Spectral rendering

In Computer Graphics, spectral rendering is where a scene's light transport is modeled considering the whole span of wavelengths instead of R,G,B values (still relating on geometric optic, which ignore wave phase). The motivation is that real colors of the physical world are spectrum; trichromatic colors are only inherent to Human Visual System. Many phenomenons are poorly represented through trichromy:

• A "green" light can have a spectrum with a peak in green, or peaks at yellow and blue. Indeed, plainty of different spectrums are perceived equivalent.
• Similarly for a transparent sheet filtering white light into "green".
• Similarly for a surface coat which reflect white light into "green".
• Still, the reflection and filtering of the light is wavelength-wise.
• Similarly for a series of filters, or for multiple scattering in a transparent volume, or for multiple reflections between coated surfaces. Thick layers of colored volumes as well as accumulated reflection at the corner or two bright coated walls tend to "purify" the material spectrum to its peaks since it is a power law of the base spectrum with the number of bounces. Thus, how "green" fluids or walls should looks like in this conditions is undetermined without spectrum.
• The refractive index is wavelength-dependant, which means that white rays are decomposed into rays going in different directions depending of the wavelength. Getting a rainbow caustic or arc requests managing wavelengths.

As an example, certain properties of tomatoes make them appear differently under sunlight than under fluorescent light. Using the blackbody radiation equations to simulate sunlight or the emission spectrum of a fluorescent bulb in combination with the tomato's spectral reflectance curve, more accurate images of each scenario can be produced. Chlorophyll, paint pigments, some bricks, are often strongly wavelength-dependent. Besides "temperature color" of incandescent light, LED, fluorescent and fluo-compact lights have "pathologic" spectrums made of isolated peaks which thus interact very differently than sun light with matter color. Thus the importance of accurate simulation for architectural, museographic or even night-driving simulation.

Some specific aspects can be dealt with using local evaluations (local light-material interaction), approximations or interpolations (.e.g, refraction). Still, the base method consist in replacing the triple R,G,B by a given larger sampling of frequencies, or by random chose of photon wavelengths. This process is thus a lot slower than classical trichromic rendering. Spectral rendering is often used in ray tracing or photon mapping to more accurately simulate the scene with demanding coat material and lighting characteristics, often for comparison with an actual photograph to test the rendering algorithm (as in a Cornell Box) or to simulate different portions of the electromagnetic spectrum for the purpose of scientific work.

Implementations

For example, Arion,^[1] FluidRay^[2] fryrender,^[3] Indigo Renderer,^[4] LuxRender,^[5] mental ray,^[6] Mitsuba,^[7] Octane Render,^[8] Spectral Studio,^[9] Thea Render^[10] and Ocean^[11] describe themselves as spectral renderers.

References

External links

• Cornell Box photo comparison