code: function (size, unitSize, color1, color2) {
          size = size || 512;
          unitSize = unitSize || 64;
          color1 = color1 || 'black';
          color2 = color2 || 'white';
  
          var repeat = Math.ceil(size / unitSize);
  
          var canvas = document.createElement('canvas');
          canvas.width = size;
          canvas.height = size;
          var ctx = canvas.getContext('2d');
          ctx.fillStyle = color2;
          ctx.fillRect(0, 0, size, size);
  
          ctx.fillStyle = color1;
          for (var i = 0; i < repeat; i++) {
              for (var j = 0; j < repeat; j++) {
                  var isFill = j % 2 ? (i % 2) : (i % 2 - 1);
                  if (isFill) {
                      ctx.fillRect(i * unitSize, j * unitSize, unitSize, unitSize);
                  }
              }
          }
  
          var texture = new Texture2D({
              image: canvas,
              anisotropic: 8
          });
  
          return texture;
      }
docs: Create a chessboard texture
  @param  {number} [size]
  @param  {number} [unitSize]
  @param  {string} [color1]
  @param  {string} [color2]
  @return {clay.Texture2D}