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.  
+This could for instance be when multiple images of the same dimension and format needs to be stored or when higher precision or range is needed.  
+Multiple images can be stored in the KTX file using arrays and referenced using the `layer` property.  
+
+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.  
+
+One intended usecase is to make it easier for implementations to optimize texture uniform sampler usage by grouping samplers by dimension and format.  
+For instance, if model uses an atlasmap and the resulting baseColor, normal and orm textures are the same size and format they can be stored as different layers in the same KTX file.  
+This will then give a hint to the runtime implementation that these textures can be grouped together, potentially using less uniform samplers.  
+
+## 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, 
+                    "layer": 0
+                }
+            }
+        }
+    ],
+    "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 encoded depending on the target format and material texture slot usage.    
+When this extension is used with material texture slots that expect sRGB-encoded values (RGB or RGBA format) and the texture format is 8 bits per pixel, the texture format must be one of VK_FORMAT_B8G8R8_SRGB or VK_FORMAT_B8G8R8A8_SRGB and contain an sRGB encoded image.  
+When this extension is used with material texture slosts that expect sRGB-encoded values (RGB or RGBA format) and the texture format is > 8 bits per pixel the texture is encoded using linear transfer function.  
+The texture format must then be one of the following:  
+VK_FORMAT_R16G16B16A16_UNORM  
+VK_FORMAT_R16G16B16_UNORM  
+
+The range limitation on the target texture remains unchanged.  
+
+
+>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.  
+
+## Non Color Texture Sources
+
+When this extension is used with material texture slots that expect linear non-color texture values, such as normal or metallic/roughness values, the texture format must be one of the VK_FORMAT_XXX_UNORM formats.  
+
+>Implementation Note: For the core glTF 2.0 specification, these texture slots include normalTexture, occlusionTexture and metallicRoughnessTexture.  
+For material extensions, it's usually texture slots named *Texture, *RoughnessTexture or similar.    
+
+## 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_B8G8R8A8_SRGB  
+VK_FORMAT_R8G8B8A8_UNORM  
+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_B8G8R8_SRGB  
+VK_FORMAT_R8G8B8_UNORM
+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_R8G8_UNORM
+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_R8_UNORM
+VK_FORMAT_R16_SFLOAT  
+VK_FORMAT_R16_UNORM  
+VK_FORMAT_R32_SFLOAT  
+
+**Implementation Note**  
+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
+
+- `vkFormat` MUST be one of:  
+VK_FORMAT_R8G8B8A8_UNORM  
+VK_FORMAT_R8G8B8_UNORM  
+VK_FORMAT_R8G8_UNORM  
+VK_FORMAT_R8_UNORM  
+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  
+
+- `supercompressionScheme` MUST be one of:  
+2 (Zstandard)  
+3 (ZLIB)  
+
+
+
+
+### 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 for Material Textures
+
+When a texture referencing a KTX v2 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.
+
+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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+IP Policy must become Adopters and confirm the implementation as conformant under
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+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": {}
+      }
+}