书名：代码本色：用编程模拟自然系统
作者：Daniel Shiffman
译者：周晗彬
ISBN：978-7-115-36947-5
目录

4.13　图像纹理和加法混合

1、粒子系统图像纹理

  既然谈到粒子系统，我们还是有必要探讨一下用粒子纹理化渲染图像的例子。在你在设计特效时，如何选择粒子的绘制方式往往是个难题。

  在开始编码前，我们应该先准备好纹理图像。我建议你使用PNG格式的图像，因为在绘制图像时，Processing会为你保留PNG的alpha通道（透明度），这是你将这些纹理作为粒子叠加渲染时所必须的。一旦你将PNG图像放到Sketch的data目录中，后续只要写少量代码就可以完成这个功能。

2、示例

示例代码4-8　粒子系统图像纹理

ParticleSystem ps;

void setup() {
  size(640,360);
  PImage img = loadImage("texture.png");
  ps = new ParticleSystem(0,new PVector(width/2,height-75),img);
}

void draw() {
  background(0);
  
  // Calculate a "wind" force based on mouse horizontal position
  float dx = map(mouseX,0,width,-0.2,0.2);
  PVector wind = new PVector(dx,0);
  ps.applyForce(wind);
  ps.run();
  for (int i = 0; i < 2; i++) {
    ps.addParticle();
  }
  
  // Draw an arrow representing the wind force
  drawVector(wind, new PVector(width/2,50,0),500);

}

// Renders a vector object 'v' as an arrow and a position 'loc'
void drawVector(PVector v, PVector pos, float scayl) {
  pushMatrix();
  float arrowsize = 4;
  // Translate to position to render vector
  translate(pos.x,pos.y);
  stroke(255);
  // Call vector heading function to get direction (note that pointing up is a heading of 0) and rotate
  rotate(v.heading2D());
  // Calculate length of vector & scale it to be bigger or smaller if necessary
  float len = v.mag()*scayl;
  // Draw three lines to make an arrow (draw pointing up since we've rotate to the proper direction)
  line(0,0,len,0);
  line(len,0,len-arrowsize,+arrowsize/2);
  line(len,0,len-arrowsize,-arrowsize/2);
  popMatrix();
}

Particle.pde

class Particle {
  PVector pos;
  PVector vel;
  PVector acc;
  float lifespan;
  PImage img;

  Particle(PVector l,PImage img_) {
    acc = new PVector(0,0);
    float vx = randomGaussian()*0.3;
    float vy = randomGaussian()*0.3 - 1.0;
    vel = new PVector(vx,vy);
    pos = l.get();
    lifespan = 100.0;
    img = img_;
  }

  void run() {
    update();
    render();
  }
  
  // Method to apply a force vector to the Particle object
  // Note we are ignoring "mass" here
  void applyForce(PVector f) {
    acc.add(f);
  }  

  // Method to update position
  void update() {
    vel.add(acc);
    pos.add(vel);
    lifespan -= 2.5;
    acc.mult(0); // clear Acceleration
  }

  // Method to display
  void render() {
    imageMode(CENTER);
    tint(255,lifespan);
    image(img,pos.x,pos.y);
    // Drawing a circle instead
    // fill(255,lifespan);
    // noStroke();
    // ellipse(pos.x,pos.y,img.width,img.height);
  }

  // Is the particle still useful?
  boolean isDead() {
    if (lifespan <= 0.0) {
      return true;
    } else {
      return false;
    }
  }
}

ParticleSystem.pde

class ParticleSystem {

  ArrayList<Particle> particles;    // An arraylist for all the particles
  PVector origin;        // An origin point for where particles are birthed
  PImage img;

  ParticleSystem(int num, PVector v, PImage img_) {
    particles = new ArrayList<Particle>();              // Initialize the arraylist
    origin = v.get();                        // Store the origin point
    img = img_;
    for (int i = 0; i < num; i++) {
      particles.add(new Particle(origin, img));    // Add "num" amount of particles to the arraylist
    }
  }

  void run() {
    for (int i = particles.size()-1; i >= 0; i--) {
      Particle p = particles.get(i);
      p.run();
      if (p.isDead()) {
        particles.remove(i);
      }
    }
  }

  // Method to add a force vector to all particles currently in the system
  void applyForce(PVector dir) {
    // Enhanced loop!!!
    for (Particle p: particles) {
      p.applyForce(dir);
    }
  }  

  void addParticle() {
    particles.add(new Particle(origin, img));
  }

}

3、运行结果