Directx11 advanced tutorial PBR (1) summary of physical phenomena of light

 

PBR

PBR namely Physically Based Rendering Abbreviation , That is, physical based rendering , And traditional Phong Rendering experience is different , It is really a rendering based on the summary of physical phenomena , Of course, it is not entirely physically correct , It is only based on the physical correctness of myopia . Let's talk about the physical phenomena of light

 

light (Light)

The essence of physical coloring is the interaction between light and various materials , And then through various ways into people's eyes .  Light is an electromagnetic wave , It vibrates up and down in the direction of its propagation .

Light has  “ wavelength ” This attribute ( The distance from crest to crest ). Light has a range of wavelengths , But the human eye can only see light with a small part of the wavelength (400mm - 700mm), This part of the wavelength is called visible light .

 

The influence of various substances on light an attribute definition of light interaction : Refractive index . Refractive index is related to wavelength , There is a functional mapping relationship . The refractive index of a substance to light affects the propagation speed of light and whether light is absorbed . 

 

The spread of light

 

Same medium (Homogeneous Media)--- Spread in a straight line

The simplest interaction between light and matter is the propagation of light in the same medium . Of course, different substances have different absorption and refractive indices of light , As a result, the absorption rate of light is also different . Light travels in a straight line in the same medium , Does not change the direction of transmission , However, the color and intensity may change in various media environments . The following is the phenomenon of light propagation in different and the same media .

 

Heterogeneous media (heterogeneous medium)-- scattering

Different media have different light refractive index , When light passes from one medium into another with a different refractive index , Light will be scattered :

1. The direction of light changes , The intensity of light does not change
2. The direction of light changes depending on the refractive index of the two materials
3. A wide range of material refractive index changes , Will cause more random , Wide range scattering

Physical appearance

When light propagates in a single material , Light is absorbed , The height and color of the light change , But the direction does not change . When light travels from one medium to another , Light is scattered , The direction of light changes , But the intensity of light does not change . Light absorption and light scattering occur simultaneously in most media , The appearance of a substance depends on the amount of light absorbed and the amount of light scattered . Of course, there is a third phenomenon of light , Spontaneous light , It is the object itself that produces light , This also affects the appearance of the substance , Such as light source , Light absorbing substances have this property at night , But in general, self illumination is rare in materials .

The appearance of matter =  The amount of light absorbed , The amount of light scattered , Spontaneous light ( Ordinary objects do not have self illumination )

 

Scattering at the surface boundary -- refraction

There are two media , Suppose that one of the two media is air , One is the other , They have different refractive indices , And suppose that there is a flat boundary between the two media , The scattering of light from one substance into another is called “ refraction ”. Pay attention , The flatness here is the ideal flatness , Many games are about equal to flat .

At this time, there are two phenomena , A beam of light enters another medium and continues to travel , This is the direction of refraction ; The other beam of light changes direction , Continue to propagate in this medium . This is reflection . Of course, this is a matter of a moment on the surface , How to propagate in a refractive medium is another matter , Not necessarily straight-line propagation . Next, we will continue to explain what may happen to refracted light .

The mathematical relations of various parameters of this refraction phenomenon are Fresnel (Fresnel equations) Formula to describe .

  Non optically flat microsurface reflection

The surface of most real-world objects cannot be regarded as ideally flat , But as an extremely small independent flat surface , This is the micro surface theory . Each micro surface is reflecting the received incident light alone , This is shown below :

The flatter the surface, the more consistent the direction of light reflected by the micro surface , The rougher the surface, the more scattered the direction of light reflected , As shown below , Above is a smooth surface , The rough surface below .

Subsurface scattering

As mentioned above, when refracted light enters another medium , Not necessarily straight-line propagation . Depends on the composition of the refracting material , Various phenomena may occur . For example, metal has a high light absorption rate ( Free electrons absorb light ), Light is absorbed as soon as it enters the metal . And for electrolytes or insulators , Will continue to scatter between the internal particles , Finally, it is re emitted from the medium , This is subsurface scattering .

If you pay close attention , It can be seen that many points scattered by the subsurface are not the same as the light incident point , And there is a certain distance . Some are far away , Some are relatively close , If the distance is less than pixels, it can be regarded as the same point in graphics , If the distance is greater than pixels, it cannot be ignored . In graphics , Many materials are ideal for re illuminating the same point as the incident point , This is called diffuse reflection (diffuse). You can think so ： Diffuse reflection is an idealistic subsurface scattering .

But in our tutorial PBR The theory mainly talks about diffuse reflection , Instead of talking about the general subsurface scattering phenomenon .

 

References

【1】Background: Physics and Math of Shadin