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Light Source

The light refracting into the pool can be from a fixed point source from which light is spreading radially, or ambient light scattering from all directions. Usually, the light incident on a real water surface is a combination of both. Because I use rays for modeling light, I classify sources into two categories. The first category includes point sources, from which light is represented as rays traveling to the surface and refracting onto the pool floor (specular-to-specular transport (Watt, 1990)). In the second category, ambient light refracts through the water surface and forms rays (diffuse-to-specular transport (Watt, 1990)). I approach the second case with the notion of an exploding surface.

For a point source, I start by computing the incident ray $ \vec s$ on each refraction point on the surface. Snell's law can be used in the plane containing both $ \vec s$ and $ \vec n$ to find the refracted ray $ \vec r$ into the water. First, however, the incident ray must be checked to insure that it is not coming from below the surface. This occurs when the source is located very close to the surface creating shadow zones behind high water ripples where there are no rays incident on the surface. Figure 3 shows the refraction of rays into water as well as some shadow zones.

fig-snells3d
fig-snells3d
Figure 3.
3D ray tracing simulation with a point source. [ER]
[pdf] [png]

As mentioned earlier, I use the exploding surface concept for the diffuse-to-specular mechanism of light transport through the surface. This concept is analogous to the exploding reflector concept that Claerbout (1985) uses as an introductory model for imaging. The refracted ray from each point on the exploding surface travels in the same direction as the surface normal at that point; i.e. $ \vec r=\vec n$. This is an easier approach to the modeling than working with a point source, but it is less accurate.


next up previous [pdf]

Next: Ray Representation Up: Forward Simulation Previous: Surface Representation

2009-04-13