(function(global) {

  'use strict';

  var fabric  = global.fabric || (global.fabric = { }),
      filters = fabric.Image.filters,
      createClass = fabric.util.createClass;

  /**
   * Gamma filter class
   * @class fabric.Image.filters.Gamma
   * @memberOf fabric.Image.filters
   * @extends fabric.Image.filters.BaseFilter
   * @see {@link fabric.Image.filters.Gamma#initialize} for constructor definition
   * @see {@link http://fabricjs.com/image-filters|ImageFilters demo}
   * @example
   * var filter = new fabric.Image.filters.Gamma({
   *   gamma: [1, 0.5, 2.1]
   * });
   * object.filters.push(filter);
   * object.applyFilters();
   */
  filters.Gamma = createClass(filters.BaseFilter, /** @lends fabric.Image.filters.Gamma.prototype */ {

    /**
     * Filter type
     * @param {String} type
     * @default
     */
    type: 'Gamma',

    fragmentSource: 'precision highp float;\n' +
      'uniform sampler2D uTexture;\n' +
      'uniform vec3 uGamma;\n' +
      'varying vec2 vTexCoord;\n' +
      'void main() {\n' +
        'vec4 color = texture2D(uTexture, vTexCoord);\n' +
        'vec3 correction = (1.0 / uGamma);\n' +
        'color.r = pow(color.r, correction.r);\n' +
        'color.g = pow(color.g, correction.g);\n' +
        'color.b = pow(color.b, correction.b);\n' +
        'gl_FragColor = color;\n' +
        'gl_FragColor.rgb *= color.a;\n' +
      '}',

    /**
     * Gamma array value, from 0.01 to 2.2.
     * @param {Array} gamma
     * @default
     */
    gamma: [1, 1, 1],

    /**
     * Describe the property that is the filter parameter
     * @param {String} m
     * @default
     */
    mainParameter: 'gamma',

    /**
     * Constructor
     * @param {Object} [options] Options object
     */
    initialize: function(options) {
      this.gamma = [1, 1, 1];
      filters.BaseFilter.prototype.initialize.call(this, options);
    },

    /**
     * Apply the Gamma operation to a Uint8Array representing the pixels of an image.
     *
     * @param {Object} options
     * @param {ImageData} options.imageData The Uint8Array to be filtered.
     */
    applyTo2d: function(options) {
      var imageData = options.imageData, data = imageData.data,
          gamma = this.gamma, len = data.length,
          rInv = 1 / gamma[0], gInv = 1 / gamma[1],
          bInv = 1 / gamma[2], i;

      if (!this.rVals) {
        // eslint-disable-next-line
        this.rVals = new Uint8Array(256);
        // eslint-disable-next-line
        this.gVals = new Uint8Array(256);
        // eslint-disable-next-line
        this.bVals = new Uint8Array(256);
      }

      // This is an optimization - pre-compute a look-up table for each color channel
      // instead of performing these pow calls for each pixel in the image.
      for (i = 0, len = 256; i < len; i++) {
        this.rVals[i] = Math.pow(i / 255, rInv) * 255;
        this.gVals[i] = Math.pow(i / 255, gInv) * 255;
        this.bVals[i] = Math.pow(i / 255, bInv) * 255;
      }
      for (i = 0, len = data.length; i < len; i += 4) {
        data[i] = this.rVals[data[i]];
        data[i + 1] = this.gVals[data[i + 1]];
        data[i + 2] = this.bVals[data[i + 2]];
      }
    },

    /**
     * Return WebGL uniform locations for this filter's shader.
     *
     * @param {WebGLRenderingContext} gl The GL canvas context used to compile this filter's shader.
     * @param {WebGLShaderProgram} program This filter's compiled shader program.
     */
    getUniformLocations: function(gl, program) {
      return {
        uGamma: gl.getUniformLocation(program, 'uGamma'),
      };
    },

    /**
     * Send data from this filter to its shader program's uniforms.
     *
     * @param {WebGLRenderingContext} gl The GL canvas context used to compile this filter's shader.
     * @param {Object} uniformLocations A map of string uniform names to WebGLUniformLocation objects
     */
    sendUniformData: function(gl, uniformLocations) {
      gl.uniform3fv(uniformLocations.uGamma, this.gamma);
    },
  });

  /**
   * Returns filter instance from an object representation
   * @static
   * @param {Object} object Object to create an instance from
   * @param {function} [callback] to be invoked after filter creation
   * @return {fabric.Image.filters.Gamma} Instance of fabric.Image.filters.Gamma
   */
  fabric.Image.filters.Gamma.fromObject = fabric.Image.filters.BaseFilter.fromObject;

})(typeof exports !== 'undefined' ? exports : this);