I have always been interested in understanding how organisms
in this world are able to protect itself from disease and infection.
During my studies as an undergrad in biology, I was able to take
classes in not oly human physiology but zoological physiology.
In this I learned that skin, mostly of mammals, is the largest
organ that we have. Skin, viewed normally, seems to be a very
smooth surface. But as you get to the microscopic level, you see
this vast terrain of great fissures running across the surface.
It seems to look like the base of a giant drought washed lake,
with cracks running in random direction. Below are a few images
of human skin at a microscopic level.
By focusing in on the the cracks on the surface of the skin I
would want to create a procedural shader that takes into account
as the camera gets closer to the object that the shader is applied
to, the cracks increase in displacement. Essentially I would like
to create a shader that changes based on the camera's ray length.
Voronoi Cell Noise (A. K. A. Worley Noise)
I decide to start this project by delving into cellular pattern
noise. I believe that this will lead me towards something that
might resemble a microscopic skin texture. This cellular noise
is known as Voronoi or Worley noise.
Georgy Voronoi (1868 - 1908) : Came up with the matimatical theory
a member x of a set X of points in the Euclidean plane,
the locus of points in the plane that are closer to x
than to any other member of X."
This theory allows for a bounding the region closer
to a point than to any adjacent point. The polygons are drawn
so that the lines are of equal distance between two adjacent points.
Visually, this can create an interesting non-sequential patterning
on a surface.
Steven Worley in 1996 presented a paper at SIGGRAPH that revelutionized
the idea of cellular noise. With his new computations we are now
able to expand the theory of G. Voronoi to include 3 dimensions.
With this mathmatical computation we can create all different types
of surface and displacement shaders.
Creating A Basic Cell Noise
displacement Shader :
Starting this project entailed having a basic understanding
of Worley noise and how it works. Once I was able to get that
under my belt I created a library file that allows me to call
to the Worley noise function within the shader which in turn allows
for a less crowded code, which are both posted at the end of this
webpage. A few of the basic parameters that need to be understood
number of cells : Self explanatory, this creates
more or less cells across the surface.
line width : Self explanatory, this controls the
width of the drawn lines.
scalesize : The amount of smoothing between the
bottom and the top edges of the lines. ( this only is in effect
when dealing with displacement. This also lends torward uniform
lines within the funtion.
With these basic parameters I was able to create
a simple displacement shader that I could control the number of
cells, linewidth, and smoothstep. Below are images that show these
parameters in action.
As you can see from above, by adjusting the number of cells within
the shader creates a multitude of differing patterns on the surface
of the object.
More complex displacement shader :
Working with the basic diplacement shader that I created above
I then turned toward a more complex displacement that could somewhat
simulate the look of microscopic skin. I decided to use a buildup
of cellular noise to create the effect. Since the cells are being
displaced up from the objects surface I can retrieve the amount
of cellular displacement from the object from rest. Once having
this I basic the next set or octave of cell noise on the amount
a set amount of displacement from the first octave. Below is a
sequence showing the displacement in multiple octaves.
Now that I have a basic displacement shader that sort of resembles
skin (at least reptile skin) I moved on to figuring out how to
deal with displacement based off of the distance of the object
from the camera.
Displacement Based On Camera Distance :
This was not an easy trick to pull off. I did start with the
idea of just basing the displacement on time. Essentially, key
framing the Km value over time to simulate the increase of the
diplacement on the object. To get a more accurate idea of what
it would look like on a character, I borrowed Howard Coulby's
female model he designed in Maya. Once I had the character imported
into Houdini as a .bgeo file I was able to apply the displacement
shader and see the results of the initial test. Below is a sequence
showing displacement over time as the camera is moving toward
the character. This in no way is based on camera distance from
So instead of trying to deal with the idea of writing into the
shader a way of calculating the distance the camera is from the
object, I relied on Houdini. By writing an expression based on
the coordinate origin (vtorigin) of the object and the camera,
I am able to get the distance from the camera. Thanks to Hubert
Weldon for his help with understanding how vtorigin works. With
this expression inserted into the Km value I am able to control
the amount of displacement, if any, of the shader applied to the
object. Below is a sequence that shows this in action.
For a basic skin shader based off of the camera distance from
the object, I feel that I completed the task that I set out to
accomplish at the beginning of this page. As for the problems
that arose, which were minimal, I seemed to be able to find a
fitting fix in the end. I would like to explore the voronoi noise
more indepth so that I would be able to create a displacement
that is based on a 3 sided polygon instead of 4 sided. I believe
that this would give rise to a more "realistic" human
skin shader. Other areas to delve into would be sub-surface scattering
and color based on the bump height.
click here for the library
click here for the displacement shader
Side note :
I did start to play around with some of the parameters in my
displacement shader. I found that by increasing the width of the
lines within the voronoi noise, I was able to get an effect of
the displacement growing on the surface of the object. Below is
a sequence showing this happening.
Apodaca, A. and Gritz, L. Advanced Renderman: Creating CGI
for Motion Pictures.
London: Academic Press, 2000.
Ebert, D., Musgrave, F., Peachey, D., Perlin, K., and Worley,
S. Texturing and Modelling: A Procedural Approach, 3 edition.
San Francisco: Elsevier Science, 2003.
Chris Gold "The Voronoi Web Site" April 2005. http://www.voronoi.com/