3DTiles介绍（⼀）
原⽂地址：
Spec status
The 3D Tiles spec is pre-1.0 (indicated by "version": "0.0" in tilet.json). We expect a draft 1.0 version and the Cesium implementation to stabilize in 2017。
七子之歌翻译Draft 1.0 Plans
Topic Status
The tilet's spatial hierarchy
⽡⽚集的空间层次
Solid ba, will add features as needed
(*.b3dm)
Textured terrain and surfaces, 3D building exteriors and interiors, massive models,
...
纹理地形和表⾯，3D建筑物外部和内部，海量模型...
Solid ba, only minor, if any, changes expected (*.i3dm)
Trees, windmills, bolts, ...
树⽊，直升机，螺栓，...
Solid ba, only minor, if any, changes expected (*.pnts)
Massive amount of points
网络营销的层次点云
Solid ba, only minor, if any, changes expected (*.vctr)
Polygons, polylines, and placemarks
⾯，线和地标
⚪ In progress,
(*.cmpt)
Combine heterogeneous tile formats
兼容异构⽡⽚格式
Solid ba, only minor, if any, changes expected
Style features using per-feature metadata Solid ba, will add features/functions as needed,
汤唯韩国获奖
Introduction简介
In 3D Tiles, a tilet is a t of tiles organized in a spatial data structure, the tree. Each tile has a bounding volume completely enclosing its contents. The tree has spatial coherence; the content for child tiles are completely inside the parent's bounding volume. To allow flexibility, the tree can be any spatial data structure with spatial coherence, including k-d trees, quadtrees, octrees, and grids.
在3D Tiles中，⽡⽚集是⽤树形空间数据结构组织的⽡⽚集合。每个⽡⽚都有⼀个包围体完全包围它
的内容(content)。树具有空间相⼲性，⼦⽡⽚的内容完全包含在⽗⽡⽚的包围体内。为了满⾜灵活性的需求，树可以是任何具有空间相⼲性的空间数据结构，包括k-d树，四叉树(quadtrees)，⼋叉树(octrees)，格⽹(grids)。
To support tight fitting volumes for a variety of datats from regularly divided terrain to cities not aligned with a line of longitude or latitude to arbitrary point clouds, the bounding volume may be an oriented bounding box, a bounding sphere, or a geographic region defined by minimum and maximum longitudes, latitudes, and heights.belligerent
为了使各种各样数据集（从规则分割的地形，到不沿经纬线对齐的城市，到任意点云）的包围体紧凑，包围体可以是有向包围盒，包围球，最⼤最⼩经纬度和⾼程定义的地理区域。
A tile references a feature or t of features, such as 3D models reprenting buildings or trees, points in a point cloud, or polygons, polylines, and points in a vector datat. The features may be batched together into esntially a single feature to reduce client-side load time and WebGL draw call overhead.
⼀个⽡⽚代表⼀个要素或⼀个要素集，例如，以建筑物、绿化为代表的3D 模型，点云中的点，和向量数据集中的点、多边形、折线。这些要素可能被批处理成单个要素，以便减少客户端加载时间和We
bGL绘制调⽤的开销。
Tile metadata ⽡⽚元数据
The metadata for each tile - not the actual contents - are defined in JSON. For example:
每个⽡⽚的元数据并不是实际的内容数据，元数据定义在JSON⽂件中。例如：
{
"boundingVolume": {
"region": [
-1.2419052957251926,
0.7395016240301894,
-1.2415404171917719,
tryout0.7396563300150859,
0,
lunar station
20.4
]
},
"geometricError": 43.88464075650763,
管理会计系统"refine" : "ADD",芬兰语在线翻译
analytics"content": {
"boundingVolume": {
"region": [
-1.2418882438584018,
0.7395016240301894,
-
1.2415422846940714,
0.7396461198389616,
0,
19.4
]
},
"url": "2/0/0.b3dm"
},
contain"children": [...]
}
ion property is an array of six numbers that define the bounding geographic
region in WGS84 / EPSG:4326 coordinates with the order[west, south, east, north, minimum height, maximum height]. Longitudes and latitudes are in radians, and heights are in meters above (or below) the. Besides region, other bounding volumes, such as box and sphere, may be ud.
