Framebuffer light intensity mapping extension - draft spec

extensions/2.0/Khronos/KHR_framebuffer_value_mapping/README.md

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+# KHR_framebuffer_value_mapping
+
+## Contributors
+
+Rickard Sahlin, <mailto:[email protected]> 
+
+Copyright (C) 2024 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 2.01 spec.
+
+## Exclusions
+
+## Termonilogy
+
+This extension uses the following terms:
+
+Framebuffer : One or more memory buffers used by the target platform to store pixel values prior to being output to display. 
+framebuffer_max_value : The maximum value that can be stored in the framebuffer, for an 8 bit unorm buffer this is 1.0. 
+
+
+## Overview
+
+This extension is intended for implementations that targets a light emitting display with the goal of outputting interactive framerates in a physically correct manner. 
+The purpose is to define how light calculation output values are mapped to framebuffer values. 
+
+There are two main goals for this extension: 
+
+1: Provide a method to linearly scale RGB values to the framebuffer format of the target device - called lightScaleFactor. 
+
+2: Define how to limit values prior to writing them to framebuffer when the scaled value exceeds the maximum range of the framebuffer - called RGBThreshold. 
+
+
+## Calculation of the lightScaleFactor 
+
+The light scale factor is calculated from the extension "maxLight" value. 
+This value defines the light intensity (lm/m2) value that will be mapped to the maximum value of the framebuffer. 
+
+
+float lightScaleFactor = framebuffer_max_value / maxLight 
+
+Each calculated RGB pixel value shall be scaled by the lightScaleFactor at some point prior to being output to display. 
+Exactly how this is done is an implementation detail, the normal usecase would be to scale the value in the fragment shader. 
+
+## Limit of RGB pixel values before output to framebuffer 
+
+If, after applying the lightScaleFactor, RGB values exceed the maximum value of the framebuffer then the RGB triplet must be scaled. 
+The scaling shall be done using the max component value from the RGB triplet, ensuring that color is retained. 
+
+pseudocode for scaling. 
+vec3 scaledRGB = RGB / max(R, max(G,B)); 
+
+
+## Defining an asset to use KHR_framebuffer_value_mapping
+
+The extension is put in the scene declaration. 
+All uses of the scene, for instance in a usecase with glTFX, shall take the extension into account. 
+This does not apply to uses of data within the scene, for instance Nodes. 
+
+
+```json
+{
+ "extensionsUsed": [
+ "KHR_framebuffer_value_mapping"
+ ],
+ "scene": 0,
+ "extensions": {
+ "KHR_framebuffer_value_mapping": {
+ "maxLight": 10000
+ }
+ }
+}
+
+```
+
+
+
+## Appendix: Full Khronos Copyright Statement
+
+Copyright 2024 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,
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+merchantability, fitness for a particular purpose, non-infringement of any
+intellectual property, correctness, accuracy, completeness, timeliness, and
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extensions/2.0/Khronos/KHR_framebuffer_value_mapping/schema/scene.KHR_framebuffer_value_mapping.schema.json

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+{
+ "$schema": "http://json-schema.org/draft-04/schema",
+ "title": "KHR_framebuffer_value_mapping glTF Scene Extension",
+ "type": "object",
+ "description": "glTF extension that defines the mapping of light intensity values to frambuffer values.",
+ "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+ "properties": {
+ "maxLight": {
+ "type": "number",
+ "description": "Light intensity value mapped to max framebuffer value.",
+ "gltf_detailedDescription": "The value defines how calculated light intensity values, in the form of RGB triplets, shall be mapped to the framebuffer. RGB values are scaled by (max_framebuffer_value / maxWhite)"
+ },
+ "extensions": { },
+ "extras": { }
+ }
+}