INTEL_lights_sunsky: a procedural sun-sky model as environmental light source

extensions/2.0/Vendor/INTEL_lights_sunsky/README.md

@@ -0,0 +1,123 @@
+# INTEL_lights_sunsky
+
+## Contributors
+
+-   Johannes Günther, Intel, [@johguenther](https://github.com/johguenther)
+-   Bruce Cherniak, Intel, [@BruceCherniak](https://github.com/BruceCherniak)
+-   Isha Sharma, Intel, [@isharma25](https://github.com/isharma25)
+
+## Status
+
+Draft
+
+## Dependencies
+
+Written against the glTF 2.0 spec.
+
+## Overview
+
+This extension defines parameters for a procedural sun-sky model, an
+environmental light source, which provides natural illumination for the
+scene and which is optionally visible in the background.
+
+<figure>
+<img src="./sunsky.png" alt="Sun and sky at day and at dusk." />
+<figcaption aria-hidden="true">Two different settings of the sunsky
+light, affecting illumination and background; left: clear sky at day,
+right: hazy sky at dusk.</figcaption>
+</figure>
+
+## Specifying a Sunsky Light
+
+Specifying sunsky lights is similar to
+[`KHR_lights_punctual`](../../Khronos/KHR_lights_punctual): the
+top-level glTF 2.0 object is extended by a `lights` array, each element
+a parameter struct defining a sunsky light.
+
+```json
+"extensions": {
+    "INTEL_lights_sunsky": {
+        "lights": [
+            {
+                "visible": true,
+                "intensity": 0.025,
+                "elevation": 1.1,
+                "azimuth": 2.5,
+                "turbidity": 2.0,
+                "albedo": 0.3
+            }
+        ]
+    }
+}
+```
+A sunsky light is instantiated by referencing it by a node, which also
+allows for rotation and orientation of the light (e.g., placing the
+zenith at +Z in the world).
+
+```json
+"nodes" : [
+    {
+        "extensions" : {
+            "INTEL_lights_sunsky" : {
+                "light" : 0
+            }
+        }
+    }            
+]
+```
+
+## Properties
+
+No property is required, all have default values.
+
+| Property   | Description  | Default |
+|:-----------|:-------------|:--------|
+| `visible`  | whether seen in the background | `true` |
+| `intensity`| brightness scaling factor | `0.025` |
+| `elevation`| angle (in `-π/2`..`π/2`) of the sun to the horizon, i.e., to the plane with normal +Y (the zenith) | `π/2` |
+| `azimuth`  | angle (in `0`..`2π`) of the sun to +X in the horizon plane; increasing azimuth is a counterclockwise rotation around +Y | `0` |
+| `turbidity`| atmospheric turbidity due to particles, between `1` (clear sky) and `10` (hazy)| `3` |
+| `albedo`   | ground reflectance, in `0`..`1` | `0.3` |
+
+The [physical-based sun-sky models](#resources) provide absolute
+radiance values, which are too bright for typical scenarios without
+tone mapping, which is why the default `intensity` scales the intrinsic
+radiance into “normal” dynamic range.
+
+Note that the `azimuth` angle is redundant to a rotation of the node the
+sunsky light is attached to, it is supported for convenience.
+
+Some sky models only provide data for the upper hemisphere (and thus
+only support `elevation >= 0`), with an unnatural, sharp cut at the
+horizon. Implementations using such models can extend the sky dome by
+stretching (interpolating) the horizon. The amount of stretch, the
+fraction of the lower hemisphere to cover can be controlled by the
+optional parameter `horizonExtension` (in `0`..`1`, default `0.01`).
+
+
+## glTF Schema Updates
+
+### JSON Schema
+
+-   [glTF.INTEL_lights_sunsky.schema.json](schema/glTF.INTEL_lights_sunsky.schema.json)
+-   [light.schema.json](schema/light.schema.json)
+-   [node.INTEL_lights_sunsky.schema.json](schema/node.INTEL_lights_sunsky.schema.json)
+
+## Known Implementations
+
+-   Intel [OSPRay Studio](https://www.ospray.org/ospray_studio/), since
+    v0.11.0
+
+## Resources
+
+-   Lukáš Hošek, Alexander Wilkie, “[An Analytic Model for Full Spectral
+    Sky-Dome
+    Radiance](https://cgg.mff.cuni.cz/projects/SkylightModelling/)”, ACM
+    Transactions on Graphics (Proceedings of ACM SIGGRAPH 2012), 31(4),
+    2012
+-   Alexander Wilkie, Petr Vevoda, Thomas Bashford-Rogers, Lukáš Hošek,
+    Tomáš Iser, Monika Kolářová, Tobias Rittig, and Jaroslav Křivánek,
+    “[A Fitted Radiance and Attenuation Model for Realistic
+    Atmospheres](https://doi.org/10.1145/3450626.3459758)”, ACM
+    Transactions on Graphics (Proceedings of SIGGRAPH 2021),
+    40(4), 2021.

