text: /*global define*/
  define([
          '../Core/defined',
          '../Core/defineProperties',
          '../Core/DeveloperError',
          '../Core/loadCRN',
          '../Core/loadImage',
          '../Core/loadImageViaBlob',
          '../Core/loadKTX',
          '../Core/throttleRequestByServer'
      ], function(
          defined,
          defineProperties,
          DeveloperError,
          loadCRN,
          loadImage,
          loadImageViaBlob,
          loadKTX,
          throttleRequestByServer) {
      'use strict';
  
      /**
       * Provides imagery to be displayed on the surface of an ellipsoid.  This type describes an
       * interface and is not intended to be instantiated directly.
       *
       * @alias ImageryProvider
       * @constructor
       *
       * @see ArcGisMapServerImageryProvider
       * @see SingleTileImageryProvider
       * @see BingMapsImageryProvider
       * @see GoogleEarthImageryProvider
       * @see MapboxImageryProvider
       * @see createOpenStreetMapImageryProvider
       * @see WebMapTileServiceImageryProvider
       * @see WebMapServiceImageryProvider
       *
       * @demo {@link http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Imagery%20Layers.html|Cesium Sandcastle Imagery Layers Demo}
       * @demo {@link http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Imagery%20Layers%20Manipulation.html|Cesium Sandcastle Imagery Manipulation Demo}
       */
      function ImageryProvider() {
          /**
           * The default alpha blending value of this provider, with 0.0 representing fully transparent and
           * 1.0 representing fully opaque.
           *
           * @type {Number}
           * @default undefined
           */
          this.defaultAlpha = undefined;
  
          /**
           * The default brightness of this provider.  1.0 uses the unmodified imagery color.  Less than 1.0
           * makes the imagery darker while greater than 1.0 makes it brighter.
           *
           * @type {Number}
           * @default undefined
           */
          this.defaultBrightness = undefined;
  
          /**
           * The default contrast of this provider.  1.0 uses the unmodified imagery color.  Less than 1.0 reduces
           * the contrast while greater than 1.0 increases it.
           *
           * @type {Number}
           * @default undefined
           */
          this.defaultContrast = undefined;
  
          /**
           * The default hue of this provider in radians. 0.0 uses the unmodified imagery color.
           *
           * @type {Number}
           * @default undefined
           */
          this.defaultHue = undefined;
  
          /**
           * The default saturation of this provider. 1.0 uses the unmodified imagery color. Less than 1.0 reduces the
           * saturation while greater than 1.0 increases it.
           *
           * @type {Number}
           * @default undefined
           */
          this.defaultSaturation = undefined;
  
          /**
           * The default gamma correction to apply to this provider.  1.0 uses the unmodified imagery color.
           *
           * @type {Number}
           * @default undefined
           */
          this.defaultGamma = undefined;
  
          DeveloperError.throwInstantiationError();
      }
  
      defineProperties(ImageryProvider.prototype, {
          /**
           * Gets a value indicating whether or not the provider is ready for use.
           * @memberof ImageryProvider.prototype
           * @type {Boolean}
           * @readonly
           */
          ready : {
              get : DeveloperError.throwInstantiationError
          },
  
          /**
           * Gets a promise that resolves to true when the provider is ready for use.
           * @memberof ImageryProvider.prototype
           * @type {Promise.<Boolean>}
           * @readonly
           */
          readyPromise : {
              get : DeveloperError.throwInstantiationError
          },
  
          /**
           * Gets the rectangle, in radians, of the imagery provided by the instance.  This function should
           * not be called before {@link ImageryProvider#ready} returns true.
           * @memberof ImageryProvider.prototype
           * @type {Rectangle}
           * @readonly
           */
          rectangle: {
              get : DeveloperError.throwInstantiationError
          },
  
          /**
           * Gets the width of each tile, in pixels.  This function should
           * not be called before {@link ImageryProvider#ready} returns true.
           * @memberof ImageryProvider.prototype
           * @type {Number}
           * @readonly
           */
          tileWidth : {
              get : DeveloperError.throwInstantiationError
          },
  
          /**
           * Gets the height of each tile, in pixels.  This function should
           * not be called before {@link ImageryProvider#ready} returns true.
           * @memberof ImageryProvider.prototype
           * @type {Number}
           * @readonly
           */
          tileHeight : {
              get : DeveloperError.throwInstantiationError
          },
  
          /**
           * Gets the maximum level-of-detail that can be requested.  This function should
           * not be called before {@link ImageryProvider#ready} returns true.
           * @memberof ImageryProvider.prototype
           * @type {Number}
           * @readonly
           */
          maximumLevel : {
              get : DeveloperError.throwInstantiationError
          },
  
          /**
           * Gets the minimum level-of-detail that can be requested.  This function should
           * not be called before {@link ImageryProvider#ready} returns true. Generally,
           * a minimum level should only be used when the rectangle of the imagery is small
           * enough that the number of tiles at the minimum level is small.  An imagery
           * provider with more than a few tiles at the minimum level will lead to
           * rendering problems.
           * @memberof ImageryProvider.prototype
           * @type {Number}
           * @readonly
           */
          minimumLevel : {
              get : DeveloperError.throwInstantiationError
          },
  
          /**
           * Gets the tiling scheme used by the provider.  This function should
           * not be called before {@link ImageryProvider#ready} returns true.
           * @memberof ImageryProvider.prototype
           * @type {TilingScheme}
           * @readonly
           */
          tilingScheme : {
              get : DeveloperError.throwInstantiationError
          },
  
