An occlusion is the strongest cue and
it is fun to play with it with optical illusions, because of that strength.
Illumination helps us perceive the orientation of surfaces and
there's two kinds of illuminations that we tend to use.
One is diffuse illumination,
which is brightest when a surface is facing a light source.
And we usually see this from soft objects, rough objects, things like cloth or
pencil erasers or other surfaces that are scattering light in many directions.
And then there's specular illumination and that adds these white highlights and
we usually see these on shiny objects, metallic objects.
And these highlights are added in regions of the surface that are facing in
a mirror reflection that would reflect the light source towards the eye.
And so, we can combine these when we're doing data visualization.
This is an example of particles that are leaving trails to show the simulation
of the flow of blood in the abdominal aorta and we're using certain cues.
For example, occlusion that some of these trails are in front of other trails as you
can see this green trail is in front of the blue trail.
We're using lighting, because you can see that these trails are cylindrical.
They have a brighter side and the darker side, because of diffused sliding.
Also loosing perspectives, we see larger things here and
smaller things more detailed at the back.
And all of those add up to give you better sense of the shape of this aorta and
the blood flow inside of it.
There's some other cues that are important.
One is shadowing.
Shadowing is an inclusion cue from a second viewpoint where that second
viewpoint is at the light source.
So here, I've got two two-dimensional images.
In both cases, we can think of these due to perspective as a flat blue
plain with a red-orange ball on top of it, but
we don't know if this red orange ball is hovering over the middle of the plain or
if it's laying on the plain toward the back of it.
If I had a shadow, you can see on the left case here,
the red orange ball is laying on the blue plane towards its back.
But in this case, which is the exact same image I've just changed the shadow,
we can see that this blue ball is hovering over the middle of the blue square.
And so in, for example, this visualization of a molecule,
you can use these shadows to get a better sense of depth due to the directions
that the shadows are being cast from in understood light source direction.
There's also a perspective in addition to being able to see more detail
far away with perspective.
Perspective is based on size constancy.
That objects are the same size, but farther ones appear smaller.
You don't think that the object is changing its size.
You understand that the visual system is looking at a larger area
that's further away, because of the way it gets projected onto our retina or
onto our image plane in the case of computer graphics.
In computer graphics, the way we do this is we actually make things that
are farther away smaller and make things that are closer to us larger by
actually moving the vertices and then projecting onto a projection plane.
And so to get this perspective view where you've got the front of the table larger
than the back of the table, what we've actually done is actually made
the primitives of the table, larger in the front and smaller at the back.
But computer graphics does this for you automatically, so
you don't actually have to do this.