Scene Layers provide a structured way for clients to store and visualize large volumes of 3D data. I3S organizes information into node hierarchies that contain features with geometry, textures and attributes.

A Scene Layer is characterized by a combination of layer type and profile. The layer type describes the kind of geospatial data stored within it. The layer profile includes additional details on the specific I3S rules for implementation.

The I3S format is declarative and extendable and can be used to represent different types of 3D data. The following layer types have been specified and the validated via implementation and production deployments:

I3S is designed to support large datasets of 3D content ranging from local to global extent containing detailed features. Clients can visualize scene layers by taking advantage of the multi-LoD (level of detail) representation and symbology to create the right user experience for visualizing the 3D content. The I3S format continues to evolve and new functionality continues to be added.

I3S is organized as nodes, which are structured into node pages. The node page includes the bounding volume, child reference, count and the level of detail selection. Nodes contain all the information to describe features including geometry, attributes and textures. Scene Layers can be created in cartesian 3D or in global 3D world coordinate reference systems. I3S Scene Layers can be delivered to web, mobile and desktop clients. Most users will interact with Scene Layers using applications with cloud or server-based information. In these cases, the scene layer content is on the server and is provided to clients through a RESTful interface. These web addressable resources provide access to the scene layer, nodes, and associated resources. Alternatively, a scene layer can be delivered as a Scene Layer Package. This is a single file that includes the complete node tree and all necessary resources in an archive.

The Indexed 3d Scene Layer (I3S) and Scene Layer Package (*.slpk) are open formats and not dependent on any vendor specific solution, technology, or products.[3] The specification for accessing I3S resources as Scene Service REST endpoints is also described in this standard as open formats.

Not required in an OGC Community Standard.

The following normative documents contain provisions that, through reference in this text, constitute provisions of this document. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. For undated references, the latest edition of the normative document referred to applies.

Normative

OGC SF [99-036/ISO 19125]: Geographic information - Simple feature access - Part 1: Common architecture. 2005. https://www.iso.org/obp/ui#iso:std:iso:19162:ed-2:v1:en August 17, 2019.

OGC WKT CRS [12-063r5/ISO 19162:2019]: Geographic information — Well-known text representation of coordinate reference systems. 2019. http://portal.opengeospatial.org/files/?artifact_id=4700 (May 15, 2017)

Informative

"Octree". https://en.wikipedia.org/wiki/Octree. Not Published (N.P.), 2016. Web. 20 Oct. 2016.

"Quadtree". https://en.wikipedia.org/wiki/Quadtree. N.P. 2017. Web. 20 Jan. 2017

"R-Trees". https://en.wikipedia.org/wiki/R-tree. N.P. 2017. Web. 20 Jan. 2017

This document uses the terms defined in Sub-clause 5.3 of [OGC 06-121r8], which is based on the ISO/IEC Directives, Part 2, Rules for the structure and drafting of International Standards. In particular, the word “shall” (not “must”) is the verb form used to indicate a requirement to be strictly followed to conform to this standard.

For the purposes of this document, the following additional terms and definitions apply.

3D Model ^[1] Three-dimensional (3D) models represent a physical body using a collection of points in 3D space, connected by various geometric entities such as triangles, lines, curved surfaces, etc.

ArrayBuffer In JavaScript, the ArrayBuffer object is used to represent a generic, fixed-length raw binary data buffer. ArrayBuffer is an abstract type that is the base for the following types: DataView, Float32Array, Float64Array, Int8Array, Int16Array, Int32Array, Uint8Array, Uint8ClampedArray, Uint16Array, Uint32Array.

Bin Size A histogram displays numerical data by grouping data into "bins" of equal width. Each bin is plotted as a bar whose height corresponds to how many data points are in that bin. Bins are also sometimes called "intervals", "classes", or "buckets". (Khan Academy July 6, 2022)

Bounding Volume (BV) ^[2] A closed volume that completely contains the union of the objects in the set. Bounding volumes are used to improve the efficiency of geometrical operations by using simple volumes to contain more complex objects.

Bounding Volume Hierarchy (BVH) ^[3] A tree structure on a set of geometric objects. All geometric objects are wrapped in bounding volumes that form the leaf nodes of the tree.

Community Standard An OGC Community Standard is an official position of the OGC endorsing a specification or standard developed external to the OGC.

Face 2-dimensional topological primitive. In solid geometry, a face is a flat (planar) surface that forms part of the boundary of a solid object; a three-dimensional solid bounded exclusively by flat faces is a polyhedron.

faceRanges Indicates the range of faces (triangles in a mesh) associated with a particular object (feature).