The geometricError property is a nonnegative number that defines the error, in meters, introduced if this tile
is rendered and its children are not. At runtime, the geometric error is ud to compute Screen-Space Error (SSE), i.e., the error measured in pixels. The SSE determines Hierarchical Level of Detail (HLOD) refinement, i.e., if a tile is sufficiently detailed for the current view or if its children should be considered.
geometricError属性是⼀个⾮负数字，定义了⼀个以⽶为单位的误差，如果当前⽡⽚被渲染⽽它的⼦⽡⽚不被渲染，则引⼊这个误差。运⾏时，此误差⽤来计算SSE，即⽤像素测量的误差。SSE⽤来判定HLOD细化，即，⼀个⽡⽚在当前视图是否⾜够精细，它的⼦⽡⽚是否需要考虑。
An optional viewerRequestVolume property (not shown above) defines a volume, using the same schema
as boundingVolume, that the viewer must be inside of before the tile's content will be requested and before
the tile will be refined bad on geometricError. See the ction.
可选属性viewerRequestVolume使⽤和boundingVolume相同的概要，在⽡⽚内容被请求之前和⽡⽚被精细（依据geometricError属性）之前，视景器必须被包含在这个体中。
The refine property is a string that is either"REPLACE" for replacement refinement or"ADD" for additive refinement. It is required for the root tile of a tilet; it is optional for all other tiles. When refine is omitted, it is inherited from the parent tile.
refine属性是这两个字符串之⼀：“REPLACE”(置换细化)，“ADD”(附加细化)。这个属性对于⽡⽚集根⽡⽚是必须的，对其他⽡⽚是可选的。当省略refine属性时，从⽗⽡⽚继承。
The content property is an object that contains metadata about the tile's content and a link to the
content属性是⼀个对象，包含关于⽡⽚内容的元数据和⼀个到⽡⽚的连接。content.url是⼀个字符串，指向⽡⽚数据，连接可以是绝对或相对url。例⼦中的url，2/0/0.b3dm，是⽡⽚地图服务(TMS)命名规则，{z}/{y}/{x}.extension。
The url can be another tilet.json file to create a tilet of tilets. See.
url可以是另外⼀个tilet.json⽂件，⽤来创建⽡⽚集的⼦⽡⽚集。
A file extension is not required for content.url. A content's can be identified by the magic field in its header, or otherwi as an external tilet if the content is JSON.
content.url的⽂件扩展名不是必须的。content的⽡⽚格式(.b3dm, .i3dm, .pnts, .vctr, .cmpt)可以通过在其header处的magic字段进⾏判断，或者如果是JSON⽂件就作为外部⽡⽚集对待。(A tile is compod of a header ction immediately followed by a body ction. 参加各⽡⽚格式的Layout)
content.boundingVolume defines an optional bounding volume similar to the top-level boundingVolu
me property. But unlike the top-level boundingVolume property,content.boundingVolume is a tightly fit bounding volume enclosing just the tile's contents. This is ud for replacement refinement;boundingVolume provides spatial coherence and content.boundingVolume enables tight view frustum culling. The screenshot below shows the bounding volumes for the root tile for.boundingVolume, shown in red, enclos the entire area of the
tilet;content.boundingVolume shown in blue, enclos just the four features (models) in the root tile.
content.boundingVolume属性定义⼀个可选的包围体属性，类似顶层的boundingVolume属性。不同的是，它是⼀个紧凑的包围体，仅仅包含⽡⽚内容。它⽤来作置换细化，boundingVolume提供空间相⼲性，content.boundingVolume允许紧凑视锥体筛选。下图中，红⾊包围体代表根⽡⽚的boundingVolume，它包围了⽡⽚集的整个区域，蓝⾊包围体展⽰content.boundingVolume，它仅仅包围了根⽡⽚中的四个要素（模型）。
content is optional. When it is not defined, the tile's bounding volume is still ud for culling (e).
content是可选的。当没有定义content属性时，⽡⽚的包围体依然⽤作筛选。
An optional transform property (not shown above) defines a 4x4 affine transformation matrix that transforms the tile's content,boundingVolume, and viewerRequestVolume as described in the ction.
可选的transform属性定义⼀个4X4的仿射变换矩阵，⽤来变换⽡⽚的content，boundingVolum和viewerRequestVolume。
children is an array of objects that define child tiles. See the.
children属性是定义⼦⽡⽚的对象数组。
