# Radiance specularity and roughness value examples
Most Radiance material definitions require a specularity and roughness
value to be set. Most 3D rendering programs have some sort of
specularity and roughness parameter, but these are not necessarily
linked back to a measurable physical property. In Radiance, these
values are measurable.
However, unlike the diffuse RGB reflectance, which can be estimated
through greyscale charts and through calibration with `macbethcal` and
a Macbeth ColourChecker, it is a bit harder to measure specularity and
roughness.
## How to measure specularity
Specularity is not clearly defined in the Radiance reference manual.
However, it is basically the ratio of reflection that is specular and
not diffuse. For instance, the total surface reflectance of an object
is given as so.
```
total surface reflectance = diffuse reflectance + specular reflectance
```
An integrating sphere-based spectrometer measures the diffuse
reflectance by giving a measurement with the specular component
excluded (i.e. what's left is the diffuse component). It also measures
the total surface reflectance by giving a measurement with the
specular component included (i.e. diffuse + specular reflectance).
Therefore you can calculate specularity as so:
```
specularity = specular component / total surface reflectance
specular component = total surface reflectance - diffuse reflectance
specularity = (SCI - SCE) / SCI
```
However, not all of us have access to a spectrometer. Unfortunately,
the only alternative is to use more heuristic methods. One way is to
simulate an object under controlled conditions and compare it to a
physical sample. Another way is simply to make an educated guess. The
good news is that our environment isn't filled with extremely specular
materials, and so a slight inaccuracy may not harm the simulation too
much, depending on what you are simulating.
## How to measure roughness
Roughness is the root-mean-squared (RMS) facet slope of the surface.
This is the microscopic surface roughness: the more jagged the facets
are, the rougher it is and more blurry reflections will appear. Note
that this is not the roughness that is visible to the naked eye.
Unless you have access to a very powerful microscope, again, it's best
to stick to a heuristic approach and fiddle with the number until you
get what you like.
## An intuitive example of specularity and roughness
There are a few rules of thumb when guessing these numbers, which I've
taken from the Radiance reference manual:
- Most plastic (i.e. non-metallic) surfaces have a specularity of .0
to .1
- Metallic surfaces generally have a specularity from .8 to .99, but
are usually .9 or greater
- Both plastic and metallic surfaces rarely exceed a roughness of .2
The
[Siggraph 1998 course](
http://radsite.lbl.gov/radiance/refer/s98c33.pdf)
gives some really great graphical examples of the difference in
specular and roughness values. I've copied the images here for
reference, but they are not produced by me and are the fine work and
copyrighted by Rob Shakespeare.
This set of images describes specularity and roughness values for
plastic materials:
Here is the same for metallic materials:
As great as these images are, I find some bugbears. Firstly, the
values tend to jump around a bit instead of linearly. Secondly,
although I'm sure it was a fantastic work of art in 1998, 20 years
later we might need a more modernised test image, perhaps using
higher-polygon models that we might typically use nowadays.
I've recreated this benchmark image with this in mind. The principles
are as follows:
- Show more complex geometry in the test image
- Show more colours in the test image
- Show a linear range of values
- Place the geometry in a more complex environment
I've used a basic sphere, the
[Utah teapot](
http://www.holmes3d.net/graphics/teapot/?graphics/teapot),
the [Stanford bunny](
https://graphics.stanford.edu/data/3Dscanrep/),
and
[Suzanne, the Blender monkey](
https://docs.blender.org/manual/en/latest/modeling/meshes/primitives.html).
The materials are taken from the galvanised sheet metal that comes
with the sample `materials.rad` file with Radiance, and the remaining
red, green (foliage), and blue (sky blue) from the Macbeth
ColourChecker chart. The range of values are those that follow the
rules of thumb above. Also, The geometry is placed on top of a
displaced rocky ground, with a skymap environment, where the sky
itself has been swapped out for a `gensky` sky.
All images are rendered using this publicly available
[Radiance test scene repository](
https://gitlab.com/dionmoult/radiance-test-scene).
After a rendering, they are processed with `pcond -h`.
To start with, here is a comparison for plastic materials with
different levels of specularity and roughness. I recommend you
[download the high resolution version](plastic.png), to fully
appreciate the differences.
Plastics differ from metallic materials in that the specular
highlights of plastics are white, whereas the specular highlights of
metallic materials are tinted. Here's an example.
So here is a final chart of examples for metallic materials. Again,
you can [download the high resolution version here](metal.png) to see
the details.
As you can see, the effects can be quite subtle to notice, so I
wouldn't stress too much if the value isn't perfectly right. Feel free
to use the repository to create your own test images.
