(function(global) {

  'use strict';

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

  /**
   * Contrast filter class
   * @class fabric.Image.filters.Contrast
   * @memberOf fabric.Image.filters
   * @extends fabric.Image.filters.BaseFilter
   * @see {@link fabric.Image.filters.Contrast#initialize} for constructor definition
   * @see {@link http://fabricjs.com/image-filters|ImageFilters demo}
   * @example
   * var filter = new fabric.Image.filters.Contrast({
   *   contrast: 0.25
   * });
   * object.filters.push(filter);
   * object.applyFilters();
   */
  filters.Contrast = createClass(filters.BaseFilter, /** @lends fabric.Image.filters.Contrast.prototype */ {

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

    fragmentSource: 'precision highp float;\n' +
      'uniform sampler2D uTexture;\n' +
      'uniform float uContrast;\n' +
      'varying vec2 vTexCoord;\n' +
      'void main() {\n' +
        'vec4 color = texture2D(uTexture, vTexCoord);\n' +
        'float contrastF = 1.015 * (uContrast + 1.0) / (1.0 * (1.015 - uContrast));\n' +
        'color.rgb = contrastF * (color.rgb - 0.5) + 0.5;\n' +
        'gl_FragColor = color;\n' +
      '}',

    /**
     * contrast value, range from -1 to 1.
     * @param {Number} contrast
     * @default 0
     */
    contrast: 0,

    mainParameter: 'contrast',

    /**
     * Constructor
     * @memberOf fabric.Image.filters.Contrast.prototype
     * @param {Object} [options] Options object
     * @param {Number} [options.contrast=0] Value to contrast the image up (-1...1)
     */

    /**
      * Apply the Contrast 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) {
      if (this.contrast === 0) {
        return;
      }
      var imageData = options.imageData, i, len,
          data = imageData.data, len = data.length,
          contrast = Math.floor(this.contrast * 255),
          contrastF = 259 * (contrast + 255) / (255 * (259 - contrast));

      for (i = 0; i < len; i += 4) {
        data[i] = contrastF * (data[i] - 128) + 128;
        data[i + 1] = contrastF * (data[i + 1] - 128) + 128;
        data[i + 2] = contrastF * (data[i + 2] - 128) + 128;
      }
    },

    /**
     * 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 {
        uContrast: gl.getUniformLocation(program, 'uContrast'),
      };
    },

    /**
     * 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.uniform1f(uniformLocations.uContrast, this.contrast);
    },
  });

  /**
   * 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.Contrast} Instance of fabric.Image.filters.Contrast
   */
  fabric.Image.filters.Contrast.fromObject = fabric.Image.filters.BaseFilter.fromObject;

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