title: Can anyone help me understand how to simulate fluids?
body: <p>I'm trying to make a program that simulates the physics of fluids in Processing. In the IDE there's an included example:</p>
  
  <pre><code>    /**
       * Fluid 
       * by Glen Murphy. 
       * 
       * Click and drag the mouse to move the simulated fluid.
       * Adjust the "res" variable below to change resolution.
       * Code has not been optimised, and will run fairly slowly.
       */
  
      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 &lt; pnum; i++) {
          p[i] = new particle(random(res,width-res),random(res,height-res));
        }
        for (int i = 0; i &lt;= lwidth; i++) {
          for (int u = 0; u &lt;= 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 &lt; lwidth; i++) {
          for (int u = 0; u &lt; lheight; u++) {
            vbuf[i][u].updatebuf(i,u);
            v[i][u].col = 32;
          }
        }
        for (int i = 0; i &lt; pnum-1; i++) {
          p[i].updatepos();
        }
        for (int i = 0; i &lt; lwidth; i++) {
          for (int u = 0; u &lt; 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 &gt; 0 &amp;&amp; x &lt; width &amp;&amp; y &gt; 0 &amp;&amp; y &lt; 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&gt;0 &amp;&amp; i&lt;lwidth &amp;&amp; u&gt;0 &amp;&amp; u&lt;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&gt;0 &amp;&amp; i&lt;lwidth &amp;&amp; u&gt;0 &amp;&amp; u&lt;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 &lt; penSize) {
              if (dist &lt; 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 &gt; 255) col = 255;
          if (i&gt;0 &amp;&amp; i&lt;lwidth-1 &amp;&amp; u&gt;0 &amp;&amp; u&lt;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);
        }
  
  }
  </code></pre>
  
  <p>It's not really commented and I'm somewhat new to programming, so I have no idea where to start as far as understanding it. Is there any good reading on fluid physics? I'm more interesting in the visual effect than the accuracy of the simulation. </p>