Particles and sound effect system in games104 music 12 game engine

Particle basis

There are various emitters in the particle system （emitter）, Emitters emit particles （particle）.

• Particles have positions 、 Speed 、 Size, size 、 Color and life cycle 3D Model .
• In the life cycle of a particle , Include the production of （Spawn）、 Interaction with the environment and death . A very important point in design is to control the number of particles in the scene .
• Particle emitters have three functions ：
  – Specify the rules for particle generation
  – Specify the logic of particle simulation
  – Describes how to render particles
• Particle systems often have more than one type of emitter , For example, there are flames in the system of fire pile 、 Mars and smoke are three kinds of launchers
• About the production of particles , Location can be divided into single point generation 、 Produce on a region and in mesh There are three kinds of ; The mode can also be divided into continuous generation and intermittent generation
• Particle simulation ：
  1、 Conventional stress
  
  2、 How particles move , Generally, explicit Euler method is used . The force state determines the acceleration , Updates that determine speed , Speed determines position updates
  3、 Simulation time , Except gravity , You can also add the rotation of the particles 、 Change of color 、 Changes in size and collisions with the environment
• There are generally three types of particle shapes ：
  1、Billboard Particle. This particle is made up of some patches , Its shape actually has only one side , But it always keeps facing the camera , So it looks like a 3D The particles of the universe . If this particle It doesn't matter if you are small , But if it is large, the suggestion is that its shape will also change with time , Otherwise, it's fake
  2、Mesh Particle. When you want to simulate scattered particles, you often use 3D mesh As particles , Then set random attributes of different categories for them , Thus, it is easier for artists to achieve the desired effect
  3、Ribbon Particle. Spline particle , In fact, the particle is the control point on the spline , Then the complete strip is obtained through connection . It is often used in places such as the shadow of weapons waving . Interpolation is required during the connection of control points , Otherwise, the shape between each connection point is discontinuous （ It looks like quadrangles put together ）. Generally use centripetal catmull-rom interpolation （ Add additional blocks between particles and the number can be selected by yourself , But for CPU The requirements will become higher ）

Particle Renderer

Common transparent fusion problems also need to be solved in particle rendering , Sort from far to near .
There are two ways to sort particles , One is the global approach , Sort completely by individual particles , It's very accurate, but it costs a lot ; The other is by level , The order from simple to complex is according to the particle system 、 according to emitter Sort and in emitter Sort within .
say concretely , If you sort by particles , Sort by the distance from the particles to the camera ; If sorted by system or emitter , Then press Bounding box Sort .

On the other hand , The reason why particle rendering can be expensive is “ Full resolution particles ”. When we render a normal scene , because Z-buffer With the help of the , In fact, you only need to render the first object of the scene you touch （ Other occluded objects do not need to be rendered ）, But particle effects sometimes （ In the worst case ） In an instant, it will produce particles with multiple layers of the whole resolution , And they do not have a complete occlusion relationship , So it is equivalent to a huge amount of rendering at once .
The solution is ：
Lower resolution down sampling for particle rendering , This is irrelevant to the original scene , Get the color and transparency of the particles alpha. And then the upper sampling is merged into the original scene .

GPU The particle

As can be seen from the above , Particle computing consumes a lot of power , So put it in GPU Is a solution , There are three reasons. ：

• High parallelism , Suitable for the simulation of a large number of particles
• You can release CPU Power consumption to calculate the game itself
• It is convenient to obtain depth buffer for occlusion judgment

But one difficulty is that particles have a life cycle , Will continue to produce and disappear , So how to GPU Implementing particles in is a difficult problem .

Solution ：

• Intial State
  First create a particle pool , Design a data structure , Set the total number of particles the system contains , The position of each particle 、 Speed etc. . There are two more list, One is Dead List, Record the currently dead particles , Initially include all particle numbers , The other one is alive list, Record the currently living particles , Initially empty .
  such as emitter fired 5 A particle , Take... From the end of the pool 5 A particle （ Serial number ） Put in alive list, At the same time dead list After 5 Serial numbers are cleared .

• Simulate:
  When time jumps to the next tick when , It will create a new one alive list1, And search in order alive list Particles in , If a particle is found dead , The particle number will be moved to the death list , And skip this particle when rendering , If you're still alive, copy it alive list1 in .
  This operation is because compute shader The invention of became easy , because compute shader It can perform atomic level operations .
  meanwhile , After updating the live particles , It can also be used GPU Conduct frustum culling View culling （ For living particles ）, And calculate their distance , Write distance buffer.

• Sort, Render and Swap Alive Lists
  next , You also need to sort 、 Rendering and swapping alive list：
  1、 Sort . According to distance buffer Sort the live culled particles
  2、 Rendering . Render the sorted particles
  3、 In exchange for . In exchange for alive list1 and alive list, Update survival list .

• Specifically, sort .GPU The sorting of is similar to the merging algorithm , The complexity is nlogn. The approach taken is for the target sequence （ After ordering ） Set a thread at each location of the , Consider which of the two lists it should get from . This is easier than having a thread at each source list location to consider where to insert , Because the latter will make “ Writing process ” Jump to jump to discontinuous .

• At the same time, collision detection can be carried out by using depth buffer , Specifically ：
  1、 Project the current position of the particle to the screen space texture coordinates of the previous frame （ It is equivalent to projecting to the camera coordinate system of the previous frame ？）
  2、 Read the depth value in the depth texture map of the previous frame
  3、 Check 1 and 2 Depth value in , Judge whether it has collided but not completely penetrated （ Thickness values are used ）
  4、 If a collision happens , Calculate the normal direction of the collision surface and the direction of the particle bounce

Particle application

• Directly use particles to simulate objects , For example, birds 、 Passers by walking under the big map （ Because it's small ）, There are also bones , But basically each vertex is limited by only one node （ Simplified human body ）.
  under these circumstances , We can replace the acceleration and other behaviors of the original example with more complex human posture actions , It's equivalent to each particle Set up a state machine . So let's take this particle The various states and corresponding attributes such as speed are set to a texture map . When particle When the attributes of the texture map change, the corresponding state is found in the texture map and constantly switched .
• Navigation Texture. On the basis of the above , You can use a particle to simulate a navigation texture map . say concretely , utilize SDF To prevent people from entering the building , Then you can set a direction texture map in turn , When we set the destination and initial velocity for a particle , It will be based on DT Our field drives us to our destination . Of course , Some randomness can be added to the process .
  
  The application of particle systems in games is “ Preset ”, That is, it is set at the beginning particle Possible behavior , And use stack To represent and store ; And then there was “ Based on graph “ The design of the , Reduce the amount of code and increase flexibility ; The best is a mixture of the two , Here's the picture ：
  

Sound foundation

Too lazy to write. … It's better to watch video , Very broken

3D audio rendering

Too lazy to write. … It's better to watch video , Very broken