Billboard.js

/*global define*/
define([
        '../Core/BoundingRectangle',
        '../Core/Cartesian2',
        '../Core/Cartesian3',
        '../Core/Cartesian4',
        '../Core/Cartographic',
        '../Core/Color',
        '../Core/createGuid',
        '../Core/defaultValue',
        '../Core/defined',
        '../Core/defineProperties',
        '../Core/DeveloperError',
        '../Core/Matrix4',
        '../Core/NearFarScalar',
        './HeightReference',
        './HorizontalOrigin',
        './SceneMode',
        './SceneTransforms',
        './VerticalOrigin'
    ], function(
        BoundingRectangle,
        Cartesian2,
        Cartesian3,
        Cartesian4,
        Cartographic,
        Color,
        createGuid,
        defaultValue,
        defined,
        defineProperties,
        DeveloperError,
        Matrix4,
        NearFarScalar,
        HeightReference,
        HorizontalOrigin,
        SceneMode,
        SceneTransforms,
        VerticalOrigin) {
    'use strict';

    /**
     * A viewport-aligned image positioned in the 3D scene, that is created
     * and rendered using a {@link BillboardCollection}.  A billboard is created and its initial
     * properties are set by calling {@link BillboardCollection#add}.
     * <br /><br />
     * <div align='center'>
     * <img src='images/Billboard.png' width='400' height='300' /><br />
     * Example billboards
     * </div>
     *
     * @alias Billboard
     *
     * @performance Reading a property, e.g., {@link Billboard#show}, is constant time.
     * Assigning to a property is constant time but results in
     * CPU to GPU traffic when {@link BillboardCollection#update} is called.  The per-billboard traffic is
     * the same regardless of how many properties were updated.  If most billboards in a collection need to be
     * updated, it may be more efficient to clear the collection with {@link BillboardCollection#removeAll}
     * and add new billboards instead of modifying each one.
     *
     * @exception {DeveloperError} scaleByDistance.far must be greater than scaleByDistance.near
     * @exception {DeveloperError} translucencyByDistance.far must be greater than translucencyByDistance.near
     * @exception {DeveloperError} pixelOffsetScaleByDistance.far must be greater than pixelOffsetScaleByDistance.near
     *
     * @see BillboardCollection
     * @see BillboardCollection#add
     * @see Label
     *
     * @internalConstructor
     *
     * @demo {@link http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Billboards.html|Cesium Sandcastle Billboard Demo}
     */
    function Billboard(options, billboardCollection) {
        options = defaultValue(options, defaultValue.EMPTY_OBJECT);

        //>>includeStart('debug', pragmas.debug);
        if (defined(options.scaleByDistance) && options.scaleByDistance.far <= options.scaleByDistance.near) {
            throw new DeveloperError('scaleByDistance.far must be greater than scaleByDistance.near.');
        }
        if (defined(options.translucencyByDistance) && options.translucencyByDistance.far <= options.translucencyByDistance.near) {
            throw new DeveloperError('translucencyByDistance.far must be greater than translucencyByDistance.near.');
        }
        if (defined(options.pixelOffsetScaleByDistance) && options.pixelOffsetScaleByDistance.far <= options.pixelOffsetScaleByDistance.near) {
            throw new DeveloperError('pixelOffsetScaleByDistance.far must be greater than pixelOffsetScaleByDistance.near.');
        }
        //>>includeEnd('debug');

        this._show = defaultValue(options.show, true);
        this._position = Cartesian3.clone(defaultValue(options.position, Cartesian3.ZERO));
        this._actualPosition = Cartesian3.clone(this._position); // For columbus view and 2D
        this._pixelOffset = Cartesian2.clone(defaultValue(options.pixelOffset, Cartesian2.ZERO));
        this._translate = new Cartesian2(0.0, 0.0); // used by labels for glyph vertex translation
        this._eyeOffset = Cartesian3.clone(defaultValue(options.eyeOffset, Cartesian3.ZERO));
        this._heightReference = defaultValue(options.heightReference, HeightReference.NONE);
        this._verticalOrigin = defaultValue(options.verticalOrigin, VerticalOrigin.CENTER);
        this._horizontalOrigin = defaultValue(options.horizontalOrigin, HorizontalOrigin.CENTER);
        this._scale = defaultValue(options.scale, 1.0);
        this._color = Color.clone(defaultValue(options.color, Color.WHITE));
        this._rotation = defaultValue(options.rotation, 0.0);
        this._alignedAxis = Cartesian3.clone(defaultValue(options.alignedAxis, Cartesian3.ZERO));
        this._width = options.width;
        this._height = options.height;
        this._scaleByDistance = options.scaleByDistance;
        this._translucencyByDistance = options.translucencyByDistance;
        this._pixelOffsetScaleByDistance = options.pixelOffsetScaleByDistance;
        this._sizeInMeters = defaultValue(options.sizeInMeters, false);
        this._id = options.id;
        this._collection = defaultValue(options.collection, billboardCollection);

        this._pickId = undefined;
        this._pickPrimitive = defaultValue(options._pickPrimitive, this);
        this._billboardCollection = billboardCollection;
        this._dirty = false;
        this._index = -1; //Used only by BillboardCollection

        this._imageIndex = -1;
        this._imageIndexPromise = undefined;
        this._imageId = undefined;
        this._image = undefined;
        this._imageSubRegion = undefined;
        this._imageWidth = undefined;
        this._imageHeight = undefined;

