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Batched 3D Model

Contributors

Overview

Batched 3D Model allows offline batching of heterogeneous 3D models, such as different buildings in a city, for efficient streaming to a web client for rendering and interaction. Efficiency comes from transferring multiple models in a single request and rendering them in the least number of WebGL draw calls necessary. Using the core 3D Tiles spec language, each model is a feature.

Per-model properties, such as IDs, enable individual models to be identified and updated at runtime, e.g., show/hide, highlight color, etc. Properties may be used, for example, to query a web service to access metadata, such as passing a building's ID to get its address. Or a property might be referenced on-the-fly for changing a model's appearance, e.g., changing highlight color based on a property value.

Batched 3D Model, or just the batch, is a binary blob in little endian accessed in JavaScript as an ArrayBuffer.

Layout

A tile is composed of two sections: a header immediately followed by a body.

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

Header

The 24-byte header contains the following fields:

Field name Data type Description
magic 4-byte ANSI string "b3dm". This can be used to identify the arraybuffer as a Batched 3D Model tile.
version uint32 The version of the Batched 3D Model format. It is currently 1.
byteLength uint32 The length of the entire tile, including the header, in bytes.
batchTableJSONByteLength uint32 The length of the batch table JSON section in bytes. Zero indicates 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.
batchLength unit32 The number of distinguishable models, also called features, in the batch. If the Binary glTF does not have a batchId attribute, this field must be 0.
The body section immediately follows the header section, and is composed of two fields: Batch Table and Binary glTF.

Code for reading the header can be found in Batched3DModelTileContent in the Cesium implementation of 3D Tiles.

Batch Table

The Batch Table contains per-model application-specific metadata, indexable by batchId, that can be used for declarative styling and application-specific use cases such as populating a UI or issuing a REST API request. In the Binary glTF section, each vertex has an numeric batchId attribute in the integer range [0, number of models in the batch - 1]. The batchId indicates the model to which the vertex belongs. This allows models to be batched together and still be identifiable.

See the Batch Table reference for more information.

Binary glTF

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

Binary glTF immediately follows the batch table. It begins 24 + batchTableByteLength bytes from the start of the arraybuffer and continues for the rest of arraybuffer. It may embed all of its geometry, texture, and animations, or it may refer to external sources for some or all of these data.

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

As described above, each vertex has a batchId attribute indicating the model to which it belongs. For example, vertices for a batch with three models may look like this:

batchId:  [0,   0,   0,   ..., 1,   1,   1,   ..., 2,   2,   2,   ...]
position: [xyz, xyz, xyz, ..., xyz, xyz, xyz, ..., xyz, xyz, xyz, ...]
normal:   [xyz, xyz, xyz, ..., xyz, xyz, xyz, ..., xyz, xyz, xyz, ...]

Vertices do not need to be ordered by batchId so the following is also OK:

batchId:  [0,   1,   2,   ..., 2,   1,   0,   ..., 1,   2,   0,   ...]
position: [xyz, xyz, xyz, ..., xyz, xyz, xyz, ..., xyz, xyz, xyz, ...]
normal:   [xyz, xyz, xyz, ..., xyz, xyz, xyz, ..., xyz, xyz, xyz, ...]

Note that a vertex can't belong to more than one model; in that case, the vertex needs to be duplicated so the batchIds can be assigned.

The batchId is identified by the glTF technique parameter semantic _BATCHID. For example:

"technique": {
    "attributes": {
        "a_batchId": "batchId"
    },
    "parameters": {
        "batchId": {
            "semantic": "_BATCHID",
            "type": 5126
        }
    }
}

For this example the attribute is named a_batchId, and is declared in the vertex shader as:

attribute float a_batchId;

The vertex shader can be modified at runtime to use a_batchId to access individual models in the batch, e.g., to change their color.

When a Batch Table is present or the batchLength property is greater than 0, the batchId attribute (with the parameter semantic _BATCHID) is required; otherwise, it is not.

Although not strictly required, clients may find the glTF CESIUM_RTC extension useful for high-precision rendering.

File Extension

.b3dm

MIME Type

TODO, #60

application/octet-stream