Instanced 3D Model

Contributors

Sean Lilley, @lilleyse
Patrick Cozzi, @pjcozzi
Rob Taglang, @lasalvavida

Overview

Instanced 3D Model is a tile format for efficient streaming and rendering of a large number of models, called instances, with slight variations. In the simplest case, the same tree model, for example, may be located - or instanced - in several places. Each instance references the same model, and has per-instance properties, such as position. Using the core 3D Tiles spec language, each instance is a feature.

In addition to trees, Instanced 3D Model is useful for exterior features such as fire hydrants, sewer caps, lamps, and traffic lights, and interior CAD features such as bolts, valves, and electric outlets.

A Composite tile can be used to create tiles with different types of instanced models, e.g., trees and traffic lights.

Instanced 3D Model maps well to the ANGLE_instanced_arrays extension for efficient rendering with WebGL.

Layout

A tile is composed of a header section immediately followed by a body section.

Figure 1: Instanced 3D Model layout (dashes indicate optional fields).

Header

The 32-byte header contains the following fields:

Field name	Data type	Description
`magic`	4-byte ANSI string	`"i3dm"`. This can be used to identify the arraybuffer as an Instanced 3D Model tile.
`version`	`uint32`	The version of the Instanced 3D Model format. It is currently `1`.
`byteLength`	`uint32`	The length of the entire tile, including the header, in bytes.
`featureTableJSONByteLength`	`uint32`	The length of the feature table JSON section in bytes.
`featureTableBinaryByteLength`	`uint32`	The length of the feature table binary section in bytes. If `featureTableJSONByteLength` is zero, this will also be zero.
`batchTableJSONByteLength`	`uint32`	The length of the batch table JSON section in bytes. Zero indicates that there is no batch table.
`batchTableBinaryByteLength`	`uint32`	The length of the batch table binary section in bytes. If `batchTableJSONByteLength` is zero, this will also be zero.
`gltfFormat`	`uint32`	Indicates the format of the glTF field of the body. `0` indicates it is a url, `1` indicates it is embedded binary glTF. See the glTF section below.

If featureTableJSONByteLength equals zero, or there is no glTF, the tile does not need to be rendered.

The body section immediately follows the header section, and is composed of three fields: Feature Table, Batch Table, and glTF.

Code for reading the header can be found in Instanced3DModelTileContent in the Cesium implementation of 3D Tiles.

Feature Table

Contains values for i3dm semantics used to create instanced models. More information is available in the Feature Table specification.

The i3dm Feature Table JSON Schema is defined in i3dm.featureTable.schema.json.

Semantics

Instance Semantics

These semantics map to an array of feature values that are used to create instances. The length of these arrays must be the same for all semantics and is equal to the number of instances. The value for each instance semantic must be a reference to the Feature Table Binary Body; they cannot be embedded in the Feature Table JSON Header.

If a semantic has a dependency on another semantic, that semantic must be defined. If both SCALE and SCALE_NON_UNIFORM are defined for an instance, both scaling operations will be applied. If both POSITION and POSITION_QUANTIZED are defined for an instance, the higher precision POSITION will be used. If NORMAL_UP, NORMAL_RIGHT, NORMAL_UP_OCT32P, and NORMAL_RIGHT_OCT32P are defined for an instance, the higher precision NORMAL_UP, and NORMAL_RIGHT will be used.

| Semantic | Data Type | Description | Required | | --- | --- | --- | --- | --- | | POSITION | float32[3] | A 3-component array of numbers containing x, y, and z Cartesian coordinates for the position of the instance. | ✅ Yes, unless POSITION_QUANTIZED is defined. | | POSITION_QUANTIZED | uint16[3] | A 3-component array of numbers containing x, y, and z in quantized Cartesian coordinates for the position of the instance. | ✅ Yes, unless POSITION is defined. | | NORMAL_UP | float32[3]| A unit vector defining the up direction for the orientation of the instance. | 🔴 No, unless NORMAL_RIGHT is defined. | | NORMAL_RIGHT | float32[3] | A unit vector defining the right direction for the orientation of the instance. Must be orthogonal to up. | 🔴 No, unless NORMAL_UP is defined. | | NORMAL_UP_OCT32P | uint16[2] | An oct-encoded unit vector with 32-bits of precision defining the up direction for the orientation of the instance. | 🔴 No, unless NORMAL_RIGHT_OCT32P is defined. | | NORMAL_RIGHT_OCT32P | uint16[2] | An oct-encoded unit vector with 32-bits of precision defining the right direction for the orientation of the instance. Must be orthogonal to up. | 🔴 No, unless NORMAL_UP_OCT32P is defined. | | SCALE | float32 | A number defining a scale to apply to all axes of the instance. | 🔴 No. | | SCALE_NON_UNIFORM | float32[3] | A 3-component array of numbers defining the scale to apply to the x, y, and z axes of the instance. | 🔴 No. | | BATCH_ID | uint8, unit16 (default), or uint32 | The batchId of the instance that can be used to retrieve metadata from the Batch Table. | 🔴 No. |

Global Semantics

These semantics define global properties for all instances.