        var image = options.image;
        var imageId = options.imageId;
        if (defined(image)) {
            if (!defined(imageId)) {
                if (typeof image === 'string') {
                    imageId = image;
                } else if (defined(image.src)) {
                    imageId = image.src;
                } else {
                    imageId = createGuid();
                }
            }

            this._imageId = imageId;
            this._image = image;
        }

        if (defined(options.imageSubRegion)) {
            this._imageId = imageId;
            this._imageSubRegion = options.imageSubRegion;
        }

        if (defined(this._billboardCollection._textureAtlas)) {
            this._loadImage();
        }

        this._actualClampedPosition = undefined;
        this._removeCallbackFunc = undefined;
        this._mode = SceneMode.SCENE3D;

        this._updateClamping();
    }

    var SHOW_INDEX = Billboard.SHOW_INDEX = 0;
    var POSITION_INDEX = Billboard.POSITION_INDEX = 1;
    var PIXEL_OFFSET_INDEX = Billboard.PIXEL_OFFSET_INDEX = 2;
    var EYE_OFFSET_INDEX = Billboard.EYE_OFFSET_INDEX = 3;
    var HORIZONTAL_ORIGIN_INDEX = Billboard.HORIZONTAL_ORIGIN_INDEX = 4;
    var VERTICAL_ORIGIN_INDEX = Billboard.VERTICAL_ORIGIN_INDEX = 5;
    var SCALE_INDEX = Billboard.SCALE_INDEX = 6;
    var IMAGE_INDEX_INDEX = Billboard.IMAGE_INDEX_INDEX = 7;
    var COLOR_INDEX = Billboard.COLOR_INDEX = 8;
    var ROTATION_INDEX = Billboard.ROTATION_INDEX = 9;
    var ALIGNED_AXIS_INDEX = Billboard.ALIGNED_AXIS_INDEX = 10;
    var SCALE_BY_DISTANCE_INDEX = Billboard.SCALE_BY_DISTANCE_INDEX = 11;
    var TRANSLUCENCY_BY_DISTANCE_INDEX = Billboard.TRANSLUCENCY_BY_DISTANCE_INDEX = 12;
    var PIXEL_OFFSET_SCALE_BY_DISTANCE_INDEX = Billboard.PIXEL_OFFSET_SCALE_BY_DISTANCE_INDEX = 13;
    Billboard.NUMBER_OF_PROPERTIES = 14;

    function makeDirty(billboard, propertyChanged) {
        var billboardCollection = billboard._billboardCollection;
        if (defined(billboardCollection)) {
            billboardCollection._updateBillboard(billboard, propertyChanged);
            billboard._dirty = true;
        }
    }

    defineProperties(Billboard.prototype, {
        /**
         * Determines if this billboard will be shown.  Use this to hide or show a billboard, instead
         * of removing it and re-adding it to the collection.
         * @memberof Billboard.prototype
         * @type {Boolean}
         */
        show : {
            get : function() {
                return this._show;
            },
            set : function(value) {
                //>>includeStart('debug', pragmas.debug);
                if (!defined(value)) {
                    throw new DeveloperError('value is required.');
                }
                //>>includeEnd('debug');

                if (this._show !== value) {
                    this._show = value;
                    makeDirty(this, SHOW_INDEX);
                }
            }
        },

        /**
        * Gets or sets the Cartesian position of this billboard.
        * @memberof Billboard.prototype
        * @type {Cartesian3}
        */
        position : {
            get : function() {
                return this._position;
            },
            set : function(value) {
                //>>includeStart('debug', pragmas.debug)
                if (!defined(value)) {
                    throw new DeveloperError('value is required.');
                }
                //>>includeEnd('debug');

                var position = this._position;
                if (!Cartesian3.equals(position, value)) {
                    Cartesian3.clone(value, position);
                    Cartesian3.clone(value, this._actualPosition);

                    this._updateClamping();
                    makeDirty(this, POSITION_INDEX);
                }
            }
        },

        /**
         * Gets or sets the height reference of this billboard.
         * @memberof Billboard.prototype
         * @type {HeightReference}
         * @default HeightReference.NONE
         */
        heightReference : {
            get : function() {
                return this._heightReference;
            },
            set : function(value) {
                //>>includeStart('debug', pragmas.debug)
                if (!defined(value)) {
                    throw new DeveloperError('value is required.');
                }
                //>>includeEnd('debug');

                var heightReference = this._heightReference;
                if (value !== heightReference) {
                    this._heightReference = value;
                    this._updateClamping();
                    makeDirty(this, POSITION_INDEX);
                }
            }
        },

