know how light behaves for any artist to draw things correctly. In the real world, light bounces off everything. It would be impossible to emulate it in any media 100% of the time. Therefore, the best that we can do is simplify it to 3 point lighting.
Given its infinite possibilities, 3 point lighting is used as a means to identify natural light into very specific behaviors that act on an object. Each will property will act and interact with one another. Thus, It is imperative that you know these three directional lighting properties of key, fill, and rim light.
Whether you are drawing, painting, or animation, creating natural light using this concept can only benefit you as an artist--especially if you are looking for how to draw shading in the most correct way possible. So let's start figuring out what we need to learn.
The main light is called the key light. This will be the strongest light to show the entire composition where your light source is coming from. It can be the sun or a single point of strong light in an enclosed space.
Next, we will stack on the fill light on the other side of the key light. In the natural world, there is always light. Since light bounces in all direction, this is to emulate the scattering of light in the real world.
To bring out your subject further, you can use a rim light. The rim light is located right behind the main subject and exists mainly to separate the subject from the background by creating a halo outline around the subject.
Don’t forget about the cast shadows either! You will see that the cast shadow is now faintly in front and rear of the object it is highlighting.
A fill light cast shadow is faint and very weak. Sometimes, it's barely noticeable when compared to the cast shadow from the key light.
Just remember that the further away the cast shadows are, its darkness will fade out as the area around the cast shadows will receive more light.
By definition, a shadow exists because light is absent. How the cast shadows are generated will be dependent on the angle of the light source.
In art, this can be simulated by reflecting shadows based on perspective lines, the horizon, and where the key light source is.
In most drawings, there is a combination of six areas that most artists use: behind the object, in front of the object, above the object, below the object, to the left of the object, and to the right of the object.
All shading will need to be contained within perspective lines, as it meets the horizon line, and the confines generated by the light source lines.
Another thing to note is that cast shadows will fade out from the base of the object, like a gradient, the further out the shadows extend.
Anything you draw will require a mental image of these perspective lines as well as 3 point lighting . So it's a good idea to get comfortable with this concept.
In real media, you would shade in shadows and other dark areas where light can't reach. In digital media, it's actually much easier to draw light than it is to draw shadows.
Blocking in form already generates all the dark values and textures needed to apply a light source towards.
From there, it's just a matter of applying a light color to areas where you think light will hit. It saves time since all the texture work is already there. All you are really doing is make existing objects light up.
Using the concept outlined with 3 point lighting, apply a key light, fill light, and rim light to the objects that you wish to bring out.
Digital art makes it easy due to layering. A layer will allow you to draw multiple objects that are independent of one another. The advantage is that you can edit each object at will without affecting the entire composition.
With that type of freedom, it means you have an unlimited way to play around with the composition before finalizing it.
Shading and highlighting go hand in hand when you finalizing your drawing. Drawing light and dark values on a different layer can help test your composition.
Changing opacities and blend modes on a different layer is a common and safe method of to see different points of lighting without editing the original drawing.
In terms of effects, a generic chalk brush can be used to create hard shadowed edges as well as draw. On the flip side, round brushes can help create a sense of smoothness in the highlighting.
When you do shading and highlighting, you also need to think about the topology of your subject. If there are indents or out dents, you need to angle your shading to fit these types of surfaces so the cast shadows look natural.
At the end of the day, shading is a concept that requires a bit of practicing to get down depending on how comfortable you are with using your drawing tools.
However, it can be quite fun so long as you are thinking about drawing light instead of focusing on creating cast shadows.
Regardless, I hope these tips on how to draw shading are helpful. So long as consideration for light sources, perspectives, and bringing light out are met, you should be able to pick this up very quickly.