// Creates a texture scale that combines pattern, color, and size.
function createTextureScale(){
  var patternScale,
      colorScale,
      sizeScale,
      patternAccessor,
      colorAccessor,
      sizeAccessor,
      texturesCache = {};

  function my(d){

    // Extract data values using accessors.
    var patternValue = patternAccessor(d),
        colorValue = colorAccessor(d),
        sizeValue = sizeAccessor(d),

        // Use data values to look up the texture.
        key = [patternValue, colorValue, sizeValue].join(","),
        texture = texturesCache[key],

        pattern, color, size;
    
    // Create a new texture the first time each unique
    // (texture, color, size) combination is encountered.
    if(!texture){

      // Evaluate scaled values for pattern, color, and size.
      pattern = patternScale(patternValue);
      color = colorScale(colorValue);
      size = sizeScale(sizeValue);

      // Create the base texture with the pattern generator.
      texture = pattern();

      // Apply color.
      texture = colorizeTexture(texture, color);

      // Apply size.
      texture = texture.size(size);

      // Initialize the texture.
      svg.call(texture);

      // Store the texture for future reuse.
      texturesCache[key] = texture;
    }

    // Return the url so this function can be passed directly into the fill attribute.
    return texture.url();
  }

  // Makes the given texture appear as the given color.
  function colorizeTexture(texture, color){

    // Use stroke, present on all patterns.
    var texture = texture.stroke(color);

    // Use fill, present only on some textures (e.g. "circles", not "lines").
    if(texture.fill){
      texture.fill(color);
    }

    return texture;
  }

  // API design inspired by http://bost.ocks.org/mike/chart/
  my.patternScale = function(value){
    if (!arguments.length) return patternScale;
    patternScale = value;
    return my;
  };

  my.colorScale = function(value){
    if (!arguments.length) return colorScale;
    colorScale = value;
    return my;
  };

  my.sizeScale = function(value){
    if (!arguments.length) return sizeScale;
    sizeScale = value;
    return my;
  };

  my.patternAccessor = function(value){
    if (!arguments.length) return patternAccessor;
    patternAccessor = value;
    return my;
  };

  my.colorAccessor = function(value){
    if (!arguments.length) return colorAccessor;
    colorAccessor = value;
    return my;
  };

  my.sizeAccessor = function(value){
    if (!arguments.length) return sizeAccessor;
    sizeAccessor = value;
    return my;
  };

  return my;
}