        /**
         * Gets or sets the pixel offset in screen space from the origin of this billboard.  This is commonly used
         * to align multiple billboards and labels at the same position, e.g., an image and text.  The
         * screen space origin is the top, left corner of the canvas; <code>x</code> increases from
         * left to right, and <code>y</code> increases from top to bottom.
         * <br /><br />
         * <div align='center'>
         * <table border='0' cellpadding='5'><tr>
         * <td align='center'><code>default</code><br/><img src='images/Billboard.setPixelOffset.default.png' width='250' height='188' /></td>
         * <td align='center'><code>b.pixeloffset = new Cartesian2(50, 25);</code><br/><img src='images/Billboard.setPixelOffset.x50y-25.png' width='250' height='188' /></td>
         * </tr></table>
         * The billboard's origin is indicated by the yellow point.
         * </div>
         * @memberof Billboard.prototype
         * @type {Cartesian2}
         */
        pixelOffset : {
            get : function() {
                return this._pixelOffset;
            },
            set : function(value) {
                //>>includeStart('debug', pragmas.debug);
                if (!defined(value)) {
                    throw new DeveloperError('value is required.');
                }
                //>>includeEnd('debug');

                var pixelOffset = this._pixelOffset;
                if (!Cartesian2.equals(pixelOffset, value)) {
                    Cartesian2.clone(value, pixelOffset);
                    makeDirty(this, PIXEL_OFFSET_INDEX);
                }
            }
        },

        /**
         * Gets or sets near and far scaling properties of a Billboard based on the billboard's distance from the camera.
         * A billboard's scale will interpolate between the {@link NearFarScalar#nearValue} and
         * {@link NearFarScalar#farValue} while the camera distance falls within the upper and lower bounds
         * of the specified {@link NearFarScalar#near} and {@link NearFarScalar#far}.
         * Outside of these ranges the billboard's scale remains clamped to the nearest bound.  If undefined,
         * scaleByDistance will be disabled.
         * @memberof Billboard.prototype
         * @type {NearFarScalar}
         *
         * @example
         * // Example 1.
         * // Set a billboard's scaleByDistance to scale by 1.5 when the
         * // camera is 1500 meters from the billboard and disappear as
         * // the camera distance approaches 8.0e6 meters.
         * b.scaleByDistance = new Cesium.NearFarScalar(1.5e2, 1.5, 8.0e6, 0.0);
         *
         * @example
         * // Example 2.
         * // disable scaling by distance
         * b.scaleByDistance = undefined;
         */
        scaleByDistance : {
            get : function() {
                return this._scaleByDistance;
            },
            set : function(value) {
                //>>includeStart('debug', pragmas.debug);
                if (defined(value) && value.far <= value.near) {
                    throw new DeveloperError('far distance must be greater than near distance.');
                }
                //>>includeEnd('debug');

                var scaleByDistance = this._scaleByDistance;
                if (!NearFarScalar.equals(scaleByDistance, value)) {
                    this._scaleByDistance = NearFarScalar.clone(value, scaleByDistance);
                    makeDirty(this, SCALE_BY_DISTANCE_INDEX);
                }
            }
        },

        /**
         * Gets or sets near and far translucency properties of a Billboard based on the billboard's distance from the camera.
         * A billboard's translucency will interpolate between the {@link NearFarScalar#nearValue} and
         * {@link NearFarScalar#farValue} while the camera distance falls within the upper and lower bounds
         * of the specified {@link NearFarScalar#near} and {@link NearFarScalar#far}.
         * Outside of these ranges the billboard's translucency remains clamped to the nearest bound.  If undefined,
         * translucencyByDistance will be disabled.
         * @memberof Billboard.prototype
         * @type {NearFarScalar}
         *
         * @example
         * // Example 1.
         * // Set a billboard's translucency to 1.0 when the
         * // camera is 1500 meters from the billboard and disappear as
         * // the camera distance approaches 8.0e6 meters.
         * b.translucencyByDistance = new Cesium.NearFarScalar(1.5e2, 1.0, 8.0e6, 0.0);
         *
         * @example
         * // Example 2.
         * // disable translucency by distance
         * b.translucencyByDistance = undefined;
         */
        translucencyByDistance : {
            get : function() {
                return this._translucencyByDistance;
            },
            set : function(value) {
                //>>includeStart('debug', pragmas.debug);
                if (defined(value) && value.far <= value.near) {
                    throw new DeveloperError('far distance must be greater than near distance.');
                }
                //>>includeEnd('debug');

                var translucencyByDistance = this._translucencyByDistance;
                if (!NearFarScalar.equals(translucencyByDistance, value)) {
                    this._translucencyByDistance = NearFarScalar.clone(value, translucencyByDistance);
                    makeDirty(this, TRANSLUCENCY_BY_DISTANCE_INDEX);
                }
            }
        },

        /**
         * Gets or sets near and far pixel offset scaling properties of a Billboard based on the billboard's distance from the camera.
         * A billboard's pixel offset will be scaled between the {@link NearFarScalar#nearValue} and
         * {@link NearFarScalar#farValue} while the camera distance falls within the upper and lower bounds
         * of the specified {@link NearFarScalar#near} and {@link NearFarScalar#far}.
         * Outside of these ranges the billboard's pixel offset scale remains clamped to the nearest bound.  If undefined,
         * pixelOffsetScaleByDistance will be disabled.
         * @memberof Billboard.prototype
         * @type {NearFarScalar}
         *
         * @example
         * // Example 1.
         * // Set a billboard's pixel offset scale to 0.0 when the
         * // camera is 1500 meters from the billboard and scale pixel offset to 10.0 pixels
         * // in the y direction the camera distance approaches 8.0e6 meters.
         * b.pixelOffset = new Cesium.Cartesian2(0.0, 1.0);
         * b.pixelOffsetScaleByDistance = new Cesium.NearFarScalar(1.5e2, 0.0, 8.0e6, 10.0);
         *
         * @example
         * // Example 2.
         * // disable pixel offset by distance
         * b.pixelOffsetScaleByDistance = undefined;
         */
        pixelOffsetScaleByDistance : {
            get : function() {
                return this._pixelOffsetScaleByDistance;
            },
            set : function(value) {
                //>>includeStart('debug', pragmas.debug);
                if (defined(value) && value.far <= value.near) {
                    throw new DeveloperError('far distance must be greater than near distance.');
                }
                //>>includeEnd('debug');

                var pixelOffsetScaleByDistance = this._pixelOffsetScaleByDistance;
                if (!NearFarScalar.equals(pixelOffsetScaleByDistance, value)) {
                    this._pixelOffsetScaleByDistance = NearFarScalar.clone(value, pixelOffsetScaleByDistance);
                    makeDirty(this, PIXEL_OFFSET_SCALE_BY_DISTANCE_INDEX);
                }
            }
        },

