Add KHR_texture_astc extension

extensions/2.0/Khronos/KHR_texture_astc/README.md

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+<!--
+Copyright 2024 The Khronos Group Inc.
+SPDX-License-Identifier: LicenseRef-KhronosSpecCopyright
+-->
+
+# KHR_texture_astc
+
+## Contributors
+
+- Wasim Abbas [@wasimabbas-arm](https://github.com/wasimabbas-arm)
+- Jan-Harald Fredriksen [@janharaldfredriksen-arm](https://github.com/janharaldfredriksen-arm)
+- Peter Harris [@solidpixel](https://github.com/solidpixel)
+- Mark Callow [@MarkCallow](https://github.com/MarkCallow)
+- Alexey Knyazev [@lexaknyazev](https://github.com/lexaknyazev)
+
+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 spec.
+
+## Overview
+
+This extension adds the ability to specify textures using (ASTC compressed images)[https://github.com/ARM-software/astc-encoder/blob/main/Docs/FormatOverview.md] in KTX v2 containers. An implementation of this extension can use such images as an alternative to the PNG or JPEG images available in glTF 2.0 for more efficient asset transmission, reducing GPU memory footprint, higher texture throughput and faster execution.
+
+When this extension is used, it's allowed to use value `image/ktx2` for the `mimeType` property of images that are referenced by the `source` property of `KHR_texture_astc` texture extension object.
+
+When running on supported platforms, engines can directly upload ASTC data without transcoding it to other compressed formats. This means that all ASTC block sizes and quality presets are supported unlike other texture extensions that focus on portability of compressed texture payloads.
+
+Platforms that do not have hardware ASTC support would have to either use other texture sources or decompress ASTC data manually.
+
+## glTF Schema Updates
+
+The `KHR_texture_astc` extension is added to the `textures` object and specifies a `source` property that points to the index of the `image` which defines a reference to the KTX v2 file with the ASTC compressed image.
+
+The following glTF will load `astc_image.ktx2` in clients that support this extension, and fall back to `image.png` otherwise.
+
+```json
+{
+    "asset": {
+        "version": "2.0"
+    },
+    "extensionsUsed": [
+        "KHR_texture_astc"
+    ],
+    "textures": [
+        {
+            "source": 0,
+            "extensions": {
+                "KHR_texture_astc": {
+                    "source": 1
+                }
+            }
+        }
+    ],
+    "images": [
+        {
+            "uri": "image.png"
+        },
+        {
+            "uri": "astc_image.ktx2"
+        }
+    ]
+}
+```
+
+When used in the glTF Binary (GLB) format the `image` that points to the KTX v2 resource uses the `mimeType` value of `image/ktx2`.
+
+```json
+{
+    "asset": {
+        "version": "2.0"
+    },
+    "extensionsUsed": [
+        "KHR_texture_astc"
+    ],
+    "textures": [
+        {
+            "source": 0,
+            "extensions": {
+                "KHR_texture_astc": {
+                    "source": 1
+                }
+            }
+        }
+    ],
+    "images": [
+        {
+            "mimeType": "image/png",
+            "bufferView": 1
+        },
+        {
+            "mimeType": "image/ktx2",
+            "bufferView": 2
+        }
+    ]
+}
+```
+
+### Using Without a Fallback
+
+To use KTX v2 image with ASTC compression without a fallback, define `KHR_texture_astc` in both `extensionsUsed` and `extensionsRequired`. The `texture` object will then have its `source` property omitted as shown below.
+
+```json
+{
+    "asset": {
+        "version": "2.0"
+    },
+    "extensionsUsed": [
+        "KHR_texture_astc"
+    ],
+    "extensionsRequired": [
+        "KHR_texture_astc"
+    ],
+    "textures": [
+        {
+            "extensions": {
+                "KHR_texture_astc": {
+                    "source": 0
+                }
+            }
+        }
+    ],
+    "images": [
+        {
+            "uri": "astc_image.ktx2"
+        }
+    ]
+}
+```
+
+### JSON Schema
+
+[texture.KHR_texture_astc.schema.json](schema/texture.KHR_texture_astc.schema.json)
+
+## KTX v2 files with ASTC compression 
+
+To cover a broad range of use cases, this extension allows different ASTC block sizes for LDR image types while HDR and 3D ASTC images are not supported. These can be determined from the vkFormat of the KTX image.
+
+### KTX header fields for ASTC payloads
+  - ASTC block sizes can be determined from the vkFormat of the KTX image. It will always be an LDR ASTC image.
+  - DFD `colorModel` MUST be `KHR_DF_MODEL_ASTC`.
+  - DFD `channelId` MUST be `KHR_DF_CHANNEL_ASTC_DATA`.
+
+### Additional requirements
+
+Regardless of the format used, these additional restrictions apply for compatibility reasons:
+
+- Orientation metadata (`KTXorientation`) MUST be `rd` or omitted.
+- Color space information in the DFD MUST match the expected usage, namely:
+  - For textures with **color data** (e.g., base color maps),
+    - `colorPrimaries` MUST be `KHR_DF_PRIMARIES_BT709`;
+    - `transferFunction` MUST be `KHR_DF_TRANSFER_SRGB`.
+  - For textures with **non-color data** (e.g., normal maps),
+    - `colorPrimaries` MUST be `KHR_DF_PRIMARIES_UNSPECIFIED`;
+    - `transferFunction` MUST be `KHR_DF_TRANSFER_LINEAR`.
+- When a texture refers to a sampler with mipmap minification, the KTX image MUST contain a full mip pyramid.
+- When a texture referencing a KTX v2 file with ASTC compressed image is used for glTF 2.0 material maps (both color and non-color), the KTX v2 image MUST be of **2D** type as defined in the KTX v2 Specification, Section 4.1.
+- `KHR_DF_FLAG_ALPHA_PREMULTIPLIED` flag MUST NOT be set unless the material's specification requires premultiplied alpha.
+
+## Known Implementations
+
+Authoring:
+
+TBA
+
+
+Viewing:
+
+TBA
+
+
+## Resources
+
+[KTX File Format Specification, version 2](https://github.khronos.org/KTX-Specification/)
+
+[KTX Reference Software](https://github.com/KhronosGroup/KTX-Software/)
+
+[ASTC encoder](https://github.com/ARM-software/astc-encoder)
+
+## Appendix: Full Khronos Copyright Statement
+
+Copyright 2023 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
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+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
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+see https://www.khronos.org/adopters.
+
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+
+Khronos® and Vulkan® are registered trademarks, and ANARI™, WebGL™, glTF™, NNEF™, OpenVX™,
+SPIR™, SPIR-V™, SYCL™, OpenVG™ and 3D Commerce™ are trademarks of The Khronos Group Inc.
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+and OpenGL® is a registered trademark and the OpenGL ES™ and OpenGL SC™ logos are trademarks
+of Hewlett Packard Enterprise used under license by Khronos. ASTC is a trademark of
+ARM Holdings PLC. All other product names, trademarks, and/or company names are used solely
+for identification and belong to their respective owners.

extensions/2.0/Khronos/KHR_texture_astc/schema/texture.KHR_texture_astc.schema.json

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+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "KHR_texture_astc glTF Texture Extension",
+    "type": "object",
+    "description": "glTF extension to specify textures using the KTX v2 images with ASTC compression.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "source": {
+          "allOf": [ { "$ref": "glTFid.schema.json" } ],
+          "description": "The index of the image which points to a KTX v2 resource with ASTC compression."
+        },
+        "extensions": {},
+        "extras": {}
+      }
+}