KHR_accessor_float64 Draft Proposal

extensions/2.0/Khronos/KHR_accessor_float64/README.md

@@ -0,0 +1,130 @@
+<!--
+Copyright 2024 The Khronos Group Inc.
+SPDX-License-Identifier: LicenseRef-KhronosSpecCopyright
+-->
+
+# KHR\_accessor\_float64
+
+## Contributors
+
+- Alexey Knyazev, [@lexaknyazev](https://github.com/lexaknyazev)
+- Sean, [@spnda](https://github.com/spnda)
+
+Copyright 2024 The Khronos Group Inc.
+See [Appendix](#appendix-full-khronos-copyright-statement) for full Khronos Copyright Statement.
+
+## Status
+
+Draft
+
+## Dependencies
+
+Written against the glTF 2.0 Specification.
+
+## Overview
+
+The base glTF 2.0 Specification supports only one floating-point component type - IEEE-754 _binary32_ (also known as single-precision). Although this is sufficient for most applications, there are use cases that benefit from increased range and precision provided by the 64-bit floating-point data type (also known as double-precision).
+
+This extension expands the set of recognized accessor component types with a new enum value `5130` that represents IEEE-754 _binary64_.
+
+This extension allows usage of such accessors in all places where the single-precision floating-point component type is accepted by the base glTF 2.0 Specification or extensions.
+
+Because this extension does not provide a way to specify alternative data representations, assets that use this extension **MUST** specify it in the `extensionsRequired` array.
+
+## Extending Accessors
+
+When the extension is supported, the following data type is valid for accessors in addition to the types defined in [Section 3.6.2.2](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#accessor-data-types).
+
+| `componentType` | Data Type | Signed | Bits |
+|-----------------|-----------|--------|------|
+| `5130`          | _double_  | Signed | 64   |
+
+Such accessors **MUST** adhere to the following additional rules:
+
+- their `normalized` property **MUST NOT** be set to true;
+- their effective byte offsets, as defined in [Section 3.6.2.4](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#data-alignment), **MUST** be aligned to 8-byte boundaries.
+
+When referring to data in a GLB-stored buffer, the start of the buffer **SHOULD** be aligned to an 8-byte boundary by adding necessary trailing padding bytes to the JSON chunk as defined in [Section 4.4.3.2](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#structured-json-content).
+
+## Extending Accessor Usage
+
+When the extension is supported, it is valid to use _double_ component type accessors in **all** places where _float_ component type accessors are valid.
+
+The following sections cover only the base glTF 2.0 Specification and ratified extensions.
+
+### Mesh Attributes
+
+When the extension is supported, _double_ component type accessors are valid for all mesh and morph target attributes that support _float_ component type accessors as defined in [Section 3.7.2.1](https://www.khronos.org/registry/glTF/specs/2.0/glTF-2.0.html#meshes-overview) and [Section 3.7.2.2](https://www.khronos.org/registry/glTF/specs/2.0/glTF-2.0.html#morph-targets) respectively.
+
+#### Interaction with `EXT_mesh_gpu_instancing`
+
+When the extension is supported, _double_ component type accessors are valid for all instancing semantics defined in the `EXT_mesh_gpu_instancing` extension.
+
+### Skins
+
+When the extension is supported, _double_ component type accessors are valid for inverse-bind matrices defined in [Section 3.7.3.1](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#skins-overview).
+
+### Animation Samplers
+
+When the extension is supported, _double_ component type accessors are valid for animation sampler inputs and outputs defined in [Section 3.11](https://www.khronos.org/registry/glTF/specs/2.0/glTF-2.0.html#animations).
+
+Interpolation of double-precision keyframe data **MUST** be performed with double-precision arithmetic.
+
+#### Interaction with `KHR_animation_pointer`
+
+When the extension is supported, _double_ component type accessors are valid for keyframe data of all Object Model properties of `float*` data types.
+
+## Compatibility
+
+Since not all graphics environments support double-precision floating-point data, implementations **MAY** employ different lossy strategies to render such assets, e.g., converting mesh attribute data to single-precision values when uploading data to GPUs.
+
+When a lossy conversion is involved, implementations **SHOULD** warn their users that the asset's rendering can be incorrect.
+
+Given transmission size and portability limitations, asset creators **SHOULD** use _double_ component type accessors only when absolutely necessary.
+
+## Appendix: Full Khronos Copyright Statement
+
+Copyright 2024 The Khronos Group Inc.
+
+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.
+
+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.
+
+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.
+
+This specification has been created under the Khronos Intellectual Property Rights
+Policy, which is Attachment A of the Khronos Group Membership Agreement available at
+https://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/conformance/adopters/file-format-adopter-program.
+
+Where this Specification identifies specific sections of external references, only those
+specifically identified sections define normative functionality. The Khronos Intellectual
+Property Rights Policy excludes external references to materials and associated enabling
+technology not created by Khronos from the Scope of this Specification, and any licenses
+that may be required to implement such referenced materials and associated technologies
+must be obtained separately and may involve royalty payments.
+
+Khronos® is a registered trademark, and glTF™ is a trademark of The Khronos Group Inc. All
+other product names, trademarks, and/or company names are used solely for identification
+and belong to their respective owners.