        /**
         * Gets or sets the 3D Cartesian offset applied to this billboard in eye coordinates.  Eye coordinates is a left-handed
         * coordinate system, where <code>x</code> points towards the viewer's right, <code>y</code> points up, and
         * <code>z</code> points into the screen.  Eye coordinates use the same scale as world and model coordinates,
         * which is typically meters.
         * <br /><br />
         * An eye offset is commonly used to arrange multiple billboards or objects at the same position, e.g., to
         * arrange a billboard above its corresponding 3D model.
         * <br /><br />
         * Below, the billboard is positioned at the center of the Earth but an eye offset makes it always
         * appear on top of the Earth regardless of the viewer's or Earth's orientation.
         * <br /><br />
         * <div align='center'>
         * <table border='0' cellpadding='5'><tr>
         * <td align='center'><img src='images/Billboard.setEyeOffset.one.png' width='250' height='188' /></td>
         * <td align='center'><img src='images/Billboard.setEyeOffset.two.png' width='250' height='188' /></td>
         * </tr></table>
         * <code>b.eyeOffset = new Cartesian3(0.0, 8000000.0, 0.0);</code><br /><br />
         * </div>
         * @memberof Billboard.prototype
         * @type {Cartesian3}
         */
        eyeOffset : {
            get : function() {
                return this._eyeOffset;
            },
            set : function(value) {
                //>>includeStart('debug', pragmas.debug);
                if (!defined(value)) {
                    throw new DeveloperError('value is required.');
                }
                //>>includeEnd('debug');

                var eyeOffset = this._eyeOffset;
                if (!Cartesian3.equals(eyeOffset, value)) {
                    Cartesian3.clone(value, eyeOffset);
                    makeDirty(this, EYE_OFFSET_INDEX);
                }
            }
        },

        /**
         * Gets or sets the horizontal origin of this billboard, which determines if the billboard is
         * to the left, center, or right of its position.
         * <br /><br />
         * <div align='center'>
         * <img src='images/Billboard.setHorizontalOrigin.png' width='400' height='300' /><br />
         * </div>
         * @memberof Billboard.prototype
         * @type {HorizontalOrigin}
         * @example
         * // Use a bottom, left origin
         * b.horizontalOrigin = Cesium.HorizontalOrigin.LEFT;
         * b.verticalOrigin = Cesium.VerticalOrigin.BOTTOM;
         */
        horizontalOrigin : {
            get : function() {
                return this._horizontalOrigin;
            },
            set : function(value) {
                //>>includeStart('debug', pragmas.debug);
                if (!defined(value)) {
                    throw new DeveloperError('value is required.');
                }
                //>>includeEnd('debug');

                if (this._horizontalOrigin !== value) {
                    this._horizontalOrigin = value;
                    makeDirty(this, HORIZONTAL_ORIGIN_INDEX);
                }
            }
        },

        /**
         * Gets or sets the vertical origin of this billboard, which determines if the billboard is
         * to the above, below, or at the center of its position.
         * <br /><br />
         * <div align='center'>
         * <img src='images/Billboard.setVerticalOrigin.png' width='400' height='300' /><br />
         * </div>
         * @memberof Billboard.prototype
         * @type {VerticalOrigin}
         * @example
         * // Use a bottom, left origin
         * b.horizontalOrigin = Cesium.HorizontalOrigin.LEFT;
         * b.verticalOrigin = Cesium.VerticalOrigin.BOTTOM;
         */
        verticalOrigin : {
            get : function() {
                return this._verticalOrigin;
            },
            set : function(value) {
                //>>includeStart('debug', pragmas.debug);
                if (!defined(value)) {
                    throw new DeveloperError('value is required.');
                }
                //>>includeEnd('debug');

                if (this._verticalOrigin !== value) {
                    this._verticalOrigin = value;
                    makeDirty(this, VERTICAL_ORIGIN_INDEX);
                }
            }
        },

        /**
         * Gets or sets the uniform scale that is multiplied with the billboard's image size in pixels.
         * A scale of <code>1.0</code> does not change the size of the billboard; a scale greater than
         * <code>1.0</code> enlarges the billboard; a positive scale less than <code>1.0</code> shrinks
         * the billboard.
         * <br /><br />
         * <div align='center'>
         * <img src='images/Billboard.setScale.png' width='400' height='300' /><br/>
         * From left to right in the above image, the scales are <code>0.5</code>, <code>1.0</code>,
         * and <code>2.0</code>.
         * </div>
         * @memberof Billboard.prototype
         * @type {Number}
         */
        scale : {
            get : function() {
                return this._scale;
            },
            set : function(value) {
                //>>includeStart('debug', pragmas.debug);
                if (!defined(value)) {
                    throw new DeveloperError('value is required.');
                }
                //>>includeEnd('debug');

                if (this._scale !== value) {
                    this._scale = value;
                    makeDirty(this, SCALE_INDEX);
                }
            }
        },