Semantic	Data Type	Description	Required
`INSTANCES_LENGTH`	`uint32`	The number of instances to generate. The length of each array value for an instance semantic should be equal to this.	✅ Yes.
`QUANTIZED_VOLUME_OFFSET`	`float32[3]`	A 3-component array of numbers defining the offset for the quantized volume.	🔴 No, unless `POSITION_QUANTIZED` is defined.
`QUANTIZED_VOLUME_SCALE`	`float32[3]`	A 3-component array of numbers defining the scale for the quantized volume.	🔴 No, unless `POSITION_QUANTIZED` is defined.
`EAST_NORTH_UP`	`boolean`	When `true` and per-instance orientation is not defined, each instance will default to the `east/north/up` reference frame's orientation on the `WGS84` ellipsoid.	🔴 No.

Examples using these semantics can be found in the examples section.

Instance Orientation

An instance's orientation is defined by an orthonormal basis created by an up and right vector. The orientation will be transformed by the [tile transform] (../../README.md#tile-transform).

The x vector in the standard basis maps onto the right vector in the transformed basis, and the y vector maps on to the up vector. The z vector would map onto a forward vector, but it is omitted because it will always be the cross product of right and up.

Figure 2: A box in the standard basis.

Figure 3: A box transformed into a rotated basis.

Oct-encoded Normal Vectors

If NORMAL_UP and NORMAL_RIGHT are not defined for an instance, its orientation may be stored as oct-encoded normals in NORMAL_UP_OCT32P and NORMAL_RIGHT_OCT32P. These define up and right using the oct-encoding described in A Survey of Efficient Representations of Independent Unit Vectors by Cigolle et al.. An implementation for encoding and decoding these unit vectors can be found in Cesium's AttributeCompression module.

Default Orientation

If NORMAL_UP and NORMAL_RIGHT or NORMAL_UP_OCT32P and NORMAL_RIGHT_OCT32P are not present, the instance will not have a custom orientation. If EAST_NORTH_UP is true the instance is assumed to be on the WGS84 ellipsoid and its orientation will default to the east/north/up reference frame at its Cartographic position. This is suitable for instanced models like trees whose orientation is always facing up from their position on the ellipsoid's surface.

Instance Position

POSITION defines the location for an instance before any tile transforms are applied.

Quantized Positions

If POSITION is not defined for an instance, its position may be stored in POSITION_QUANTIZED which defines the instance position relative to the quantized volume. If neither POSITION or POSITION_QUANTIZED are defined, the instance will not be created.

A quantized volume is defined by offset and scale to map quantized positions into model space.

Figure 4: A quantized volume based on offset and scale.

offset is stored in the global semantic QUANTIZED_VOLUME_OFFSET, and scale is stored in the global semantic QUANTIZED_VOLUME_SCALE. If those global semantics are not defined, POSITION_QUANTIZED cannot be used.

Quantized positions can be mapped to model space using the formula:

POSITION = POSITION_QUANTIZED * QUANTIZED_VOLUME_SCALE + QUANTIZED_VOLUME_OFFSET

Instance Scaling

Scaling can be applied to instances using the SCALE and SCALE_NON_UNIFORM semantics. SCALE applies a uniform scale along all axes, and SCALE_NON_UNIFORM applies scaling to the x, y, and z axes independently.

Examples

These examples show how to generate JSON and binary buffers for the feature table.

Positions Only

In this minimal example, we place 4 instances on the corners of a unit length square with the default orientation.

var featureTableJSON = {
    INSTANCES_LENGTH : 4,
    POSITION : {
        byteOffset : 0
    }
};

var featureTableBinary = new Buffer(new Float32Array([
    0.0, 0.0, 0.0, 
    1.0, 0.0, 0.0,
    0.0, 0.0, 1.0,
    1.0, 0.0, 1.0
]).buffer);

Quantized Positions and Oct-Encoded Normals

In this example, the 4 instances will be placed with an orientation up of [0.0, 1.0, 0.0] and right of [1.0, 0.0, 0.0] in oct-encoded format and they will be placed on the corners of a quantized volume that spans from -250.0 to 250.0 units in the x and z directions.

