How to Draw a Sphere
Explaining sketching without me using a pencil
I want to explain sketching from a physics point of view.
It’s important, not only for realism in your work, but just as a fundamental methodology for making new / different shapes and then considering how they’d behave based on rules of light rather than “oh, my art teacher told me to do this, so I did that on everything” (because that’s something we actually see a lot in people’s art and it’s understandable, but we can teach better.)
So let’s learn how light do!
The most basic thing I can describe is to think of light as a bunch of lasers bouncing around, because that’s almost literally what it is. In any situation where you’re unsure how light behaves, just ‘shoot’ it with a straight line and try to imagine how the geometry makes it bounce. We’ll come back to this again and again within this description.
Incidentally, this is basically how ray-tracing rendering works for computer graphics, so it’s a convenient methodology between the two.
Understanding ambient light
With very few exceptions, there’s almost always more than one light source on something. Even right now, wherever you are, look around: there’s probably electric lights in whatever room you’re in. There might be windows with sunshine, or even just skylight, or even sunlight that’s facing away from you but bouncing off some opposite wall and then reflecting back inside. There’s light coming from the monitor you’re reading this on. There’s infinite combinations of where it originates, how it bounces and so on.
So when we’re sketching it, we can do two things: try and simulate it as realistically as possible, or just understand the general idea and fake it as best as you can, realizing that no one will notice or care.
I like the latter approach.
This is the exact same sphere as the above with the sunlight turned off.
Immediately we can see where there’s brighter and darker patches. This is our first introduction to physics.
You’ll notice those patches correlate exactly to where the light would be in a reflection with a more mirror-like sphere:
When sketching, we need to think about the material itself. Is this ball metal? Rubber? Glass? Plastic? It helps, crazy as it sounds, to think about what sound it makes when you tap it with your pencil. Is it dull thud? Does it make a sharp clink sound? The sound is likely related to the glossiness of the object.
We can describe gloss as how shiny something is, ranging from full mirror to fully dull (matte). There’s a lot of other science-y albedo bits here, but I won’t bore you.
The idea is that all (normal) things reflect light. Some things reflect light in a sharp way and those laser-lines we’re shooting will bounce off directly, and some things are dull; that laser light will hit the surface and sort of scatter into fuzzy diffused light. Similar to our sound test, wherein ‘clink’ sounds are shiny objects that reflect light (and sound) in sharp ways.
And, we can draw everything in between those two extremes:
So this is the base light that’s just ambiently around us all the time. It might look different depending on the environment, but always consider that there’s some surface variation to everything in real life. Having a sphere under a single spotlight in a room actually isn’t very good drawing practice.
Anatomy of grounding
The word if you want to look into it more is called ‘ambient occlusion’ but that won’t be on the test. The basic idea is that everything has a shadow when in proximity to another object. It’s just this effect of light where, when things are near each other there’s a statistical probability that it won’t be lit up between those two geometries. So any crevice gets shadows by default, and the shadow darkness is proportional to how deep and sharp the angle of the crevice is.
This is how we ground objects to the floor, and to themselves.
The floor gets a shadow that’s a sum of the ambient occlusion shadow — that is, the darkness caused by the curve of the object above it — and also the actual lights and whatever that I lighting the scene. We’ll see that in a second.
This is a pretty good look at how rendering technologies have improved over the years too — watch old movies like Toy Story and notice they’re all shaded like the left image. Even games are just recently getting into realtime AO as a viable rendering technology (you’ll see it in settings as SSAO)
But mostly: notice how the left is sort of floating? There’s nothing tying it to the ground? But the right has a tiny black edge and some shadows sort of fading outward from it. This is ambient occlusion.
Look at the shapes themselves too — the armpit and the neck crease especially. The left might be accurately shaded according to where the light source is and most people can reasonably shade this sort of form with their pencils, but the right has occlusion there based on the trapping of light in the crevice too and gives that more firm visual read to our brains.
As the light falls into the tunnel of the ear, say, there’s less chance of it bouncing back out, based on the angles involved. Thus, it’s darker. There’s less light escaping from that area and hitting our eyes to see it.
You can scroll up to the beginning first and second photo and see how AO is affecting those two spheres, now that you can ‘see’ it as a layer.
