pigmented colorants: dependence on media and time - Cornell ...

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Figure 4.12: A sequence of images showing the aging of a statuette, illustrating
the buildup of the underlying smooth copper sulphide tarnish
and the rough green patina. Adapted from [DH96].
matched to physical samples and photographs and the K-M absorption K and
scattering S coefficients were calculated from the estimated reflectances. Also,
since K-M theory only calculates diffuse reflectance, glossy and specular reflections
were approximated to complete the approximation to the BRDF.
Dorsey et al. further explored the weathering phenomena with stone [DEJ + 99].
In the simulation, changes are not only made to the appearance of stone over time,
but to the underlying 3D model as well (Dorsey’s previous work was limited to
2D surface effects, using a stack of layered operators). As stone weathers, the
majority of the changes occur near the surface of the material. First, a digitized
mesh representing the geometry of an object was converted into voxels. Then, the
model was parameterized with surface-aligned volumes (denoted slabs) containing
a narrow region of the outer portion of the model. Each of these slabs underwent
the authors’ weathering model, which employed a series of effects within the porous
stone material. These included the flow of moisture, the transport, dissolution, and
re-crystallization of minerals, and erosion of the material. The base composition
of the stone material was a procedurally generated, volumetric texture. Input to