          /**
           * Gets the tile discard policy.  If not undefined, the discard policy is responsible
           * for filtering out "missing" tiles via its shouldDiscardImage function.  If this function
           * returns undefined, no tiles are filtered.  This function should
           * not be called before {@link ImageryProvider#ready} returns true.
           * @memberof ImageryProvider.prototype
           * @type {TileDiscardPolicy}
           * @readonly
           */
          tileDiscardPolicy : {
              get : DeveloperError.throwInstantiationError
          },
  
          /**
           * Gets an event that is raised when the imagery provider encounters an asynchronous error..  By subscribing
           * to the event, you will be notified of the error and can potentially recover from it.  Event listeners
           * are passed an instance of {@link TileProviderError}.
           * @memberof ImageryProvider.prototype
           * @type {Event}
           * @readonly
           */
          errorEvent : {
              get : DeveloperError.throwInstantiationError
          },
  
          /**
           * Gets the credit to display when this imagery provider is active.  Typically this is used to credit
           * the source of the imagery. This function should
           * not be called before {@link ImageryProvider#ready} returns true.
           * @memberof ImageryProvider.prototype
           * @type {Credit}
           * @readonly
           */
          credit : {
              get : DeveloperError.throwInstantiationError
          },
  
          /**
           * Gets the proxy used by this provider.
           * @memberof ImageryProvider.prototype
           * @type {Proxy}
           * @readonly
           */
          proxy : {
              get : DeveloperError.throwInstantiationError
          },
  
          /**
           * Gets a value indicating whether or not the images provided by this imagery provider
           * include an alpha channel.  If this property is false, an alpha channel, if present, will
           * be ignored.  If this property is true, any images without an alpha channel will be treated
           * as if their alpha is 1.0 everywhere.  When this property is false, memory usage
           * and texture upload time are reduced.
           * @memberof ImageryProvider.prototype
           * @type {Boolean}
           * @readonly
           */
          hasAlphaChannel : {
              get : DeveloperError.throwInstantiationError
          }
      });
  
      /**
       * Gets the credits to be displayed when a given tile is displayed.
       * @function
       *
       * @param {Number} x The tile X coordinate.
       * @param {Number} y The tile Y coordinate.
       * @param {Number} level The tile level;
       * @returns {Credit[]} The credits to be displayed when the tile is displayed.
       *
       * @exception {DeveloperError} <code>getTileCredits</code> must not be called before the imagery provider is ready.
       */
      ImageryProvider.prototype.getTileCredits = DeveloperError.throwInstantiationError;
  
      /**
       * Requests the image for a given tile.  This function should
       * not be called before {@link ImageryProvider#ready} returns true.
       * @function
       *
       * @param {Number} x The tile X coordinate.
       * @param {Number} y The tile Y coordinate.
       * @param {Number} level The tile level.
       * @returns {Promise.<Image|Canvas>|undefined} A promise for the image that will resolve when the image is available, or
       *          undefined if there are too many active requests to the server, and the request
       *          should be retried later.  The resolved image may be either an
       *          Image or a Canvas DOM object.
       *
       * @exception {DeveloperError} <code>requestImage</code> must not be called before the imagery provider is ready.
       */
      ImageryProvider.prototype.requestImage = DeveloperError.throwInstantiationError;
  
      /**
       * Asynchronously determines what features, if any, are located at a given longitude and latitude within
       * a tile.  This function should not be called before {@link ImageryProvider#ready} returns true.
       * This function is optional, so it may not exist on all ImageryProviders.
       *
       * @function
       *
       * @param {Number} x The tile X coordinate.
       * @param {Number} y The tile Y coordinate.
       * @param {Number} level The tile level.
       * @param {Number} longitude The longitude at which to pick features.
       * @param {Number} latitude  The latitude at which to pick features.
       * @return {Promise.<ImageryLayerFeatureInfo[]>|undefined} A promise for the picked features that will resolve when the asynchronous
       *                   picking completes.  The resolved value is an array of {@link ImageryLayerFeatureInfo}
       *                   instances.  The array may be empty if no features are found at the given location.
       *                   It may also be undefined if picking is not supported.
       *
       * @exception {DeveloperError} <code>pickFeatures</code> must not be called before the imagery provider is ready.
       */
      ImageryProvider.prototype.pickFeatures = DeveloperError.throwInstantiationError;
  
      var ktxRegex = /\.ktx$/i;
      var crnRegex = /\.crn$/i;
  
      /**
       * Loads an image from a given URL.  If the server referenced by the URL already has
       * too many requests pending, this function will instead return undefined, indicating
       * that the request should be retried later.
       *
       * @param {ImageryProvider} imageryProvider The imagery provider for the URL.
       * @param {String} url The URL of the image.
       * @returns {Promise.<Image|Canvas>|undefined} A promise for the image that will resolve when the image is available, or
       *          undefined if there are too many active requests to the server, and the request
       *          should be retried later.  The resolved image may be either an
       *          Image or a Canvas DOM object.
       */
      ImageryProvider.loadImage = function(imageryProvider, url) {
          if (ktxRegex.test(url)) {
              return throttleRequestByServer(url, loadKTX);
          } else if (crnRegex.test(url)) {
              return throttleRequestByServer(url, loadCRN);
          } else if (defined(imageryProvider.tileDiscardPolicy)) {
              return throttleRequestByServer(url, loadImageViaBlob);
          }
          return throttleRequestByServer(url, loadImage);
      };
  
      return ImageryProvider;
  });
  