extensions/2.0/Vendor/INTEL_lights_sunsky/schema/glTF.INTEL_lights_sunsky.schema.json

@@ -0,0 +1,21 @@
+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "INTEL_lights_sunsky glTF extension",
+    "type": "object",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "lights": {
+            "type": "array",
+            "items": {
+                "type": "object",
+                "$ref": "light.schema.json"
+            },
+            "minItems": 1
+        },
+        "extensions": {},
+        "extras": {}
+    },
+    "required": [
+        "lights"
+    ]
+}

extensions/2.0/Vendor/INTEL_lights_sunsky/schema/light.schema.json

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+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "light",
+    "type": "object",
+    "description": "A procedural sun-sky model, an environmental light source.",
+    "allOf": [ { "$ref": "glTFChildOfRootProperty.schema.json" } ],
+    "properties": {
+        "visible": {
+            "type": "boolean",
+            "description": "Whether seen in the background.",
+            "default": true
+        },
+        "intensity": {
+            "type": "number",
+            "description": "Brightness scaling factor.",
+            "default": 0.025,
+            "minimum": 0
+        },
+        "elevation": {
+            "type": "number",
+            "description": "Angle of the sun to the horizon, i.e., to the plane with normal +Y (the zenith).",
+            "default": 1.5707963267948966,
+            "minimum": -1.5707963267948966,
+            "maximum": 1.5707963267948966
+        },
+        "azimuth": {
+            "type": "number",
+            "description": "Angle of the sun to +X in the horizon plane; increasing azimuth is a counterclockwise rotation around +Y.",
+            "default": 0,
+            "minimum": 0,
+            "maximum": 6.283185307179586
+        },
+        "turbidity": {
+            "type": "number",
+            "description": "Atmospheric turbidity due to particles, between `1` (clear sky) and `10` (hazy).",
+            "default": 3,
+            "minimum": 1,
+            "maximum": 10
+        },
+        "albedo": {
+            "type": "number",
+            "description": "Ground reflectance.",
+            "default": 0.3,
+            "minimum": 0,
+            "maximum": 1
+        },
+        "name": {},
+        "extensions": {},
+        "extras": {}
+    }
+}

extensions/2.0/Vendor/INTEL_lights_sunsky/schema/node.INTEL_lights_sunsky.schema.json

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+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "INTEL_lights_sunsky node extension",
+    "type": "object",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "light": {
+            "allOf": [
+                {
+                    "$ref": "glTFid.schema.json"
+                }
+            ],
+            "description": "The id of the light referenced by this node."
+        },
+        "extensions": {},
+        "extras": {}
+    },
+    "required": [
+        "light"
+    ]
+}

extensions/README.md

@@ -55,6 +55,7 @@ Vendor extensions are not covered by the Khronos IP framework.
 * [AGI_stk_metadata](2.0/Vendor/AGI_stk_metadata/README.md)
 * [CESIUM_primitive_outline](2.0/Vendor/CESIUM_primitive_outline/README.md)
 * [FB_geometry_metadata](2.0/Vendor/FB_geometry_metadata/README.md)
+* [INTEL_lights_sunsky](2.0/Vendor/INTEL_lights_sunsky/README.md)
 * [MSFT_lod](2.0/Vendor/MSFT_lod/README.md)
 * [MSFT_packing_normalRoughnessMetallic](2.0/Vendor/MSFT_packing_normalRoughnessMetallic/README.md)
 * [MSFT_packing_occlusionRoughnessMetallic](2.0/Vendor/MSFT_packing_occlusionRoughnessMetallic/README.md)