int res = 2;
  int penSize = 30;
  int lwidth;
  int lheight;
  int pnum = 30000;
  vsquare[][] v;
  vbuffer[][] vbuf;
  particle[] p = new particle[pnum];
  int pcount = 0;
  int mouseXvel = 0;
  int mouseYvel = 0;
  
  void setup() 
  {
    size(200, 200);
    noStroke();
    frameRate(30);
    lwidth = width/res;
    lheight = height/res;
    v = new vsquare[lwidth+1][lheight+1];
    vbuf = new vbuffer[lwidth+1][lheight+1];
    for (int i = 0; i < pnum; i++) {
      p[i] = new particle(random(res,width-res),random(res,height-res));
    }
    for (int i = 0; i <= lwidth; i++) {
      for (int u = 0; u <= lheight; u++) {
        v[i][u] = new vsquare(i*res,u*res);
        vbuf[i][u] = new vbuffer(i*res,u*res);
      }
    }
  }
  
  void draw() 
  {
    background(#666666);
    
    int axvel = mouseX-pmouseX;
    int ayvel = mouseY-pmouseY;
  
    mouseXvel = (axvel != mouseXvel) ? axvel : 0;
    mouseYvel = (ayvel != mouseYvel) ? ayvel : 0;
  
    for (int i = 0; i < lwidth; i++) {
      for (int u = 0; u < lheight; u++) {
        vbuf[i][u].updatebuf(i,u);
        v[i][u].col = 32;
      }
    }
    for (int i = 0; i < pnum-1; i++) {
      p[i].updatepos();
    }
    for (int i = 0; i < lwidth; i++) {
      for (int u = 0; u < lheight; u++) {
        v[i][u].addbuffer(i, u);
        v[i][u].updatevels(mouseXvel, mouseYvel);
        v[i][u].display(i, u);
      }
    }
  }
  
  class particle {
    float x;
    float y;
    float xvel;
    float yvel;
    int pos;
    particle(float xIn, float yIn) {
      x = xIn;
      y = yIn;
    }
  
    void updatepos() {
      float col1;
      if (x > 0 && x < width && y > 0 && y < height) {
        int vi = (int)(x/res);
        int vu = (int)(y/res);
        vsquare o = v[vi][vu];    
        
        float ax = (x\%res)/res;
        float ay = (y\%res)/res;
        
        xvel += (1-ax)*v[vi][vu].xvel*0.05;
        yvel += (1-ay)*v[vi][vu].yvel*0.05;
        
        xvel += ax*v[vi+1][vu].xvel*0.05;
        yvel += ax*v[vi+1][vu].yvel*0.05;
        
        xvel += ay*v[vi][vu+1].xvel*0.05;
        yvel += ay*v[vi][vu+1].yvel*0.05;
  
        o.col += 4;
        
        x += xvel;
        y += yvel;
      }
      else {
        x = random(0,width);
        y = random(0,height);
        xvel = 0;
        yvel = 0;
      }
  
      xvel *= 0.5;
      yvel *= 0.5;
    }
  }
  
  class vbuffer {
    int x;
    int y;
    float xvel;
    float yvel;
    float pressurex = 0;
    float pressurey = 0;
    float pressure = 0;
  
    vbuffer(int xIn,int yIn) {
      x = xIn;
      y = yIn;
      pressurex = 0;
      pressurey = 0;
      }
  
    void updatebuf(int i, int u) {
      if (i>0 && i<lwidth && u>0 && u<lheight) {
        pressurex = (v[i-1][u-1].xvel*0.5 + v[i-1][u].xvel + v[i-1][u+1].xvel*0.5 - v[i+1][u-1].xvel*0.5 - v[i+1][u].xvel - v[i+1][u+1].xvel*0.5);
        pressurey = (v[i-1][u-1].yvel*0.5 + v[i][u-1].yvel + v[i+1][u-1].yvel*0.5 - v[i-1][u+1].yvel*0.5 - v[i][u+1].yvel - v[i+1][u+1].yvel*0.5);
        pressure = (pressurex + pressurey)*0.25;
        }
      }
    }
  
  class vsquare {
    int x;
    int y;
    float xvel;
    float yvel;
    float col;
  
    vsquare(int xIn,int yIn) {
      x = xIn;
      y = yIn;
      }
  
    void addbuffer(int i, int u) {
      if (i>0 && i<lwidth && u>0 && u<lheight) {
        xvel += (vbuf[i-1][u-1].pressure*0.5
                +vbuf[i-1][u].pressure
                +vbuf[i-1][u+1].pressure*0.5
                -vbuf[i+1][u-1].pressure*0.5
                -vbuf[i+1][u].pressure
                -vbuf[i+1][u+1].pressure*0.5
                )*0.25;
        yvel += (vbuf[i-1][u-1].pressure*0.5
                +vbuf[i][u-1].pressure
                +vbuf[i+1][u-1].pressure*0.5
                -vbuf[i-1][u+1].pressure*0.5
                -vbuf[i][u+1].pressure
                -vbuf[i+1][u+1].pressure*0.5
                )*0.25;
        }
      }
  
    void updatevels(int mvelX, int mvelY) {
      if (mousePressed) {
        float adj = x - mouseX;
        float opp = y - mouseY;
        float dist = sqrt(opp*opp + adj*adj);
        if (dist < penSize) {
          if (dist < 4) dist = penSize;
          float mod = penSize/dist;
          xvel += mvelX*mod;
          yvel += mvelY*mod;
          }
        }
  
      xvel *= 0.99;
      yvel *= 0.99;
    }
    
    void display(int i, int u) {
      float tcol = 0;
      if (col > 255) col = 255;
      if (i>0 && i<lwidth-1 && u>0 && u<lheight-1) {
        tcol = (+ v[i][u+1].col 
                + v[i+1][u].col 
                + v[i+1][u+1].col*0.5
                )*0.4;
        tcol = (int)(tcol+col*0.5);
        }
      else {
        tcol = (int)col;
        }
      fill(tcol, tcol, tcol);
      rect(x,y,res,res);
    }
  }