【翻译】【cesium】Introduction to Particle Systems(粒子系统)

目录

What is a particle system?

Particle system basics

Emitters

BoxEmitter

CircleEmitter

ConeEmitter

SphereEmitter

Configuring particle systems

Life of the particle and life of the system

Styling particles

Color

Size

Speed

UpdateCallback

Positioning

Learn more

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What is a particle system?

 Particle systems are a graphical technique that simulates complex physically-based effects. Particle systems are collections of small images that when viewed together form a more complex “fuzzy” object, such as fire, smoke, weather, or fireworks. These complex effects are controlled by specifying the behavior of individual particles using properties such as initial position, velocity, and lifespan.

粒子系统是一种图形技术，可以模拟复杂的基于物理的效果。粒子系统是很多个小图像的集合，当它们在一起观看时，会形成一个更复杂的“模糊”物体，比如火、烟、天气或烟花。这个复杂的效果是通过控制单个粒子的初始位置、速度和寿命等属性行为来实现的

Particle system effects are common in movies and video games. For example, to represent damage to a plane, a technical artist could use a particle system to represent an explosion on the plane’s engine and then render a different particle system representing a smoke trail from the plane as it crashes.

粒子系统效果在电影和电子游戏中很常见。例如，用来表示飞机的损坏，技术艺术家可以使用一个粒子系统来代表飞机引擎上的爆炸，然后渲染一个不同的粒子系统来代表飞机坠毁时的烟雾轨迹。

Particle system basics

Take a look at the code for a basic particle system:

var particleSystem = viewer.scene.primitives.add(new Cesium.ParticleSystem({
    image : '../../SampleData/smoke.png',
    imageSize : new Cesium.Cartesian2(20, 20),
    startScale : 1.0,
    endScale : 4.0,
    particleLife : 1.0,
    speed : 5.0,
    emitter : new Cesium.CircleEmitter(0.5),
    emissionRate : 5.0,
    modelMatrix : entity.computeModelMatrix(viewer.clock.startTime, new Cesium.Matrix4()),
    lifetime : 16.0
}));

The above code creates a ParticleSystem, an object parameterized to control the appearance and behavior of individual Particle objects over time. Particles, which are born from a ParticleEmitter, have a position and type, live for a set amount of time, and then die.

Some of these properties are dynamic. Notice that instead of using the available single color property scale, there is a startScale and endScale. These allow you to specify that the particle size transitions between the start and end scale over the course of the particle’s life. startColor and endColor work similarly.

Other ways of affecting the visual output include maximum and minimum attributes. For every variable with a maximum and minimum input, the actual value for that variable on the particle will be randomly assigned to be between the maximum and minimum input and stay statically at that value for the entire life of the particle. For example, use minimumSpeed and maximumSpeed as bounds for the randomly chosen speed of each particle. Attributes that allow for changes like this include imageSize, speed, life, and particleLife.

Emitters

When a particle is born, its initial position and velocity vector are controlled by the ParticleEmitter. The emitter will spawn some number of particles per second, specified by the emissionRate parameter, initialized with a random velocity dependent on the emitter type.

Cesium has various ParticleEmitter’s that you can use out of the box.

BoxEmitter

BoxEmitter initializes particles at randomly-sampled positions within a box and directs them out of one of the six box faces. It accepts a Cartesian3 parameter specifying the width, height, and depth dimensions of the box.

var particleSystem = scene.primitives.add(new Cesium.ParticleSystem({
    image : '../../SampleData/smoke.png',
    color: Cesium.Color.MAGENTA,
    emissionRate: 5.0,
    emitter: new Cesium.BoxEmitter(new Cesium.Cartesian3(5.0, 5.0, 5.0)),
    imageSize : new Cesium.Cartesian2(25.0, 25.0),
    modelMatrix : entity.computeModelMatrix(viewer.clock.startTime, new Cesium.Matrix4()),
    lifetime : 16.0
}));

CircleEmitter

CircleEmitter initializes particles at randomly-sampled positions within a circle in the direction of the emitter’s up axis. It accepts a single float parameter specifying the radius of the circle.

var particleSystem = scene.primitives.add(new Cesium.ParticleSystem({
    image : '../../SampleData/smoke.png',
    color: Cesium.Color.MAGENTA,
    emissionRate: 5.0,
    emitter: new Cesium.CircleEmitter(5.0),
    imageSize : new Cesium.Cartesian2(25.0, 25.0),
    modelMatrix : entity.computeModelMatrix(viewer.clock.startTime, new Cesium.Matrix4()),
    lifetime : 16.0
}));

 If no emitter is specified, a CircleEmitter is created by default.

ConeEmitter

ConeEmitter initializes particles at the tip of a cone and directs them at random angles out of the cone. It takes a single float parameter specifying the angle of the cone. The cone is oriented along the up axis of the emitter.

var particleSystem = scene.primitives.add(new Cesium.ParticleSystem({
    image : '../../SampleData/smoke.png',
    color: Cesium.Color.MAGENTA,
    emissionRate: 5.0,
    emitter: new Cesium.ConeEmitter(Cesium.Math.toRadians(30.0)),
    imageSize : new Cesium.Cartesian2(25.0, 25.0),
    modelMatrix : entity.computeModelMatrix(viewer.clock.startTime, new Cesium.Matrix4()),
    lifetime : 16.0
}));

SphereEmitter

SphereEmitter initializes particles at randomly-sampled positions within a sphere and directs them outwards from the center of the sphere. It takes a single float parameter specifying the radius of the sphere.

var particleSystem = scene.primitives.add(new Cesium.ParticleSystem({
    image : '../../SampleData/smoke.png',
    color: Cesium.Color.MAGENTA,
    emissionRate: 5.0,
    emitter: new Cesium.SphereEmitter(5.0),
    imageSize : new Cesium.Cartesian2(25.0, 25.0),
    modelMatrix : entity.computeModelMatrix(viewer.clock.startTime, new Cesium.Matrix4()),
    lifetime : 16.0
}));

Configuring particle systems

Particle emission rate

emissionRate controls how many particles are emitted per second, which changes the density of particles in the system.