        /**
         * Gets or sets the color that is multiplied with the billboard's texture.  This has two common use cases.  First,
         * the same white texture may be used by many different billboards, each with a different color, to create
         * colored billboards.  Second, the color's alpha component can be used to make the billboard translucent as shown below.
         * An alpha of <code>0.0</code> makes the billboard transparent, and <code>1.0</code> makes the billboard opaque.
         * <br /><br />
         * <div align='center'>
         * <table border='0' cellpadding='5'><tr>
         * <td align='center'><code>default</code><br/><img src='images/Billboard.setColor.Alpha255.png' width='250' height='188' /></td>
         * <td align='center'><code>alpha : 0.5</code><br/><img src='images/Billboard.setColor.Alpha127.png' width='250' height='188' /></td>
         * </tr></table>
         * </div>
         * <br />
         * The red, green, blue, and alpha values are indicated by <code>value</code>'s <code>red</code>, <code>green</code>,
         * <code>blue</code>, and <code>alpha</code> properties as shown in Example 1.  These components range from <code>0.0</code>
         * (no intensity) to <code>1.0</code> (full intensity).
         * @memberof Billboard.prototype
         * @type {Color}
         *
         * @example
         * // Example 1. Assign yellow.
         * b.color = Cesium.Color.YELLOW;
         *
         * @example
         * // Example 2. Make a billboard 50% translucent.
         * b.color = new Cesium.Color(1.0, 1.0, 1.0, 0.5);
         */
        color : {
            get : function() {
                return this._color;
            },
            set : function(value) {
                //>>includeStart('debug', pragmas.debug);
                if (!defined(value)) {
                    throw new DeveloperError('value is required.');
                }
                //>>includeEnd('debug');

                var color = this._color;
                if (!Color.equals(color, value)) {
                    Color.clone(value, color);
                    makeDirty(this, COLOR_INDEX);
                }
            }
        },

        /**
         * Gets or sets the rotation angle in radians.
         * @memberof Billboard.prototype
         * @type {Number}
         */
        rotation : {
            get : function() {
                return this._rotation;
            },
            set : function(value) {
                //>>includeStart('debug', pragmas.debug);
                if (!defined(value)) {
                    throw new DeveloperError('value is required.');
                }
                //>>includeEnd('debug');

                if (this._rotation !== value) {
                    this._rotation = value;
                    makeDirty(this, ROTATION_INDEX);
                }
            }
        },

        /**
         * Gets or sets the aligned axis in world space. The aligned axis is the unit vector that the billboard up vector points towards.
         * The default is the zero vector, which means the billboard is aligned to the screen up vector.
         * @memberof Billboard.prototype
         * @type {Cartesian3}
         * @example
         * // Example 1.
         * // Have the billboard up vector point north
         * billboard.alignedAxis = Cesium.Cartesian3.UNIT_Z;
         *
         * @example
         * // Example 2.
         * // Have the billboard point east.
         * billboard.alignedAxis = Cesium.Cartesian3.UNIT_Z;
         * billboard.rotation = -Cesium.Math.PI_OVER_TWO;
         *
         * @example
         * // Example 3.
         * // Reset the aligned axis
         * billboard.alignedAxis = Cesium.Cartesian3.ZERO;
         */
        alignedAxis : {
            get : function() {
                return this._alignedAxis;
            },
            set : function(value) {
                //>>includeStart('debug', pragmas.debug);
                if (!defined(value)) {
                    throw new DeveloperError('value is required.');
                }
                //>>includeEnd('debug');

                var alignedAxis = this._alignedAxis;
                if (!Cartesian3.equals(alignedAxis, value)) {
                    Cartesian3.clone(value, alignedAxis);
                    makeDirty(this, ALIGNED_AXIS_INDEX);
                }
            }
        },

        /**
         * Gets or sets a width for the billboard. If undefined, the image width will be used.
         * @memberof Billboard.prototype
         * @type {Number}
         */
        width : {
            get : function() {
                return defaultValue(this._width, this._imageWidth);
            },
            set : function(value) {
                if (this._width !== value) {
                    this._width = value;
                    makeDirty(this, IMAGE_INDEX_INDEX);
                }
            }
        },

        /**
         * Gets or sets a height for the billboard. If undefined, the image height will be used.
         * @memberof Billboard.prototype
         * @type {Number}
         */
        height : {
            get : function() {
                return defaultValue(this._height, this._imageHeight);
            },
            set : function(value) {
                if (this._height !== value) {
                    this._height = value;
                    makeDirty(this, IMAGE_INDEX_INDEX);
                }
            }
        },

        /**
         * Gets or sets if the billboard size is in meters or pixels. <code>true</code> to size the billboard in meters;
         * otherwise, the size is in pixels.
         * @memberof Billboard.prototype
         * @type {Boolean}
         * @default false
         */
        sizeInMeters : {
            get : function() {
                return this._sizeInMeters;
            },
            set : function(value) {
                if (this._sizeInMeters !== value) {
                    this._sizeInMeters = value;
                    makeDirty(this, COLOR_INDEX);
                }
            }
        },

