Cloud rendering volume cloud [theoretical basis and implementation scheme]

• I went to listen to it some time ago xzw Big guy's volume cloud offline teaching , I took several notes
• According to the notes made during the class and the review after class , I have accumulated a lot of knowledge
• The teaching content mainly includes ： Theoretical basis and implementation scheme of volume cloud rendering 、 These features are modeled from the perspective of optical features 、 Optimization scheme of rendering effect and performance
• I will share some of the lessons learned from this course below （ First published in Zhihu ）

️ Common cloud rendering schemes

be based on   texture / particle system / Geometry disturbance / Volume rendering   Cloud rendering

️ Advantages and disadvantages of different cloud rendering schemes

️ Concept of atmosphere

The so-called atmosphere is around the planet / The gaseous material shell of celestial bodies such as satellites , When simulating the visual characteristics of the earth's atmosphere , We need to consider three atmospheric components ：

1. Dielectric particles whose size is significantly smaller than the wavelength of visible light （ oxygen / nitrogen ）
2. Dielectric particles with a size close to or larger than the wavelength of visible light （ Water drop ）
3. Medium particles with special visual effects and individual density distribution （ ozone ）

️ Cloud classification

Luke Howard stay 1803 According to the type of cloud, there are seven different cloud forms ：

️ Cloud shape simulation

Clouds have amorphous characteristics , Noise is usually used to simulate this amorphous feature of cloud . For volume rendering , It is generally used 3D texture . After determining the texture dimension , Then consider the noise simulation form .DECIMA The team mentioned Perlin-Worley Noise has good cloud morphological characteristics .

Tileable Perlin-Worley 3D (shadertoy.com)​www.shadertoy.com/view/3dVXDc

Berlin noise   Based on random , On this basis, smooth interpolation is carried out by using slow moving curve , The final effect tends to be more natural , It has good spatial continuity .

Wally noise   It belongs to a kind of implementation of cellular noise , There are obvious cell characteristics , This feature better simulates the cellular body of natural cloud .

Berlin - Wally noise   Both cloud characteristics , It has good continuity and cell effect . To be specific , Berlin noise can be defined as Wally noise , Remap to 0~1 The range space of , So we can get Berlin Wally noise .

Low frequency noise and high frequency noise   In the production of volume cloud , The basic shape of clouds is often constructed with low-frequency noise , And use high-frequency noise to erode the basic shape of the cloud, so as to shape a single cloud .

Low frequency noise   It's a 4 The tunnel 3D texture ,R Channel storage Berlin - Wally noise ,G\B\A Corresponding to the gradual improvement 3 Wally noise .

High frequency noise Used to add detail to the basic shape of the cloud 3D Texture has 3 Channels , Gradually increasing storage frequency 3 Wally noise , The Wally noise of these three channels is low-frequency noise G\B\A The Wally noise stored in the three channels is different in that its frequency is relatively higher .

The follow-up will be supplemented according to the time ...