EXT_materials_bump

extensions/2.0/Vendor/EXT_materials_bump/README.md

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+# EXT\_materials\_bump
+
+## Contributors
+
+- Ben Houston, Threekit [@bhouston](https://github.com/bhouston)
+
+Copyright 2023 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.
+
+## Exclusions
+
+- This extension must not be used on a material that also uses `KHR_materials_pbrSpecularGlossiness`.
+- This extension must not be used on a material that also uses `KHR_materials_unlit`.
+
+## Overview
+
+This extension adds one parameters to the metallic-roughness material: `bump`.
+
+`bump` allows users to perturb the surface normal via a bump map. The bump map is a grayscale texture, where the value of each texel represents the height of the surface at that texel.  The height is then used to perturb the normal direction.
+
+Bump maps were introduced into computer graphics in 1978 by Blinn in "Simulation of Wrinkled Surfaces, while normal maps were introduced in 1989 by Cohen et al, "Appearance-Preserving Simplification."
+
+Benefits of bump maps over normal maps are primarily these two:
+
+1. Bump maps are much more compact that normal maps and lead to smaller glTF files as well as less GPU memory requirements.  This is because the normal map has to store a full 3D vector per texel with floating point precision, while the bump map only needs to store a single scalar value per texel.  Thus bump maps require 3 times less data than normal maps for transmission and GPU storage.  Normal maps are also very sensitive to texture compressions methods, while bump maps are less sensitive.
+
+2. Bump maps are useful for capturing small details in combination with a normal map for large scale structures.  For example, when rendering human faces, there are usually two layers of surface pertubation detail, the unwrapped normal map is used for representing wrinkles, and then a repeating bump map is used for skin pore details.  This is also the case for many other materials such as wood, metal, and stone, where there is both macro and micro details.
+
+Normal maps are more resistant to artifacts when undergoing going magnification as compared to bump maps, thus they are better suited for large scale smooth detail.  This limitation of bump maps is not significant though when bump maps are being used for detail textures where the magnification is small and they align roughly with the same texel resolution as the color map.
+
+Additionally, maybe WebGL rendering engines, including both ThreeJS and Sketchfab, already support simultaneous bump maps and normal maps in their default materials.  Without this extension, these engines can not save or load this information via glTF files.
+
+## Extending Materials
+
+The `KHR_materials_bump` extension can be specified in the `extensions` property of the material:
+
+```json
+{
+    "materials": [
+        {
+            "extensions": {
+                "KHR_materials_bump": {
+                    "bumpFactor": 0.5
+                }
+            }
+        }
+    ]
+}
+```
+
+Factor and texture are combined by multiplication to describe a single value.
+
+| |Type|Description|Required|
+|-|----|-----------|--------|
+| **bumpFactor** | `number` | The strength of the specular reflection. | No, default: `1.0`|
+| **bumpTexture** | [`textureInfo`](https://www.khronos.org/registry/glTF/specs/2.0/glTF-2.0.html#reference-textureinfo) | A texture that defines the strength of the specular reflection, stored in the red (`R`) channel. This will be multiplied by bumpFactor. | No |
+
+The `bump` parameters affect the normal used in `dielectric_brdf` of the glTF 2.0 metallic-roughness material.
+
+## Implementation
+
+*This section is non-normative.*
+
+The `bump` parameter is used to perturb the normal direction. The normal direction is perturbed by the value of the bump map at the texel corresponding to the fragment. The perturbed normal is then used in the `dielectric_brdf` function of the glTF 2.0 metallic-roughness material.
+
+### Blending Normal and Bump Perturbations
+
+In order to properly blend the bump map with a normal map, please follow the recommendation of https://blog.selfshadow.com/publications/blending-in-detail/.
+
+```glsl
+vec3 t = originalNormal.xyz * vec3( 2,  2, 2 ) + vec3( -1, -1, 0 );
+vec3 u = bumpNormal.xyz * vec3( -2, -2, 2 ) + vec3( 1,  1, -1 );
+vec3 r = normalize( t * dot(t, u) / t.z - u ); // not required if t and u are pre-normalized
+vec3 blendedNormal = r * 0.5 + 0.5;
+```
+
+## Schema
+
+- [material.EXT_materials_bump.schema.json](schema/material.EXT_materials_bump.schema.json)
+
+## References
+
+[Blinn, J.F., "Simulation of Wrinkled Surfaces" (1978)](https://www.microsoft.com/en-us/research/wp-content/uploads/1978/01/p286-blinn.pdf)
+
+[Cohen, J. et al. "Appearance-Preserving Simplification" (1989)](http://www.cs.unc.edu/~geom/APS/APS.pdf)
+
+[Waveren, J.M.P. et al. "Real-Time Normal Map DXT Compression" (2008)](https://developer.download.nvidia.com/whitepapers/2008/real-time-normal-map-dxt-compression.pdf)
+
+## 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
+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
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+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.
+
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+express or implied, regarding this specification, including, without limitation:
+merchantability, fitness for a particular purpose, non-infringement of any
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+respective owners.

extensions/2.0/Vendor/EXT_materials_bump/schema/material.EXT_materials_bump.schema.json

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+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "EXT_materials_bump glTF Material Extension",
+    "type": "object",
+    "description": "glTF extension that defines a bump which perturbs the surface normal.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "bumpFactor": {
+            "type": "number",
+            "description": "The strength of the bump displacement.",
+            "default": 1.0,
+            "minimum": 0.0,
+            "maximum": 100.0,
+            "gltf_detailedDescription": "This parameter scales the amount of normal perturbation by the bump map on the surface normal."
+        },
+        "bumpTexture": {
+            "allOf": [ { "$ref": "textureInfo.schema.json" } ],
+            "description": "A texture that defines the displacement in the red channel.",
+            "gltf_detailedDescription": "A texture that defines the displacement in the red channel. This will be multiplied by bumpFactor."
+        },
+        "extensions": { },
+        "extras": { }
+    }
+}