Gravity-related height Height dependent on the Earth’s gravity field. NOTE: This refers to in particular orthometric height or normal height, which are both approximations of the distance of a point above the mean sea level. (ISO 19111)

Height Distance of a point from a chosen reference surface measured upward along a line perpendicular to that surface. NOTE: A height below the reference surface will have a negative value.

Integrated Mesh An Integrated Mesh is a type of I3S layer that belongs to the mesh-pyramids profile. An Integrated Mesh layer type is typically used to represent and visualize geographic data captured as ‘3D Image’ representing the landscape in a seamless, highly scalable, textured mesh. Such ‘3D Image’ can integrate within its content a multitude of landscape elements including terrain surface, ground imagery, vegetation, man-made objects and structures, and water surfaces. This type of data is typically produced by automated extraction solutions operating on input data from satellite, aerial and/or drone imagery.

Level of Detail (LoD) Using different LoDs involves decreasing the complexity of a 3D model representation as it moves away from the viewer or according to other metrics such as object importance, viewpoint-relative speed or position. There are numerous approaches to defining LoDs. In GIS, LoDs typically refer to maps defined at given scales and resolutions. Typically higher levels of detail provide greater fidelity. A number of OGC standards define approaches to LoD. LiDAR Light Detection and Ranging, is a remote sensing method that uses light in the form of a pulsed laser to measure ranges (variable distances) to the Earth.

Minimum Bounding Sphere (MBS, mbs) ^[4] In mathematics, given a non-empty set of objects of finite extension in n-dimensional space, for example a set of points, a bounding sphere, enclosing sphere or enclosing ball for that set is an n-dimensional solid sphere containing all of these objects.

Near Infrared Light Generally refers to light within the wavenumber range of 12,500 to 4,000 cm-1 (wavelengths from 800 to 2,500 nm) (https://www.shimadzu.com/an/ftir/support/tips/letter9/nir1.html, March 2019)

Normal or Normals ^[5] The normal vector, often simply called the "normal," to a surface is a vector which is perpendicular to the surface at a given point. When normals are considered on closed surfaces, the inward-pointing normal (pointing towards the interior of the surface) and outward-pointing normal are usually distinguished.

Oriented Bounding Box (OBB) ^[6] In geometry, the minimum or smallest bounding or enclosing box for a point set (S) in N dimensions is the box with the smallest measure (area, volume, or hyper-volume in higher dimensions) within which all the points lie. An oriented bounding box is simply a bounding parallelepiped whose faces and edges are not parallel to the basis vectors of the frame in which they’re defined. In many applications the bounding box is aligned with the axes of the coordinate reference system and is known as an axis-aligned bounding box (AABB). To distinguish the general case from an AABB, an arbitrary bounding box is called an oriented bounding box (OBB) when an object’s local coordinate reference system is used.

Point Cloud A point cloud is a set of data points in space. Point clouds are generally produced by 3D scanners, which measure a large number of points on the external surfaces of objects around them. (Wikipedia, https://en.wikipedia.org/wiki/Point_cloud, March 2019)

Profile In I3S, specific implementation instances for specific layer definitions (point, mesh, etc.)

S3TC ^[7] S3TC is a technique for compressing images for use as textures. Standard image compression techniques like JPEG and PNG can achieve greater compression ratios than S3TC. However, S3TC is designed to be implemented in high-performance hardware. JPEG and PNG decompress images all-at-once, while S3TC allows specific sections of the image to be decompressed independently.

Scene Layer A scene layer is a type of layer that is optimized for displaying large amounts of 3D data in a scene. A scene layer displays one of four data types: points, a point cloud, 3D objects, or an integrated mesh.

Shader A small program or set of algorithms that determines how 3-D surface properties of objects are rendered, and how light interacts with the object within a 3-D computer program.

Texture In 3D graphics, the digital representation of the surface of an object. In addition to two-dimensional qualities, such as color and brightness, a texture is also encoded with three-dimensional properties, such as how transparent and reflective the object is. Once a texture has been defined, it can be wrapped around any 3-dimensional object. This is called texture mapping.

Texture Atlas ^[8] A large image containing a collection, or "atlas", of sub-images, each of which is a texture map for some part of a 2D or 3D model.

Texture Coordinates Texture coordinates define how an image (or portion of an image) gets mapped to a geometry. A texture coordinate is associated with each vertex on the geometry, and it indicates what point within the texture image should be mapped to that vertex.(SAFE Software, 4/2021)

Texture Mapping ^[9] Texture mapping is a method for defining high frequency detail, surface texture, or color information on a computer-generated graphic or 3D model.

Texture Maps A texture map is an image applied (mapped) to the surface of a shape or polygon. This may be a bitmap image or a procedural texture. They may be stored in common image file formats, referenced by 3d model formats or material definitions, and assembled into resource bundles.