Specify an array of burst objects to emit bursts of particles at specified times (as demonstrated in the animations above). This adds variety or explosions to your particle system.

Add this property to your particleSystem.

bursts : [
    new Cesium.ParticleBurst({time : 5.0, minimum : 300, maximum : 500}),
    new Cesium.ParticleBurst({time : 10.0, minimum : 50, maximum : 100}),
    new Cesium.ParticleBurst({time : 15.0, minimum : 200, maximum : 300})
]

 These bursts will emit between min and max particles at the given times.

Life of the particle and life of the system

By default, particle systems will run forever. To make a particle system run for a set duration, use lifetime to specify the duration in seconds and set loop to false.

lifetime : 16.0,
loop: false

Setting particleLife to 5.0 will set every particle in the system to have that value for particleLife. To randomize the output for each particle, use the variables minimumParticleLife and maximumParticleLife.

minimumParticleLife: 5.0,
maximumParticleLife: 10.0

Styling particles

Color

Particles are styled using a texture specified with image and a color, which can change over the particle’s lifetime to create dynamic effects.

The following code makes the smoke particles transition from green and fade out to white.

startColor : Cesium.Color.LIGHTSEAGREEN.withAlpha(0.7),
endColor : Cesium.Color.WHITE.withAlpha(0.0),

Size

The size of a particle is controlled with the imageSize. To randomize sizes, control the width in pixels with minimumImageSize.x and maximumImageSize.x and the height in pixels between minimumImageSize.y and maximumImageSize.y.

This creates square particles between 30 and 60 pixels:

minimumImageSize : new Cesium.Cartesian2(30.0, 30.0),
maximumImageSize : new Cesium.Cartesian2(60.0, 60.0)

Speed

Speed is controlled by either the speed or by both the minimumSpeed and maximumSpeed settings.

minimumSpeed: 5.0,
maximumSpeed: 10.0

UpdateCallback

Particle systems can be further customized by applying an update function. This acts as a manual updater for each particle for effects such as gravity, wind, or color changes.

Particle systems have an updateCallback that modifies properties of the particle during the simulation. This function takes a particle and a simulation time step. Most physically-based effects will modify the velocity vector to change direction or speed. Here’s an example that will make particles react to gravity:

This function computes a gravity vector and uses the acceleration of gravity to alter the velocity of the particle.

Set the force as the particle system’s updateFunction:

updateCallback : applyGravity

Positioning

Particle systems are positioned using two Matrix4 transformation matrices:

• modelMatrix : transforms the particle system from model to world coordinates.
• emitterModelMatrix : transforms the particle system emitter within the particle system’s local coordinate system.


You can use just one of these transformation matrices and leave the other as an identity matrix, but we provide both for convenience. To practice creating the matrices, let’s position our particle emitter relative to another entity.

Create an entity for our particle system to accent. Open the Hello World Sandcastle example and add the following code to add a milk truck model to the viewer:

var entity = viewer.entities.add({
    model : {
        uri : '../../SampleData/models/CesiumMilkTruck/CesiumMilkTruck-kmc.glb'
    },
    position : Cesium.Cartesian3.fromDegrees(-75.15787310614596, 39.97862668312678)
});
viewer.trackedEntity = entity;

We want to add a smoke effect coming from the rear of the truck. Create a model matrix that will position and orient the particle system the same as the milk truck entity.

modelMatrix: entity.computeModelMatrix(time, new Cesium.Matrix4())

This places the particle system at the center of the truck. To position it at the back of the truck, we can create a matrix with a translation.

function computeEmitterModelMatrix() {
    hpr = Cesium.HeadingPitchRoll.fromDegrees(0.0, 0.0, 0.0, hpr);
    trs.translation = Cesium.Cartesian3.fromElements(-4.0, 0.0, 1.4, translation);
    trs.rotation = Cesium.Quaternion.fromHeadingPitchRoll(hpr, rotation);

    return Cesium.Matrix4.fromTranslationRotationScale(trs, emitterModelMatrix);
}

Now add the particle system.

var particleSystem = viewer.scene.primitives.add(new Cesium.ParticleSystem({
    image : '../../SampleData/smoke.png',
    
    startColor : Cesium.Color.LIGHTSEAGREEN.withAlpha(0.7),
    endColor : Cesium.Color.WHITE.withAlpha(0.0),
    
    startScale : 1.0,
    endScale : 4.0,
    
    particleLife : 1.0,
    
    minimumSpeed : 1.0,
    maximumSpeed : 4.0
    
    imageSize : new Cesium.Cartesian2(25, 25),
    emissionRate : 5.0,
    lifetime : 16.0,
    
    modelMatrix : entity.computeModelMatrix(viewer.clock.startTime, new Cesium.Matrix4())
    emitterModelMatrix : computeEmitterModelMatrix()
}));

Also note that we can update the model or emitter matrices over time. For instance, if we wanted to animate the emitter position on the truck, we can modify the emitterModelMatrix while leaving the modelMatrix untouched.

See the complete code example here: Particle System demo.

Learn more

For more cool effects with Particle Systems using more advanced techniques, see the Particle System More Effects Tutorial.

 

 

For more example code see:

• Particle System Fireworks Demo
• Particle Systems Weather
• Particle Systems Tails