This is also why ink washing miniatures works to give it scale, you’re adding ambient occlusion as a physical layer to imply scale of bigger crevices. You give it that light and shadow punch that adds heft and depth.
Shadows from light sources
It’s not quite accurate, but here’s how I’ve always thought about it: there’s three areas to any form. The middle is just whatever the neutral colour of the object is. The top is called specular, and that’s where light is hitting something. The bottom then is shadow, wherever light isn’t hitting.
And these three layers go over top of the ambient lighting layer, which itself goes over the occlusion layer.
At any point on any object, we’re doing a sum total of light on that point.
So points with occlusion are darkened on the whole, but if there’s light shining directly on them the light is added and we get a value somewhere lighter.
If we look at the back left of the sphere we can see where the shadow part is overtaken by the ambient light part to make a little reflective shine. We can also see where the ambient light on the right adds to the specular shine on the right, making even more intense light at that point.
This also works as a sum of darkness: if you look right in the crack of the shadow on the right sphere there, you’ll notice there’s a shadow darkness and then it gets even darker where there’s AO added.
While we’re focused here, let’s also talk about the infamous light bounce.
This is often taught (and for good reason, it’s true) but I see a lot of tutorials get it wrong: where you put it is directly related to the light on the ground. It is, after all, a bouncing of light from a light patch on the floor, and that patch location is defined by the light source.
So, don’t lighten the whole bottom of the sphere ‘just because’ but rather think about where the light is and how it’s bouncing. It won’t likely be on the backside of the sphere because there’s shadow there and the shadow is dark, so there isn’t light coming off that point up towards the sphere point (and bouncing again to your eyes) but there might be from an angle just oblique to the shadow itself.
There’s a second bounce too: you’ll see the floor in front of the sphere is slightly brightened warm. This is where the light is bouncing off the floor, off the sphere and back off the floor again to reach you the viewer.
As with all these considerations, the reflectivity of both the floor and sphere material come into play. Those glossy surfaces transmit more light, so you get conserved energy and more bounces (both as a number and as a brightness — remember that each bounce takes a little bit of energy away, and matte objects scatter more light, so those rays get less powerful quicker.)
A word on colour
You might have noticed that my lighting has been orange and blue.
It’s blue! What gives!
The sky is blue.
Literally, that’s all it is.
When we talk about ambient light and also have the power to strip away light sources like we can in 3D (and less so in real life) then what we’re left with is a very blue light; there’s a lot of sky-blue light rays bouncing around.
If we look at the ambient light, it’s sort of warm tone because unfortunately that’s baked into my lighting map: there’s those bridge things and they’re being hit by sunlight (which is warm) so they show up as warm in the reflections.
Similarly, if you scroll up and look at the shadows of those spheres you’ll notice it’s blue. This is that sum thing again: there’s actually ambient light hitting those shadows so they’re not fully 100% black.
What they are is a lack of light from that sun light. So they still are subject to the same ambient light as you see to the left here, which, you’ll note, is blue.
It’s fairly realistic: go to any parking lot at 3 pm and notice that shadows tend to be blue. The sunlight makes the ground and cars and whatever warm with its light, but the areas in lack of sunlight still get some ambient skylight.
We can make this more or less contrasty in how we style things (and what sort of lights we’re using / environments we’re in!) but outdoors are typically subject to this warm / cool dichotomy.
You’ll also also notice that ambient occlusion is generally black. It really is the absence of light on the whole, so it’s literally darkness incarnate, ie not-hued.
So when we look at that shadow + AO render, you’ll see the shadow is blue, but the AO areas are darkened to greyscale as part of that sum thing.
The sum approach actually works with all colours, I just didn’t want to get too much into it.
And that brings us back to the beginning!
We have a sphere, which is more or less glossy, and it has both ambient light and ambient shading plus the light source specular and shading.
Shadows are blue, the bounced light in front is warm. AO is grey. Bounced light from the floor comes up only where you’d imagine lines from the light source itself (or ambient light) might come from.
Every value is a sum of light and shadow layers fighting.
Your eyes are basically cameras: every ‘pixel’ point on any scene, on any object can be read as a discrete individual value of lightness (and of colour) and they’re the sum of the world you see bouncing around and eventually hitting your eyes.
Oh, and you can generally think about how glossy an object is by the sound it makes when you tap it with your pencil.
