Core Shadows, Cast Shadows and
Terminators By Neil Blevins Created On: Sept 16th 2025 Software: Any
If you've ever taken a traditional art class, you may have heard terms
like Cast Shadow, Core Shadow, or Terminator. These terms are not only
useful in traditional art, but also digital 2D art, and 3D art as well.
This quick lesson will explain these concepts, and how they relate to
the real world.
A Shadow Is A Shadow
First off, all shadows in the physical world are actually the same
thing, light not hitting a surface, which causes it to go dark. But
when making artwork, either in 2D or 3D, it's often easier to break
shadows into sub categories, and treat each one separately. But that
being said, always remember that any sort of shadow is the same
phenomena, a darkened area
where light has not struck an object, regardless of how we go about
replicating them or categorizing them in our art.
Setup
1
To explain some of these concepts, we'll start with a simple example, a
sphere sitting on a black table
with a single light source.
Light
The light hits the sphere, causing that area to be bright. Where the
surface of the sphere is directly facing the light, you get maximum
brightness. As the surface starts to turn away from the direction of
the light, the sphere starts getting darker.
Terminator
At the point where the
surface is 90 degrees from the light direction, you get the Terminator
Line. The terminator line is where your Core Shadow starts in this
example.
Core Shadow
Since no light reaches the surface starting at the terminator line and
going past 90 degrees, that's the Core Shadow Area, which in this
example would be totally black.
Setup
2
So this example is similar, except we are using a white table under the
sphere.
Doing so adds 2 new things to the equation: "Bounce Light" (also called
Reflected Light) and a "Cast Shadow".
Bounce Light
While the area of the sphere pointing away from the light source should
be fully dark, when the light hits a white table instead of a black
table, the white table will "bounce light" towards the underside of the
sphere surface. This causes low intensity brightness under the sphere.
The reason it's low intensity is because the surface it hits first
(like the white table) absorbs some of the light energy. So the light
bouncing from the table onto the underside of the sphere is less
powerful than the original light that hit it. The Terminator Line is at
the same spot as the previous example. But the Core Shadow Area is
now smaller because it's receiving some light from the bounce light.
The Core Shadow is always the darkest area of shadow on the subject.
On a side note, for those of you who use 3d software, bounce light is
one of the features that separates old school lighting from modern
raytracing. In the earlier days of computer graphics, rendering systems
couldn't simulate bounce light, and so your light source would create
direct lighting and core shadows only. People cheated bounce light by
placing low intensity lights in the shadow area, or even using ambient
lighting, but the cheat didn't always look terribly realistic. Years
later, when computers became faster, people started moving over to
raytrace renderers that could do a set of calculations called "Global
Illumination", which, among other things, could now properly simulate
bounce light accurately.
Cast Shadows
Now on to Cast
Shadows, which are shadows that one object projects onto another, or
onto itself. The
easiest example is when you're standing outside, the sun creates a cast
shadow of you standing there on the ground opposite of where the sun is.
Cast Shadows with a Sky
You might notice a cast shadow can be darker closer to the object and
brighter further away. This is usually due to the sky (this can also
happen due to light bouncing around your room). The sky creates
a type of cast shadow called "occlusion", which is a soft darkening
under objects. So in our outdoor example, the sky provides soft
illumination that also produces a soft shadow called occlusion. And the
sun provides strong direct light that also causes a much sharper cast
shadow. They combine together to form an area of darkness under your
subject that combines 2 shadows
.
The Material Matters
So in our 2 examples, we're assuming our sphere is made of a matte hard
material (like say wood or plastic). However, if your sphere is
something else, like human skin, or even a styrofoam sphere, you can
sometimes get some light that leaks past the terminator line. This is
due to Subsurface Scattering. You can read more about this phenomena
here: Translucency
and Sub-Surface
Scattering In The Real World In short, instead of light only
bouncing off a surface, some light enters the surface than bounces out
past the terminator line. So when observing shadows, keep in mind that
the object's material will also play in a part in the final placement
of the shadows.
2D and 3D
Now that you've learned the theory, you can use this knowledge to paint
light in 2D. Start by adding your core shadow on your subject (keeping
in mind any bounce light).
Then you can calculate your object's cast shadow on any surface it
hits, including potentially itself, assuming your subject is more
complex than a sphere. Like for example, an arm will have a cast shadow
that hits your body as well as hitting the ground.
Most 3d applications have a similar split, for example, in blender, if
you place a light source near a sphere, it will automatically calculate
the core shadow, but you have to check the "shadow" check box to have
it calculate a cast shadow.
Exercise 1
This is a classic traditional drawing exercise. Place a white sphere on
a white surface with a single light source, and then replicate what you
see. All the items you'll need can be purchased at any good art store.
And feel free to use traditional pencils / pastels, or digital, either
will achieve the same results. Bonus points for labeling each of the
shadow types in the final composition like the diagrams above.
Get the items mentioned in Exercise 1, and before setting them up, draw
how you think the cast shadow will look like on the sphere if the light
is directly above. After you draw, set up the scenario I just described
in real life, and see how close your came the drawing the correct cast
shadow. Now draw the same scenario, but place the light from a 45
degree angle from the side. Then confirm with the real light how
accurate your drawing guess is. Keep choosing new light positions, do a
sketch of what the cast shadow will look like, then confirm your
results.
A variation of this exercise can be done using 3d software, say if you
have Blender, setup the scene described, and move the digital light
around to test your guesses.
Conclusion
Hopefully this helps explain some of these terms and how they interact
to create the lighting scenario you see before you.
.
