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KHR_draco_mesh_compression

Contributors

Copyright (C) 2013-2017 The Khronos Group Inc. All Rights Reserved. glTF is a trademark of The Khronos Group Inc. See Appendix for full Khronos Copyright Statement.

Status

Complete

Dependencies

Written against the glTF 2.0 spec.

Overview

This extension defines a schema to use Draco geometry compression (non-normative) libraries in glTF format. This allows glTF to support streaming compressed geometry data instead of the raw data. This extension specification is based on Draco bitstream version 2.2 (the entirety of this specification is normative and included in Scope).

The conformance section specifies what an implementation must do when encountering this extension, and how the extension interacts with the attributes defined in the base specification.

glTF Schema Updates

If a primitive contains an extension property and the extension property defines its KHR_draco_mesh_compression property, then the Draco geometry compression must be used.

The following picture shows the structure of the schema update.

Figure 1: Structure of geometry compression extension.

The Draco extension points to the bufferView that contains the compressed data. If the uncompressed version of the asset is not provided, then KHR_draco_mesh_compression must be added to extensionsRequired.

"extensionsRequired" : [
    "KHR_draco_mesh_compression"
]

If the KHR_draco_mesh_compression property is set in extensionsRequired then the primitive must only contain the Draco compressed data. If a Draco compressed version of the asset is provided then KHR_draco_mesh_compression must be added to extensionsUsed.

Usage of the extension must be listed in the extensionsUsed.

"extensionsUsed" : [
    "KHR_draco_mesh_compression"
]

Below is an example of what part of a glTF file will look like if the Draco extension is set. Note that all other nodes stay the same except primitives:

"mesh" : {
    "primitives" : [
        {
            "attributes" : {
                "POSITION" : 11,
                "NORMAL" : 12,
                "TEXCOORD_0" : 13,
                "WEIGHTS_0" : 14,
                "JOINTS_0" : 15
            },
            "indices" : 10,
            "mode" : 4,
            "extensions" : {
                "KHR_draco_mesh_compression" : {
                    "bufferView" : 5,
                    "attributes" : {
                        "POSITION" : 0,
                        "NORMAL" : 1,
                        "TEXCOORD_0" : 2,
                        "WEIGHTS_0" : 3,
                        "JOINTS_0" : 4
                    }
                }
            }
        }
    ]
}

"bufferViews" : [
    // ...
    // bufferView of Id 5
    {
        "buffer" : 10,
        "byteOffset" : 1024,
        "byteLength" : 10000
    }
    // ...
}

bufferView

The bufferView property points to the buffer containing compressed data. The data must be passed to a mesh decoder and decompressed to a mesh.

attributes

attributes defines the attributes stored in the decompressed geometry. e.g., in the example above, POSITION, NORMAL, TEXCOORD_0, WEIGHTS_0 and JOINTS_0. Each attribute is associated with an attribute id which is its unique id in the compressed data. The attributes defined in the extension must be a subset of the attributes of the primitive. To request an attribute, loaders must be able to use the correspondent attribute id specified in the attributes to get the attribute from the compressed data.

accessors

The accessors properties corresponding to the attributes and indices of the primitives must match the decompressed data.

Restrictions on geometry type

When using this extension, the mode of primitive must be either TRIANGLES or TRIANGLE_STRIP and the mesh data will be decoded accordingly.

JSON Schema

For full details on the KHR_draco_mesh_compression extension properties, see the schema:

Conformance

Below is the recommended process when a loader encounters a glTF asset with the Draco extension set:

  • If KHR_draco_mesh_compression is in extensionsRequired and the loader does not support the Draco extension, then the loader must fail loading the asset.
  • If the loader does not support the Draco extension and KHR_draco_mesh_compression is not in extensionsRequired, then load the glTF asset ignoring KHR_draco_mesh_compression in primitive.
  • If the loader does support the Draco extension, but will not process KHR_draco_mesh_compression, then the loader must load the glTF asset ignoring KHR_draco_mesh_compression in primitive.
  • If the loader does support the Draco extension, and will process KHR_draco_mesh_compression then:
    • The loader must process KHR_draco_mesh_compression first. The loader must get the data from KHR_draco_mesh_compression's bufferView property and decompress the data using a Draco decoder to a Draco geometry.
    • Then the loader must process attributes and indices properties of the primitive. When loading each accessor, you must ignore the bufferView and byteOffset of the accessor and go to the previously decoded Draco geometry in the primitive to get the data of indices and attributes. A loader must use the decompressed data to fill the accessors or render the decompressed Draco geometry directly (e.g. ThreeJS (non-normative)).
    • If additional attributes are defined in primitive's attributes, but not defined in KHR_draco_mesh_compression's attributes, then the loader must process the additional attributes as usual.

Implementation note

This section is non-normative.

To prevent transmission of redundant data, exporters should generally write compressed Draco data into a separate buffer from the uncompressed fallback, and shared data into a third buffer. Loaders may then optimize to request only the necessary buffers.

Draco compression may change the order and number of vertices in a mesh. To satisfy the requirement that accessors properties be correct for both compressed and uncompressed data, generators should create uncompressed attributes and indices using data that has been decompressed from the Draco buffer, rather than the original source data.

Resources

This section is non-normative.

Appendix: Full Khronos Copyright Statement

Copyright 2013-2017 The Khronos Group Inc.

Some parts of this Specification are purely informative and do not define requirements necessary for compliance and so are outside the Scope of this Specification. These parts of the Specification are marked as being non-normative, or identified as Implementation Notes.

Where this Specification includes normative references to external documents, only the specifically identified sections and functionality of those external documents are in Scope. Requirements defined by external documents not created by Khronos may contain contributions from non-members of Khronos not covered by the Khronos Intellectual Property Rights Policy.

This specification is protected by copyright laws and contains material proprietary to Khronos. Except as described by these terms, it or any components may not be reproduced, republished, distributed, transmitted, displayed, broadcast or otherwise exploited in any manner without the express prior written permission of Khronos.

This specification has been created under the Khronos Intellectual Property Rights Policy, which is Attachment A of the Khronos Group Membership Agreement available at www.khronos.org/files/member_agreement.pdf. Khronos grants a conditional copyright license to use and reproduce the unmodified specification for any purpose, without fee or royalty, EXCEPT no licenses to any patent, trademark or other intellectual property rights are granted under these terms. Parties desiring to implement the specification and make use of Khronos trademarks in relation to that implementation, and receive reciprocal patent license protection under the Khronos IP Policy must become Adopters and confirm the implementation as conformant under the process defined by Khronos for this specification; see https://www.khronos.org/adopters.

Khronos makes no, and expressly disclaims any, representations or warranties, express or implied, regarding this specification, including, without limitation: merchantability, fitness for a particular purpose, non-infringement of any intellectual property, correctness, accuracy, completeness, timeliness, and reliability. Under no circumstances will Khronos, or any of its Promoters, Contributors or Members, or their respective partners, officers, directors, employees, agents or representatives be liable for any damages, whether direct, indirect, special or consequential damages for lost revenues, lost profits, or otherwise, arising from or in connection with these materials.

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