var featureTableJSON = {
    INSTANCES_LENGTH : 4,
    QUANTIZED_VOLUME_OFFSET : [-250.0, 0.0, -250.0],
    QUANTIZED_VOLUME_SCALE : [500.0, 0.0, 500.0],
    POSITION_QUANTIZED : {
        byteOffset : 0
    },
    NORMAL_UP_OCT32P : {
        byteOffset : 24
    },
    NORMAL_RIGHT_OCT32P : {
        byteOffset : 40
    }
};

var positionQuantizedBinary = new Buffer(new Uint16Array([
    0, 0, 0,
    65535, 0, 0,
    0, 0, 65535,
    65535, 0, 65535
]).buffer);

var normalUpOct32PBinary = new Buffer(new Uint16Array([
    32768, 65535,
    32768, 65535,
    32768, 65535,
    32768, 65535
]).buffer);

var normalRightOct32PBinary = new Buffer(new Uint16Array([
    65535, 32768,
    65535, 32768,
    65535, 32768,
    65535, 32768
]).buffer);

var featureTableBinary = Buffer.concat([positionQuantizedBinary, normalUpOct32PBinary, normalRightOct32PBinary]);

Batch Table

Contains metadata organized by batchId that can be used for declarative styling.

See the Batch Table reference for more information.

glTF

The glTF asset to be instanced is stored after the feature table and batch table.

glTF is the runtime asset format for WebGL. Binary glTF is an extension defining a binary container for glTF. Instanced 3D Model uses glTF 1.0 with the KHR_binary_glTF extension.

header.gltfFormat determines the format of the glTF field. When it is 0, the glTF field is

a UTF-8 string, which contains a url to a glTF model.

When the value of header.gltfFormat is 1, the glTF field is

a binary blob containing binary glTF.

In either case, header.gltfByteLength contains the length of the glTF field in bytes.

If the glTF asset is embedded, it must be 8-byte aligned so that glTF's byte-alignment guarantees are met. This can be done by padding the Feature Table or Batch Table if they are present.

File Extension

.i3dm

MIME Type

TODO, #60

application/octet-stream

Resources

A Survey of Efficient Representations of Independent Unit Vectors by Cigolle et al.
Mesh Geometry Compression for Mobile Graphics by Jongseok Lee et al.
Cesium AttributeCompression module for oct-encoding

Implementation Examples

Cesium

Generating up and right from longitude, latitude, and height

var position = Cartesian3.fromRadians(longitude, latitude, height);

// Compute the up vector
var up = new Cartesian3();
var transform = Transforms.eastNorthUpToFixedFrame(position);
var rotation = new Matrix3();
Matrix4.getRotation(transform, rotation);

// In east-north-up, the up vector is stored in the z component
Matrix3.multiplyByVector(rotation, Cartesian3.UNIT_Z, up);

// Compute the right and forward vectors
var right = new Cartesian3();
var forward = Cartesian3.clone(Cartesian3.UNIT_Z);

// You can change the orientation of a model by rotating about the y-axis first
var orient = new Matrix3();
var orientAngle = CesiumMath.fromDegrees(90.0);
Matrix3.fromRotationY(orientAngle, orient);
Matrix3.multiplyByVector(orient, forward, forward);

// Cross up and forward to get right
Cartesian3.cross(up, forward, right);
Cartesian3.normalize(right, right);

Construct a model-matrix for an instance

// Cross right and up to get forward
var forward = new Cartesian3();
Cartesian3.cross(right, up, forward);

// Place the basis into a rotation matrix
var rotation = new Matrix3();
Matrix3.setColumn(rotation, 0, right, rotation);
Matrix3.setColumn(rotation, 1, up, rotation);
Matrix3.setColumn(rotation, 2, forward, rotation);

// Get the scale
var scale = Matrix3.IDENTITY.clone();
if (defined(uniformScale)) {
    Matrix3.multiplyByScalar(scale, uniformScale, scale);
}
if (defined(nonUniformScale)) {
    Matrix3.multiplyByVector(scale, nonUniformScale, scale);
}

// Generate the model matrix
var modelMatrix = new Matrix4();
Matrix4.fromTranslationRotationScale(
    new TranslationRotationScale(position, rotation, scale),
    modelMatrix
);

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README.md

README.md

Instanced 3D Model

Contributors

Overview

Layout

Header

Feature Table

Semantics

Instance Semantics

Global Semantics

Instance Orientation

Oct-encoded Normal Vectors

Default Orientation

Instance Position

Quantized Positions

Instance Scaling

Examples

Positions Only

Quantized Positions and Oct-Encoded Normals

Batch Table

glTF

File Extension

MIME Type

Resources

Implementation Examples

Cesium

Generating up and right from longitude, latitude, and height

Construct a model-matrix for an instance

Files

README.md

Latest commit

History

README.md

File metadata and controls

Instanced 3D Model

Contributors

Overview

Layout

Header

Feature Table

Semantics

Instance Semantics

Global Semantics

Instance Orientation

Oct-encoded Normal Vectors

Default Orientation

Instance Position

Quantized Positions

Instance Scaling

Examples

Positions Only

Quantized Positions and Oct-Encoded Normals

Batch Table

glTF

File Extension

MIME Type

Resources

Implementation Examples

Cesium

Generating up and right from longitude, latitude, and height

Construct a model-matrix for an instance