Treekey Indicates both the level and sibling association of a given node. The key also directly indicates the position of the node in the tree, allowing sorting of all resources on a single dimension. In OGC Version 1.2, node ids are linearized integers converted to strings. This does not change the format of a node index document (was string and remains string). The concept of Treekeys was utilized by the node index neighbor property which was deprecated at 1.0

UV Coordinate ^[10] UV coordinates are 2D coordinates that are mapped onto a 3D model. UV coordinates are a texture’s x and y coordinates and always range from 0 to 1. Let’s take for example a 800×600 image. When we use a UV coordinate with u=0.5 and v=0.5 then the pixel at x=400 and y=300 is targeted.

UV Mapping (aka UV Unwrapping) ^[11] UV mapping is the 3D modeling process of projecting a 2D image to a 3D model’s surface for texture mapping.

Vertex ^[12] In computer graphics, a vertex is not only associated with three spatial coordinates but also with other graphical information necessary to render the object correctly, such as colors, reflectance properties, textures, and surface normals. These properties are used in rendering by a vertex shader, part of the vertex pipeline.

Vertex Attribute Computer Graphics. A vertex attribute is an input variable to a shader that is supplied with per-vertex data. Without vertex attributes, data cannot be rendered.

There are no mandatory conventions defined in this Community Standard.

This section provides background information on the design principals and background for I3S

This section contains the normative clauses and requirements for implementing I3S. In the property tables, any property highlighted with a bold font is mandatory.

I3S organizes information using a hierarchical, node-based spatial index structure in which each node’s payload may contain features with associated geometry, textures and attributes.

The purpose of any index is to allow fast access to blocks of relevant data. In an Indexed 3D Scene layer, the spatial extent of the data is split into regions, called nodes. Each node has roughly equal amounts of and is organized into a hierarchical and navigable data structure. The node index allows the client to quickly determine which data it needs and allows the server to quickly locate the data requested by any client. Node creation is capacity driven. For example, the smaller the node capacity is, typically the smaller the spatial extent.

The following clauses detail this structure.

Scene Layers include Levels of Detail (LoD) that apply to the whole layer and serve to generalize the layer. Scene Layers support LoD in a manner that preserves the identity of the individual features that are retained within any LoD.

The concept of Level of Detail (LoD) is intrinsic to the I3S standard. Scene Layers may include levels of detail that apply to the layer as whole and serve to generalize or summarize information for the layer. This is similar to image pyramids and also similar to raster and tiled vector data schemes. A node in the I3S scene layer tree could be considered the analog of a tile in a raster or vector tiling scheme. Scene layers support levels of detail in a manner that preserves the identity of the individual features that are retained within any level of detail. Levels of Detail can be used to split heavy features, thin or cluster for better visuals, and integrate externally authored files.

The I3S Level of Detail model covers several use cases, including, splitting very heavy features such as detailed building or very large features (coastlines, rivers, infrastructure), thinning/clustering for optimized visualization as well as support for representing externally authored multiple LoDs.

Note that the I3S Level of Detail concept is orthogonal to the concept of consolidated storage (batches) for a set of geometries within a level of detail. This batching is based on, for example, the concatenation of geometries/meshes into larger geometry collections/meshes to assist in optimal rendering. In all such cases the consolidated storage makes use of Geometry Array Buffers that provide access to individual geometries when needed, preserving the feature to geometry element mapping within the consolidated geometries.

A RESTful API allows access to I3S scene layers. Each scene layer profile has different components and features. For details on the API of a specific profile and version, refer to the individual README documents as provided below.

Version 1.3 support for:
3D Objects
Integrated Mesh
Points
Point Cloud
Buildings

Version 1.2 support for:
3D Objects
Integrated Mesh
Points
Point Cloud

For backwards compatibility Version 1.1 support for:
3D Objects,
Integrated Mesh,
Points.

The following media types are referenced in the I3S Community Standard.

This Annex is provided for further guidance and information on Mesh-pyramids.

What this profile is used for: This profile is implemented by the 3D Object and Integrated Mesh layer types.

Chris Andrews, Tamrat Belayneh, Jillian Foster, Javier Gutierrez, Markus Lipp, Sud Menon, Pascal Müller, Dragan Petrovic, Ronald Poirrier, Simon Reinhard, Juan Ruiz, Johannes Schmid, Ivonne Seler, Chengliang Shan,Thorsten Reitz, Ben Tan, Richard Vargas, Moxie Zhang, Bart van Andel, Fabien Dachicourt, Carl Reed

This section provides a detailed, logical-level specification for each of the resource types.