Merge pull request #91 from CesiumGS/BENTLEY_materials_point_style

BENTLEY_materials_point_style

Author: weegeekps

extensions/2.0/Vendor/BENTLEY_materials_point_style/README.md

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+<!--
+Copyright 2025 Bentley Systems, Incorporated
+SPDX-License-Identifier: CC-BY-4.0
+-->
+
+# BENTLEY_materials_point_style
+
+## Contributors
+
+* Paul Connelly, Bentley Systems, [@pmconne](https://github.com/pmconne)
+* Mark Schlosser, Bentley Systems, [@markschlosseratbentley](https://github.com/markschlosseratbentley)
+* Daniel Zhong, Bentley Systems, [@danielzhong](https://github.com/danielzhong)
+
+## Status
+
+Draft
+
+## Dependencies
+
+Written against the glTF 2.0 spec.
+
+## Overview
+
+Points are fundamental elements in many 3D modeling and computer-aided design (CAD) environments. In such visualizations, individual point primitives may differ in their styling, mainly focused around their diameter in screen pixels. Such diameter can be used to indicate a variety of meanings useful to CAD applications.
+
+This specification describes a minimal extension sufficient to meet Bentley Systems' requirements in supporting CAD-style visualizations of points using glTF.
+
+## Specifying Point Styles
+
+The `BENTLEY_materials_point_style` extension is applied to a material. When that material is used by a `POINTS` primitive (indicated by a value of `0` for [`mesh.primitive.mode`](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#_mesh_primitive_mode)), the extension dictates the diameter of those points in pixels.
+
+### Diameter
+
+The `diameter` property specifies the diameter of each point in CSS pixels. When specified, this value must be greater than zero.
+
+For each point, implementations should render the point such that a filled circle is centered at the point's CSS pixel screen position with a pixel diameter equal to `diameter`. This point faces the camera.
+
+Note: The term "CSS pixel" follows the [CSS3 definition](https://www.w3.org/TR/css-values-3/#absolute-lengths). Per that definition, the CSS pixel is defined as $1\text{px} = \frac{1}{96}\text{th}\ \text{of}\ 1\text{in}$, which this specification adopts for clarity and stability.
+
+## Implementation Notes
+
+Some graphics APIs may not require that implementations directly support point sizes greater than one. For example, [Section 3.4 of the OpenGL ES 3.0 Specification](https://registry.khronos.org/OpenGL/specs/es/3.0/es_spec_3.0.pdf) only guarantees a maximum point size of at least one via `ALIASED_POINT_SIZE_RANGE`. In such cases, points with diameters greater than one could be rendered as quads which are shaped into circles using a shader.
+
+## JSON Schema
+
+* [material.BENTLEY_materials_point_style.schema.json](schema/material.BENTLEY_materials_point_style.schema.json)
+
+## Known implementations
+
+* [iTwin.js](https://github.com/iTwin/itwinjs-core/pull/8835)

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

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+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "title": "BENTLEY_materials_point_style extension",
+  "type": "object",
+  "description": "Specifies point diameter for CAD-style visualization.",
+  "allOf": [
+    { "$ref": "glTFProperty.schema.json" }
+  ],
+  "properties": {
+    "diameter": {
+      "type": "integer",
+      "description": "Diameter of the point in screen pixels. Must be > 0.",
+      "minimum": 1
+    },
+    "extensions": {},
+    "extras": {}
+  },
+  "required": ["diameter"]
+}