Article 1

reflection

Reflection refers to the process by which light incident on a surface interacts with the surface so that it leaves on the (same) side of the incident without changing frequency. The choice of reflection function determines the appearance of the surface.

• Ideal Specular: The perfect mirror

• Ideal diffuse: Uniform reflection in all directions, covering a large area

• Glossy specular: Most light is distributed in the direction of reflection

• Retro-reflection: Reflecting light back to the light source with small coverage

scattering

How do we simulate the scattering of light. Because photons can do a lot of things on the surface.

Left to right, top to bottom: rebound, penetration, into the inner surface rebound, absorption delay rebound

As you can see, photons can do a lot of things, but we just have to keep in mind that energy is conserved. It’s actually more complicated than that, of course, because some light is absorbed. In general, we use probabilities to get photons to do these things.

Article 2

When a ray of light hits an object surface, it reflects and refracts due to the rapid change in refractive index between the surface and air:

• Reflection. The direct reflection of light at the junction of two media is called Specular reflection. Metallic specular reflection color is three-channel color, while non-metallic specular reflection color is single-channel monochrome.
• Refraction.The overall appearance of a medium is determined by the combination of absorption and scattering characteristics of light refracted from the surface.
  • Scattering.Rapid changes in refractive index cause scattering, and the direction of light changes (splitting into multiple directions), but the amount or spectral distribution of light does not change. The type of scattering that is ultimately considered depends on the scale of observation:
    • Sub-surface Scattering. The observed pixel is smaller than the scattering distance, and the scattering is treated as subsurface scattering.
    • Diffuse. The pixel is observed to be larger than the scattering distance and the scattering is treated as diffuse reflection.
    • Transmission. The emergence of refracted incident light through an object. Transmission is a special case of subsurface scattering.
  • Absorption. Regions of material with a complex refractive index cause absorption by matching the frequency of the light wave to that of the electrons in the material’s atoms. The imaginary part of the complex number determines whether light is absorbed (converted to other forms of energy) as it propagates. The amount of light in the absorbing medium decreases with the distance it travels (it may also change the color of the light if absorption occurs preferentially at certain wavelengths), and the direction of the light does not change as a result of absorption. Any color tone is usually caused by the wavelength dependence of absorption.

Scattering and absorption

• Scattering determines the degree of turbidity of the medium. For the most part, particles in both solid and liquid media are larger than the wavelength of light and tend to scatter light evenly across all visible wavelengths. High scattering produces an opaque appearance.
• Absorption determines the appearance and colour of a material. The appearance color of almost any material is usually caused by the correlation of wavelengths it absorbs.

Metals and non-metals

• Metal. The appearance of a metal depends mainly on the direct reflection of light at the interface between the two media (specular reflection). The specular reflection color of metal is three-channel color, R, G, B are different. Light refracted into the metal is absorbed almost immediately by free electrons, and there is no scattering of light refracted into the metal.
• Non-metal (no-metal). The overall appearance of a nonmetal, or dielectric, is determined by its combination of absorption and scattering properties. Similarly, nonmetals interact with light in two parts: reflection and refraction. According to the scattering and absorption characteristics of the medium type, refraction can be divided into many categories:
  • Reflection. The specular reflection color of nonmetal is single channel monochrome, that is, R=G=B.
  • Refraction. Light is refracted from a surface into a non-metallic medium, and the overall appearance of the medium is determined by the combination of its scattering and absorption properties. Different media types have different scattering and absorption properties:
• For metals, refracted light is absorbed instantly – energy is absorbed instantly by free electrons.
• In the case of a nonmetal (also called a dielectric or insulator), once light is refracted within it, it behaves as a conventional participating medium, exhibiting both absorption and scattering.

Nonmetallic medium

• Homogeneous Media. Mainly transparent medium, no refractive index change. There is no scattering, light always travels in straight lines and does not change direction. There is absorption, which reduces the intensity of light, and the farther it travels, the more it absorbs.
• Nonhomogeneous Media. It can usually be modeled as a uniform medium with embedded scattered particles. It has refractive index variations and is divided into several categories.
  • Cloudy Media. The turbid medium has weak scattering and slightly random scattering direction. Regions of matter with a complex refractive index cause absorption, depending on their composition.
  • 3. Always Media. The translucent medium has strong scattering and the scattering direction is completely random. Regions of matter with a complex refractive index cause absorption, depending on their composition.
  • Opaque Media. Opaque media and translucent media are consistent. It has strong scattering and completely random scattering direction. Regions of matter with a complex refractive index cause absorption, depending on their composition.

Diffuse reflection and subsurface scattering are essentially the same

• In addition, diffuse reflection and sub-surface scattering are actually the same physical phenomenon, essentially are refracted light sub-surface scattering results. The only difference is the scattering distance relative to the observed scale. The scattering distance is insignificant compared to the pixel, and the sub-surface scattering can be approximated as diffuse reflection. That is, the refraction of light, modeled as diffuse reflection or subsurface scattering, depends on the scale of observation, as shown below.

In the upper left corner, the pixels (a green circle with a red border) are greater than the distance the light travels before leaving the surface. In this case, it can be assumed that the outgoing light emanates from the entry point (upper right), which can be treated as diffuse reflection and treated with a locally colored model. At the bottom, the pixel is smaller than the scattering distance; If a more realistic coloring effect is required, the existence of these distances cannot be ignored and should be treated as sub-surface scattering.

Reflections and highlights