Lighting Basics: optical model

1. Prerequisite knowledge

Before explaining the lighting model , Let's first review the fragmentary and simple common sense of optics .

1、 Several common light sources ：

• The ambient light ： There is no concept of location , Formed by multiple reflections of light from the surrounding surface .
• Sunlight ： Directional light sources , Emit light at infinity , The light is parallel , Intensity does not decay with distance .
• Point source ： A light emitted at a point in a finite space , Light intensity decays with distance .
• Spotlights ： A light source with a conical effect , The light source has the direction and angle of light generation , There is a main direction of illumination , Light intensity increases with distance , Decay outward along the center （ Like a flashlight , stage lights ）.

2、 shadow

• Soft shadows ： Soft shadow border , The gradient , No serrations , Common and common .
• Hard shadow ： Sharp boundary , It usually appears at the alternation with the object or near the object when the light hits .

3、 Two basic reflections

• Diffuse reflection ： The smaller the angle between the incident light and the normal vector of the object surface, the brighter .
• Specular reflection ： The smaller the angle between the reflected light and the line of sight, the brighter .

4、 Scattering and absorption

When light hits the surface of an object , There are only two results ： scattering (scattering) And absorb (absorption).

• scattering ： Including internal scattering and external scattering .
  • Internal scattering ： refraction (refraction) And projection (transmission).
  • External scattering ： Reflection (reflection), Mainly diffuse reflection and specular reflection .
• absorption ： An object absorbs the energy of light , Cause the attenuation of light energy .

5、 Isotropy (isotropic) And anisotropy (anisotropic)

• Isotropy ： When the direction and angle of view of the light source are fixed, only the surface of the object is rotated , Reflection Can't Change .
• anisotropy ： When the direction and angle of view of the light source are fixed, only the surface of the object is rotated , The reflection changes .

6、 Direct light (DirectLight) And indirect illumination (IndirectLight)

• Direct light ： It is the light directly irradiated by the light source , Such as sunlight .
• Indirect illumination ： Is the light after one or more reflections , Such as moonlight .

2. Illumination model

Illumination model is to describe the scattering of light on an object 、 Optical phenomena such as absorption , And enter the human visual system （ Or visual sensor ） Model of .

among , We often mention in rendering To color （shading） It means that according to the material information 、 Light source information , The process of using a specific optical model to calculate the emittance along an observation direction .

Let's first introduce two basic optical models .

2.1 Basic optical model

2.1.1 Phong Optical model

Feng's illumination model (Phong) The main structure of the consists of 3 Elements make up ：

• The ambient light
• Diffuse reflection
• Specular illumination

2.1.2 Blinn-Phong Optical model

stay Phong Illumination algorithm , Specular reflection is modeled by the angle between the incident light and the normal vector of the object surface , As shown in the figure below . There is a problem with this approach , In computer computing , An included angle greater than 90° The situation of , But this kind of situation often does not exist in physics , For example, people observe things under the ground .

Blinn-Phong The model is Phong The expansion of the model , And Phong The model is very similar , But there are some differences in the treatment of specular light model , It no longer depends on the reflection vector , Instead, the so-called Half range vector (Halfway Vector), That is, a unit vector in the direction of half the angle between the light and the line of sight , When the half range vector is closer to the normal vector , The greater the specular light component , As shown in the figure below . This ensures that no matter which direction the observer looks , The angle between the half range vector and the surface normal will not exceed 90 degree .

2.2 BRDF Model

BRDF In the light model , It is used to describe the calculation when a ray of light strikes a surface from a certain direction , The intensity and direction of reflected light , among BRDF The full name is bidirectional reflection distribution function (Bidirectional Reflectance Distribution Function), In graphics, a mathematical formula is usually used to express , It also provides some parameters to adjust the material properties , Previously mentioned Phong Illumination and Blinn-Phong Light is the two most basic BRDF.

The optical model can be divided into two types ：

• Empirical model ： It's not practical , But easy to calculate , aforementioned Phong The model and its extensions are based on experience , This model cannot represent many important physical phenomena , Such as Fresnel reflection .
• The physical model ： Light based on Physics (PBR), Close to reality .

Reference resources

1、PBR Theoretical basis 3： Image based lighting （ On ）
2、 Computer graphics basic course homepage | Tsinghua University
3、 Computer graphics basic course video