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              // Two chemicals
// A (red) is continually added
// B (green) is continually removed
// there is a reaction A+B+B -> B+B+B
// A and B diffuse at different rates
// according to the rate at which A is added and B removed,
// different patterns emerge

// see http://mrob.com/pub/comp/xmorphia
// see http://www.karlsims.com/rd.html

// for a more efficent option, use WebGL shaders:
// see http://mrob.com/pub/comp/xmorphia/ogl/index.html
 

AB = new field2D(128);
AB.set(function() {
  return [
    random() < 0.9 ? 1 : 0,
    random() < 0.1 ? 1 : 0,
    0
  ];
});
AB1 = AB.clone();

AB.smooth = true;
AB1.smooth = true;

function update() {
  // the diffusion rates (mainly affects scale)
  const dA = 5/AB.width; // diffusion rate of A
  const dB = dA*0.5; // diffusion rate of B

  // reaction term:
  AB1.set(function(x, y) {
    
    // vary feed & kill rates over the XY space:
    
    // feed rate, rate at which A is added
    // feed rates try .045 to .07
    const feed = 0.01 + 0.09*y/AB.height;
    
    // kill rate, rate at which B is removed
    // kill rates try .01 to .1
    const kill = 0.047 + 0.02*x/AB.width; 
    
    // center cell:
    const C = AB.cell(x, y);
    const a = C[0];
    const b = C[1];
    // the rate of change in A and B concentration 
    // due to reaction A B B -> B B B
    // is a product of the current concentrations:
    const reaction = a * b * b;
    
    // a basic approximation of the laplacian for diffusion
    const N = AB.cell(x, y+1);
    const S = AB.cell(x, y-1);
    const E = AB.cell(x+1, y);
    const W = AB.cell(x-1, y);
    const NW = AB.cell(x+1, y+1);
    const SW = AB.cell(x+1, y-1);
    const NE = AB.cell(x-1, y+1);
    const SE = AB.cell(x-1, y-1);
    const diag = 0.25;
    const alaplacian = N[0] + S[0] + E[0] + W[0] 
                     + 0.25*(NW[0] + NE[0] + SW[0] + SE[0]) 
                     - 5*a;
    const blaplacian = N[1] + S[1] + E[1] + W[1] 
                     + 0.25*(NW[1] + NE[1] + SW[1] + SE[1]) 
                     - 5*b;
    
    // compute changes in A and B concentration:
    const da = dA*alaplacian - reaction + feed*(1 - a);
    const db = dB*blaplacian + reaction - (kill + feed)*b;
    
    // apply these changes:
    return [a + da, b + db, 0];
  });

  // swap:
  const tmp = AB1;
  AB1 = AB;
  AB = tmp;
}

function draw() {
  AB.draw();
}
            
          
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