        /**
         * Gets or sets the user-defined object returned when the billboard is picked.
         * @memberof Billboard.prototype
         * @type {Object}
         */
        id : {
            get : function() {
                return this._id;
            },
            set : function(value) {
                this._id = value;
                if (defined(this._pickId)) {
                    this._pickId.object.id = value;
                }
            }
        },

        /**
         * The primitive to return when picking this billboard.
         * @memberof Billboard.prototype
         * @private
         */
        pickPrimitive : {
            get : function() {
                return this._pickPrimitive;
            },
            set : function(value) {
                this._pickPrimitive = value;
                if (defined(this._pickId)) {
                    this._pickId.object.primitive = value;
                }
            }
        },

        /**
         * <p>
         * Gets or sets the image to be used for this billboard.  If a texture has already been created for the
         * given image, the existing texture is used.
         * </p>
         * <p>
         * This property can be set to a loaded Image, a URL which will be loaded as an Image automatically,
         * a canvas, or another billboard's image property (from the same billboard collection).
         * </p>
         *
         * @memberof Billboard.prototype
         * @type {String}
         * @example
         * // load an image from a URL
         * b.image = 'some/image/url.png';
         *
         * // assuming b1 and b2 are billboards in the same billboard collection,
         * // use the same image for both billboards.
         * b2.image = b1.image;
         */
        image : {
            get : function() {
                return this._imageId;
            },
            set : function(value) {
                if (!defined(value)) {
                    this._imageIndex = -1;
                    this._imageSubRegion = undefined;
                    this._imageId = undefined;
                    this._image = undefined;
                    this._imageIndexPromise = undefined;
                    makeDirty(this, IMAGE_INDEX_INDEX);
                } else if (typeof value === 'string') {
                    this.setImage(value, value);
                } else if (defined(value.src)) {
                    this.setImage(value.src, value);
                } else {
                    this.setImage(createGuid(), value);
                }
            }
        },

        /**
         * When <code>true</code>, this billboard is ready to render, i.e., the image
         * has been downloaded and the WebGL resources are created.
         *
         * @memberof Billboard.prototype
         *
         * @type {Boolean}
         * @readonly
         *
         * @default false
         */
        ready : {
            get : function() {
                return this._imageIndex !== -1;
            }
        },

        /**
         * Keeps track of the position of the billboard based on the height reference.
         * @memberof Billboard.prototype
         * @type {Cartesian3}
         * @private
         */
        _clampedPosition : {
            get : function() {
                return this._actualClampedPosition;
            },
            set : function(value) {
                this._actualClampedPosition = Cartesian3.clone(value, this._actualClampedPosition);
                makeDirty(this, POSITION_INDEX);
            }
        }
    });

    Billboard.prototype.getPickId = function(context) {
        if (!defined(this._pickId)) {
            this._pickId = context.createPickId({
                primitive : this._pickPrimitive,
                collection : this._collection,
                id : this._id
            });
        }

        return this._pickId;
    };

    Billboard.prototype._updateClamping = function() {
        Billboard._updateClamping(this._billboardCollection, this);
    };

    var scratchCartographic = new Cartographic();
    var scratchPosition = new Cartesian3();

    Billboard._updateClamping = function(collection, owner) {
        var scene = collection._scene;
        if (!defined(scene)) {
            //>>includeStart('debug', pragmas.debug);
            if (owner._heightReference !== HeightReference.NONE) {
                throw new DeveloperError('Height reference is not supported without a scene.');
            }
            //>>includeEnd('debug');
            return;
        }

        var globe = scene.globe;
        var ellipsoid = globe.ellipsoid;
        var surface = globe._surface;

        var mode = scene.frameState.mode;

        var modeChanged = mode !== owner._mode;
        owner._mode = mode;

        if ((owner._heightReference === HeightReference.NONE || modeChanged) && defined(owner._removeCallbackFunc)) {
            owner._removeCallbackFunc();
            owner._removeCallbackFunc = undefined;
            owner._clampedPosition = undefined;
        }

        if (owner._heightReference === HeightReference.NONE || !defined(owner._position)) {
            return;
        }

        var position = ellipsoid.cartesianToCartographic(owner._position);
        if (!defined(position)) {
            return;
        }

        if (defined(owner._removeCallbackFunc)) {
            owner._removeCallbackFunc();
        }

        function updateFunction(clampedPosition) {
            if (owner._heightReference === HeightReference.RELATIVE_TO_GROUND) {
                if (owner._mode === SceneMode.SCENE3D) {
                    var clampedCart = ellipsoid.cartesianToCartographic(clampedPosition, scratchCartographic);
                    clampedCart.height += position.height;
                    ellipsoid.cartographicToCartesian(clampedCart, clampedPosition);
                } else {
                    clampedPosition.x += position.height;
                }
            }
            owner._clampedPosition = Cartesian3.clone(clampedPosition, owner._clampedPosition);
        }
        owner._removeCallbackFunc = surface.updateHeight(position, updateFunction);

        Cartographic.clone(position, scratchCartographic);
        var height = globe.getHeight(position);
        if (defined(height)) {
            scratchCartographic.height = height;
        }

        ellipsoid.cartographicToCartesian(scratchCartographic, scratchPosition);

        updateFunction(scratchPosition);
    };

