Wet Looking Materials
By Neil Blevins
Created On: June 11th 2002
Updated On: Dec 14th 2024
Software: Blender or 3dsmax
(vray)
What makes a material look wet? Or gooey, for those of you who love
to make slimy creatures. Well, first, lets look at the problem at the
most basic level. An
object
looks wet because the surface of the object has a thin sheet of water
over
top of it. So what makes water look like water?
- Water has no obvious Diffuse
Reflectivity.
- It has a strong specular component (Specular Reflection),
especially
at glancing angles due to the fresnel equation.
- It is also refractive,
however since the sheet of wetness is so thin, the only visual
indication of refraction usually manifests itself as a darkening of the
surface. In hard surfaces like a rock, this is due to internal
reflections between the water surface and the rock surface, which dims
the light reflected back to the eye. In porous surfaces like cloth, a
similar phenomena is happening, the surface absorbs water, so when the
surface is struck by light the light scatters deeper into the material,
and so hits more particles before leaving the cloth. Each particle
absorbs a slight amount of the light, so when the light emerges from
the material back to the eye, the surface appears darker because more
of the light is absorbed in the material.
So the main two features of a wet surface vs a dry surface is a
darker color and a strong specular highlight.
Reference
If you look at these rocks, you can see the diffuse component of the
rock, and over top you can see nice sharp specular reflections of its
environment
(slightly distorted by the bumps on the rock). Since this rock in its
natural
state would have almost no specular reflections, the viewer, upon
seeing
the rock, would assume it to be wet. It's always easier to make
something
look wet if the object has no specular component to begin with.
Although I have no photo of a dry rock, you'd find the dry rock to
have a slightly lighter color than the rocks in the photos above.
Here's another example image, this is a carrot, half of which is
wet, and half of which is dry. Notice the differences in the wet and
dry sides, the surface has a strong specular highlight (a reflection of
the nearby window) and the color is slightly darker and more saturated.
One more example, some seedweed I found on the beach:
Software Agnostic Material
This type of material is sometimes refered to as an Overcoat, which
means 2 materials mixed together in some fashion. So for example, your
base material might have specular reflections, but the water on top
also has reflectivity, and so the result is one reflection sitting on
top of the other.
So to make something wet, we need to do 2 things...
- Layer a strong glossy specular reflection on top of your base
material
- darken the base material slightly if its porous
Blender Example
The Principled BSDF shader has controls for this exact thing, it's
called "Coat", as in this reflection is coated on top of the base
material.
So say you have a rock material on a boulder...
Under Coat, add a weight of 1 and IOR of 1.6 to get water
reflectivity on top...
And adjust the Tint to a dark grey to simulate the surface under the
water getting darker.
One last issue. Right now the spec is covering the bump in the rock
texture, as though the water layer is really thick. Maybe that's what
you want, but if you want a thinner looking sheet of water that's being
affected by the rocky bump, hook the bump map from your rock into the
Normal of the Coat.
3dsmax (vray) Example
The default VRayMtl has controls for this exact thing, it's
called "Coat", as in this wet reflection is coated on top of the base
material.
So say you have a rock material on a boulder...
Under Coat, change coat amount to 1.0, keep IOR at 1.6. Here's the
resulting wet rock...
This already looks pretty good, but we also want the rock to darken
where the wetness is. That's what the Coat Color parameter is for, that
reduces the brightness of the diffuse under the coat. So change the
white color to a darker grey...
One last issue. Right now the spec is covering the bump in the rock
texture, as though the water layer is really thick. Maybe that's what
you want, but if you want a thinner looking sheet of water that's being
affected by the rocky bump, check the "Lock coat bump to base bump"
checkbox.
Now the wet specular is affected by the rock's bump map.
