In this challenge we were asked to change the color of a polygonal
object based on the displacement of the object. Renderman has the
ability to pass information about a displacement shader to a surface
shader by the use of :
output varying float
Use of this allows the displacement or "hump" value to
be parsed to the surface shader which then can be used to control
where color should be placed on the displacement. To show this I
used a basic noise based displacement and parsed the information
to the surface shader using the above float command. What I got
hump > 0.5
Essentially, everything that is over 50% of displacement values
coming into the surface shader is colored red. This is a basic representation
of the shader working, but there is aliasing at the point where
the color changes. To remedy this, I resorted to using "smoothstep"
to ramp between the colors. I also included in the surface shader
the ability to control the amount of "smoothing" or "ramping"
between the two colors. This is what the shader returned with different
values for "smoothing":
As you can see, by controlling the amount of "ramp" you
get quite different effects happening to the shader. With this in
hand, I was also to control the point at which the ramp is applied
to the displacement. I then was able to move the "ramp"
up or down the displacement. This is the effect of moving the ramp
based on the percentage of the hump value:
I left the "ramp" value low so to show the movement of
the linepoint up the displacement. I made a couple of movies of
both of these ideas to be able to better show what is happening
as the values are changing.
click for displacement shader
click for surface shader
So I decided to take this idea a bit further and attempt to use
a texture map for displacement and then get a gradient between two
colors from this. I first looked for a decent texture map. I was
able to find some DEM (Digital Elevation Maps) online and with the
help of James Van Allen I converted the DEMs to TIFFs. Taking the
TIFF file into Photoshop I "squared" the map to 256 x
256. Once having a workable TIFF image, I converted it into a TX
so that renderman could recognize the map. Instead of using a noise
function in the displacement shader, I used texture function to
read the displacement and parse the information to the surface shader.
This is the displacement I ended up with:
Lowell, Or displacement
I chose this DEM for two reasons:
1. It has a great delineation between the lower part (which is
a lake) and the surrounding mountians
2. This is the DEM of Lowell, OR, which is the area where my parents
So, with this displacement, I used the parsed information and read
it in the basic surface shader above to get the correct delineation
between the lake and the mountains. With some "tweeking"
of the ramp and linepoint values I was able to get this image:
Lowell, Or movie
Made a short
movie to show the control over the linepoint. Has the effect
of the water flowing into the lower areas.
Even though this came out pretty well, I wasn't happy with just
the two colors. I then turned to the use of "spline" within
the surface shader to get more color gradients within the image.
I found that "spline" needed in and out points to control
the "curve" of the colors up through the the displacement.
I also found that spline is difficult to control, but this is the
image I was able to get using "spline" with 8 separate
Lowell, Or (spline)
As you can see from the image, the "spline" still needs
to be "tweeked" to get a better representation of where
the colors really should be. All in all, I am satified with the
click for Lowell displacement
click from Lowell