    Billboard.prototype._loadImage = function() {
        var atlas = this._billboardCollection._textureAtlas;

        var imageId = this._imageId;
        var image = this._image;
        var imageSubRegion = this._imageSubRegion;
        var imageIndexPromise;

        if (defined(image)) {
            imageIndexPromise = atlas.addImage(imageId, image);
        }
        if (defined(imageSubRegion)) {
            imageIndexPromise = atlas.addSubRegion(imageId, imageSubRegion);
        }

        this._imageIndexPromise = imageIndexPromise;

        if (!defined(imageIndexPromise)) {
            return;
        }

        var that = this;
        imageIndexPromise.then(function(index) {
            if (that._imageId !== imageId || that._image !== image || !BoundingRectangle.equals(that._imageSubRegion, imageSubRegion)) {
                // another load occurred before this one finished, ignore the index
                return;
            }

            // fill in imageWidth and imageHeight
            var textureCoordinates = atlas.textureCoordinates[index];
            that._imageWidth = atlas.texture.width * textureCoordinates.width;
            that._imageHeight = atlas.texture.height * textureCoordinates.height;

            that._imageIndex = index;
            that._ready = true;
            that._image = undefined;
            that._imageIndexPromise = undefined;
            makeDirty(that, IMAGE_INDEX_INDEX);
        }).otherwise(function(error) {
            console.error('Error loading image for billboard: ' + error);
            that._imageIndexPromise = undefined;
        });
    };

    /**
     * <p>
     * Sets the image to be used for this billboard.  If a texture has already been created for the
     * given id, the existing texture is used.
     * </p>
     * <p>
     * This function is useful for dynamically creating textures that are shared across many billboards.
     * Only the first billboard will actually call the function and create the texture, while subsequent
     * billboards created with the same id will simply re-use the existing texture.
     * </p>
     * <p>
     * To load an image from a URL, setting the {@link Billboard#image} property is more convenient.
     * </p>
     *
     * @param {String} id The id of the image.  This can be any string that uniquely identifies the image.
     * @param {Image|Canvas|String|Billboard~CreateImageCallback} image The image to load.  This parameter
     *        can either be a loaded Image or Canvas, a URL which will be loaded as an Image automatically,
     *        or a function which will be called to create the image if it hasn't been loaded already.
     * @example
     * // create a billboard image dynamically
     * function drawImage(id) {
     *   // create and draw an image using a canvas
     *   var canvas = document.createElement('canvas');
     *   var context2D = canvas.getContext('2d');
     *   // ... draw image
     *   return canvas;
     * }
     * // drawImage will be called to create the texture
     * b.setImage('myImage', drawImage);
     *
     * // subsequent billboards created in the same collection using the same id will use the existing
     * // texture, without the need to create the canvas or draw the image
     * b2.setImage('myImage', drawImage);
     */
    Billboard.prototype.setImage = function(id, image) {
        //>>includeStart('debug', pragmas.debug);
        if (!defined(id)) {
            throw new DeveloperError('id is required.');
        }
        if (!defined(image)) {
            throw new DeveloperError('image is required.');
        }
        //>>includeEnd('debug');

        if (this._imageId === id) {
            return;
        }

        this._imageIndex = -1;
        this._imageSubRegion = undefined;
        this._imageId = id;
        this._image = image;

        if (defined(this._billboardCollection._textureAtlas)) {
            this._loadImage();
        }
    };

    /**
     * Uses a sub-region of the image with the given id as the image for this billboard,
     * measured in pixels from the bottom-left.
     *
     * @param {String} id The id of the image to use.
     * @param {BoundingRectangle} subRegion The sub-region of the image.
     *
     * @exception {RuntimeError} image with id must be in the atlas
     */
    Billboard.prototype.setImageSubRegion = function(id, subRegion) {
        //>>includeStart('debug', pragmas.debug);
        if (!defined(id)) {
            throw new DeveloperError('id is required.');
        }
        if (!defined(subRegion)) {
            throw new DeveloperError('subRegion is required.');
        }
        //>>includeEnd('debug');

        if (this._imageId === id && BoundingRectangle.equals(this._imageSubRegion, subRegion)) {
            return;
        }

        this._imageIndex = -1;
        this._imageId = id;
        this._imageSubRegion = BoundingRectangle.clone(subRegion);

        if (defined(this._billboardCollection._textureAtlas)) {
            this._loadImage();
        }
    };

    Billboard.prototype._setTranslate = function(value) {
        //>>includeStart('debug', pragmas.debug);
        if (!defined(value)) {
            throw new DeveloperError('value is required.');
        }
        //>>includeEnd('debug');

        var translate = this._translate;
        if (!Cartesian2.equals(translate, value)) {
            Cartesian2.clone(value, translate);
            makeDirty(this, PIXEL_OFFSET_INDEX);
        }
    };

    Billboard.prototype._getActualPosition = function() {
        return defined(this._clampedPosition) ? this._clampedPosition : this._actualPosition;
    };

    Billboard.prototype._setActualPosition = function(value) {
        if (!(defined(this._clampedPosition))) {
            Cartesian3.clone(value, this._actualPosition);
        }
        makeDirty(this, POSITION_INDEX);
    };

    var tempCartesian3 = new Cartesian4();
    Billboard._computeActualPosition = function(billboard, position, frameState, modelMatrix) {
        if (defined(billboard._clampedPosition)) {
            if (frameState.mode !== billboard._mode) {
                billboard._updateClamping();
            }
            return billboard._clampedPosition;
        } else if (frameState.mode === SceneMode.SCENE3D) {
            return position;
        }

