KHR_texture_ktx

extensions/2.0/Khronos/KHR_texture_ktx/README.md

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+# KHR_texture_ktx
+
+## Contributors
+
+* Rickard Sahlin, Ikea  
+* Ben Houston, ThreeKit  
+* Alexey Knyazev  
+
+
+Copyright (C) 2021 The Khronos Group Inc. All Rights Reserved. glTF is a trademark of 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 KTX v2 images.  
+An implementation of this extension can use such images as an alternative to the PNG or JPEG images available in glTF 2.0 when higher precision or range is needed.  
+
+When the 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_ktx` texture extension object.  
+This extension does not define how to output the higher dynamic range result buffer to display or to a lower precision target buffer.  
+
+## glTF Schema Updates
+
+The `KHR_texture_ktx` 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 image.
+
+The following glTF will load `image.ktx2`
+
+```json
+{
+    "asset": {
+        "version": "2.0"
+    },
+    "extensionsUsed": [
+        "KHR_texture_ktx"
+    ],
+    "textures": [
+        {
+            "source": 0,
+            "extensions": {
+                "KHR_texture_ktx": {
+                    "source": 1
+                }
+            }
+        }
+    ],
+    "images": [
+        {
+            "uri": "image.png"
+        },
+        {
+            "uri": "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_ktx"
+    ],
+    "textures": [
+        {
+            "source": 0,
+            "extensions": {
+                "KHR_texture_ktx": {
+                    "source": 1
+                }
+            }
+        }
+    ],
+    "images": [
+        {
+            "mimeType": "image/png",
+            "bufferView": 1
+        },
+        {
+            "mimeType": "image/ktx2",
+            "bufferView": 2
+        }
+    ]
+}
+```
+
+### Using Without a Fallback
+
+To use KTX v2 image without a fallback, define `KHR_texture_ktx` 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_ktx"
+    ],
+    "extensionsRequired": [
+        "KHR_texture_ktx"
+    ],
+    "textures": [
+        {
+            "extensions": {
+                "KHR_texture_ktx": {
+                    "source": 0
+                }
+            }
+        }
+    ],
+    "images": [
+        {
+            "uri": "image.ktx2"
+        }
+    ]
+}
+```
+
+### JSON Schema
+
+[texture.KHR_texture_ktx.schema.json](schema/texture.KHR_texture_ktx.schema.json)
+
+## Color Texture Sources
+
+Texture data provided by this extension is always encoded using linear transfer function. When this extension is used with material texture slots that expect sRGB-encoded values, only the fallback 8-bit texture (when present) uses sRGB encoding.  
+
+>Implementation Note: For the core glTF 2.0 specification, these texture slots include emissiveTexture and baseColorTexture; for material extensions, it's usually texture slots named *ColorTexture.  
+
+## KTX v2 Images
+
+For the purposes of this extension, the following texture types and formats are defined:
+
+- **RGBA:** A texture that uses all four channels.  
+
+VK_FORMAT_R16G16B16A16_SFLOAT  
+VK_FORMAT_R16G16B16A16_UNORM  
+VK_FORMAT_R32G32B32A32_SFLOAT  
+
+
+- **RGB:** A texture that uses red, green, and blue channels. Alpha channel is unused and not sampled at runtime.  
+
+VK_FORMAT_E5B9G9R9_UFLOAT_PACK32  
+VK_FORMAT_B10G11R11_UFLOAT_PACK32  
+VK_FORMAT_R16G16B16_SFLOAT  
+VK_FORMAT_R16G16B16_UNORM  
+VK_FORMAT_R32G32B32_SFLOAT  
+
+- **RG_** A texture that uses, red and green channels. All other channels are unused and their values are not sampled at runtime.  
+
+VK_FORMAT_R16G16_SFLOAT  
+VK_FORMAT_R16G16_UNORM  
+VK_FORMAT_R32G32_SFLOAT  
+
+
+- **Red:** A texture that uses only red channel. All other channels are unused and their values are not sampled at runtime.  
+
+VK_FORMAT_R16_SFLOAT  
+VK_FORMAT_R16_UNORM  
+VK_FORMAT_R32_SFLOAT  
+
+**Implementation Notes**  
+This section is non-normative
+
+This extension defines a number of formats that are supported for images used as texture sources.  
+It does not define how these formats are stored internally, it is up to implementations to use a suitable texture format.  
+The source texture shall be used in shader (BRDF) calculations in a way that retains the increased range.  
+However, this extension does not define the way pixel data is written to framebuffer or how swapchain is presented using the increased range.  
+
+If ktx image 
+
+## KTX header fields
+
+`supercompressionScheme` MUST be 3 (ZLIB).  
+
+- vkFormat must be one of:  
+VK_FORMAT_E5B9G9R9_UFLOAT_PACK32  
+VK_FORMAT_B10G11R11_UFLOAT_PACK32  
+VK_FORMAT_R16G16B16A16_SFLOAT  
+VK_FORMAT_R16G16B16A16_UNORM  
+VK_FORMAT_R32G32B32A32_SFLOAT  
+VK_FORMAT_R16G16B16_SFLOAT  
+VK_FORMAT_R16G16B16_UNORM  
+VK_FORMAT_R32G32B32_SFLOAT
+VK_FORMAT_R16G16_SFLOAT  
+VK_FORMAT_R16G16_UNORM  
+VK_FORMAT_R32G32_SFLOAT  
+VK_FORMAT_R16_SFLOAT  
+VK_FORMAT_R16_UNORM  
+VK_FORMAT_R32_SFLOAT  
+
+
+
+### Additional requirements
+
+Regardless of the format used, these additional restrictions apply for compatibility reasons:
+
+- Swizzling metadata (`KTXswizzle`) MUST be `rgba` or omitted.
+- 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_LINEAR`.
+  - For textures with **non-color data** (e.g., normal maps),
+    - `colorPrimaries` MUST be `KHR_DF_PRIMARIES_UNSPECIFIED`;
+    - `transferFunction` MUST be `KHR_DF_TRANSFER_LINEAR`.
+
+  > **Implementation Note**: Engines may need to generate missing mipmaps at runtime when a full mip pyramid is not available.
+
+
+### Using KTX v2 Images with Basis Universal Supercompression for Material Textures
+
+When a texture referencing a KTX v2 image with Basis Universal supercompression 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.
+
+Exporters SHOULD choose the most appropriate texture format based on the texture's usage and content.
+
+## Known Implementations
+
+Authoring:
+
+
+Viewing:
+
+
+## Resources
+
+[KTX File Format Specification, version 2](https://github.khronos.org/KTX-Specification/)
+
+[KTX Reference Software](https://github.com/KhronosGroup/KTX-Software/)
+
+## Appendix: Full Khronos Copyright Statement
+
+Copyright 202§ 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,
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+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.
+
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extensions/2.0/Khronos/KHR_texture_ktx/schema/texture.KHR_texture_ktx.schema.json

@@ -0,0 +1,15 @@
+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "KHR_texture_ktx glTF extension",
+    "type": "object",
+    "description": "glTF extension to specify textures using the KTX v2 images.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "source": {
+          "allOf": [ { "$ref": "glTFid.schema.json" } ],
+          "description": "The index of the image which points to a KTX v2 resource."
+        },
+        "extensions": {},
+        "extras": {}
+      }
+}