        Matrix4.multiplyByPoint(modelMatrix, position, tempCartesian3);
        return SceneTransforms.computeActualWgs84Position(frameState, tempCartesian3);
    };

    var scratchCartesian2 = new Cartesian2();
    var scratchCartesian3 = new Cartesian3();
    var scratchComputePixelOffset = new Cartesian2();

    // This function is basically a stripped-down JavaScript version of BillboardCollectionVS.glsl
    Billboard._computeScreenSpacePosition = function(modelMatrix, position, eyeOffset, pixelOffset, scene, result) {
        // Model to world coordinates
        var positionWorld = Matrix4.multiplyByPoint(modelMatrix, position, scratchCartesian3);

        // World to window coordinates
        var positionWC = SceneTransforms.wgs84WithEyeOffsetToWindowCoordinates(scene, positionWorld, eyeOffset, result);

        // Apply pixel offset
        pixelOffset = Cartesian2.clone(pixelOffset, scratchComputePixelOffset);
        var po = Cartesian2.multiplyByScalar(pixelOffset, scene.context.uniformState.resolutionScale, scratchCartesian2);
        positionWC.x += po.x;
        positionWC.y += po.y;

        return positionWC;
    };

    var scratchPixelOffset = new Cartesian2(0.0, 0.0);

    /**
     * Computes the screen-space position of the billboard's origin, taking into account eye and pixel offsets.
     * The screen space origin is the top, left corner of the canvas; <code>x</code> increases from
     * left to right, and <code>y</code> increases from top to bottom.
     *
     * @param {Scene} scene The scene.
     * @param {Cartesian2} [result] The object onto which to store the result.
     * @returns {Cartesian2} The screen-space position of the billboard.
     *
     * @exception {DeveloperError} Billboard must be in a collection.
     *
     * @example
     * console.log(b.computeScreenSpacePosition(scene).toString());
     *
     * @see Billboard#eyeOffset
     * @see Billboard#pixelOffset
     */
    Billboard.prototype.computeScreenSpacePosition = function(scene, result) {
        var billboardCollection = this._billboardCollection;
        if (!defined(result)) {
            result = new Cartesian2();
        }

        //>>includeStart('debug', pragmas.debug);
        if (!defined(billboardCollection)) {
            throw new DeveloperError('Billboard must be in a collection.  Was it removed?');
        }
        if (!defined(scene)) {
            throw new DeveloperError('scene is required.');
        }
        //>>includeEnd('debug');

        // pixel offset for screenspace computation is the pixelOffset + screenspace translate
        Cartesian2.clone(this._pixelOffset, scratchPixelOffset);
        Cartesian2.add(scratchPixelOffset, this._translate, scratchPixelOffset);

        var modelMatrix = billboardCollection.modelMatrix;
        var actualPosition = this._getActualPosition();

        var windowCoordinates = Billboard._computeScreenSpacePosition(modelMatrix, actualPosition,
                this._eyeOffset, scratchPixelOffset, scene, result);
        return windowCoordinates;
    };

    /**
     * Determines if this billboard equals another billboard.  Billboards are equal if all their properties
     * are equal.  Billboards in different collections can be equal.
     *
     * @param {Billboard} other The billboard to compare for equality.
     * @returns {Boolean} <code>true</code> if the billboards are equal; otherwise, <code>false</code>.
     */
    Billboard.prototype.equals = function(other) {
        return this === other ||
               defined(other) &&
               this._id === other._id &&
               Cartesian3.equals(this._position, other._position) &&
               this._imageId === other._imageId &&
               this._show === other._show &&
               this._scale === other._scale &&
               this._verticalOrigin === other._verticalOrigin &&
               this._horizontalOrigin === other._horizontalOrigin &&
               this._heightReference === other._heightReference &&
               BoundingRectangle.equals(this._imageSubRegion, other._imageSubRegion) &&
               Color.equals(this._color, other._color) &&
               Cartesian2.equals(this._pixelOffset, other._pixelOffset) &&
               Cartesian2.equals(this._translate, other._translate) &&
               Cartesian3.equals(this._eyeOffset, other._eyeOffset) &&
               NearFarScalar.equals(this._scaleByDistance, other._scaleByDistance) &&
               NearFarScalar.equals(this._translucencyByDistance, other._translucencyByDistance) &&
               NearFarScalar.equals(this._pixelOffsetScaleByDistance, other._pixelOffsetScaleByDistance);
    };

    Billboard.prototype._destroy = function() {
        if (defined(this._customData)) {
            this._billboardCollection._scene.globe._surface.removeTileCustomData(this._customData);
            this._customData = undefined;
        }

        if (defined(this._removeCallbackFunc)) {
            this._removeCallbackFunc();
            this._removeCallbackFunc = undefined;
        }

        this.image = undefined;
        this._pickId = this._pickId && this._pickId.destroy();
        this._billboardCollection = undefined;
    };

    /**
     * A function that creates an image.
     * @callback Billboard~CreateImageCallback
     * @param {String} id The identifier of the image to load.
     * @returns {Image|Canvas|Promise<Image|Canvas>} The image, or a promise that will resolve to an image.
     */

    return Billboard;
});