From c35ba7fe5c9f49064532f693e46690f8669a0f9d Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?P=C3=A4r=20Winzell?= Date: Wed, 29 May 2019 15:21:30 -0700 Subject: [PATCH 1/3] FB_ngon_encoding extension. --- .../2.0/Vendor/FB_ngon_encoding/README.md | 108 ++++++++++++++++++ .../schema/glTF.FB_ngon_encoding.schema.json | 15 +++ 2 files changed, 123 insertions(+) create mode 100644 extensions/2.0/Vendor/FB_ngon_encoding/README.md create mode 100644 extensions/2.0/Vendor/FB_ngon_encoding/schema/glTF.FB_ngon_encoding.schema.json diff --git a/extensions/2.0/Vendor/FB_ngon_encoding/README.md b/extensions/2.0/Vendor/FB_ngon_encoding/README.md new file mode 100644 index 0000000000..a85346a7c3 --- /dev/null +++ b/extensions/2.0/Vendor/FB_ngon_encoding/README.md @@ -0,0 +1,108 @@ +# FB_ngon_encoding + +## Contributors + +* Pär Winzell, Facebook, [zell@fb.com](mailto:garyhsu@microsoft.com) +* Michael Bunnell, Facebook + +## Status + +Draft + +## Dependencies + +Written against the glTF 2.0 spec. + +## Overview + +While glTF can only deliver a polygon mesh after it's been decomposed into triangles, there are cases where access to the source topology is still useful. One common such case is [Catmull-Clarke subdivision surfaces](https://en.wikipedia.org/wiki/Catmull%E2%80%93Clark_subdivision_surface), which work best with quadrilaterals. + + +This extension contains **no additional data**: +``` + "meshes": [ + { + "name": "Apple", + "primitives": [ + { + "indices": 0, + "attributes": { + "POSITION": 1, + "NORMAL": 2, + "COLOR_0": 3, + "TEXCOORD_0": 4 + }, + "material": 0, + "mode": 4, + "extensions": { + "FB_ngon_encoding": {} + } + } + ] + } + ], +``` + +Rather, it marks the use of an implicit scheme where the **order of triangles and per-triangle vertex indices** holds all the information needed to reconstruct the original structure, and can be generated via a simple triangulation process: + +- For each polygon `p`, choose one identifying vertex `v(p)`, +- Break the polygon into a fan of triangles, all anchored in that same `v(p)`, + - (It's obvious that this can be done for convex polygons, e.g. quads, but the true constraint is a bit less strict.) +- Proceed with the next polygon `p'` where, importantly, `v(p') != v(p)`. + +This lays each polygon's constituent triangles down sequentially, grouped by sharing the same `triangle.vertices[0]`. Given such a sequence, it's then trivial to decode the original structure with linear iteration. A sketch of a possible implementation can be found [below](#Resources). + +Advantages of this scheme: +- The meshes remain valid glTF 2.0, +- Asset size is unchanged, since we add no new data, +- The scheme works with arbitrary mixtures of triangles, quads, and so on, +- Very simple to implement. + +There are a few geometric concerns for non-quad cases: +- Polygons that can't be decomposed into a triangle fan must first be cut into ones that can, +- Even polygons that are technically convex can yield triangles narrow enough to be nearly degenerate. + +On a pragmatic ecosystem note, we must also remember that glTF 2.0 normally allows reordering of triangles and triangle vertices (in fact, such operations are common optimisations), and so tools unfamiliar with this extension would likely destroy the structure we rely on here. Then again, they would presumably not preserve the extension in their output, so at worst one ends up with glTF that's unextended yet still valid. + + +## glTF Schema Updates + +* **JSON schema**: [glTF.FB_ngon_encoding.schema.json](schema/glTF.FB_ngon_encoding.schema.json) + +## Known Implementations + +* Facebook AR/VR asset delivery + +## Resources + +A sketch of an implementation. Output each polygon as a fan of individual triangles. Make sure that the triangles for a polygon do not start with the same vertex index as the triangles of the preceding polygon. +``` +// convert mesh to triangles +int startIndex = -1; // invalid vertex index +for (int p = 0; p < numPolygons; p++) { + int start = startIndex == polygon[p].vertex[0] ? 1 : 0; + startIndex = polygon[p].vertex[start]; + int n = polygon[p].numVertices; + for (int v = start+2; v < start+n; v++) + EmitTriangle(startIndex, polygon[p].vertex[v-1], polygon[p].vertex[v == n ? 0 : v]); +} +``` + +Reconstruction reverses the process: +``` +// convert triangles back to original polygons +int startIndex = -1; +for (int t = 0; t < numTriangles; t++) { + if (triangle[t].vertex[0] != startIndex) { + if (startIndex >= 0) + EndPolygon(); + BeginPolygon(); + startIndex = triangle.vertex[0]; + EmitVertex(startIndex); + EmitVertex(triangle[t].vertex[1]; + } + EmitVertex(triangle[t].vertex[2]); +} +if (startIndex >= 0) + EndPolygon(); +``` diff --git a/extensions/2.0/Vendor/FB_ngon_encoding/schema/glTF.FB_ngon_encoding.schema.json b/extensions/2.0/Vendor/FB_ngon_encoding/schema/glTF.FB_ngon_encoding.schema.json new file mode 100644 index 0000000000..022352d8b8 --- /dev/null +++ b/extensions/2.0/Vendor/FB_ngon_encoding/schema/glTF.FB_ngon_encoding.schema.json @@ -0,0 +1,15 @@ +{ + "$schema": "http://json-schema.org/draft-04/schema", + "title": "FB_ngon_encoding glTF extension", + "type": "object", + "description": "glTF extension for reconstruction general polygonal meshes", + "allOf": [ + { + "$ref": "glTFProperty.schema.json" + } + ], + "properties": { + "extensions": {}, + "extras": {} + } +} \ No newline at end of file From 7a2df57a2f0f2720acc65f25253fcdd4981ab6ce Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?P=C3=A4r=20Winzell?= Date: Wed, 29 May 2019 17:38:38 -0700 Subject: [PATCH 2/3] Forgot to mention FBX2glTF. --- extensions/2.0/Vendor/FB_ngon_encoding/README.md | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/extensions/2.0/Vendor/FB_ngon_encoding/README.md b/extensions/2.0/Vendor/FB_ngon_encoding/README.md index a85346a7c3..4356898e26 100644 --- a/extensions/2.0/Vendor/FB_ngon_encoding/README.md +++ b/extensions/2.0/Vendor/FB_ngon_encoding/README.md @@ -71,7 +71,8 @@ On a pragmatic ecosystem note, we must also remember that glTF 2.0 normally allo ## Known Implementations -* Facebook AR/VR asset delivery +* [FBX2glTF](https://github.com/facebookincubator/FBX2glTF) (shortly) +* Internal Facebook AR/VR development ## Resources From 295f90ca937a09fc932ddf33f5cca4d3047d91c3 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?P=C3=A4r=20Winzell?= Date: Wed, 7 Aug 2019 08:58:02 -0700 Subject: [PATCH 3/3] Replace copy-and-pasted email address with my own. --- extensions/2.0/Vendor/FB_ngon_encoding/README.md | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/extensions/2.0/Vendor/FB_ngon_encoding/README.md b/extensions/2.0/Vendor/FB_ngon_encoding/README.md index 4356898e26..86360d343a 100644 --- a/extensions/2.0/Vendor/FB_ngon_encoding/README.md +++ b/extensions/2.0/Vendor/FB_ngon_encoding/README.md @@ -2,7 +2,7 @@ ## Contributors -* Pär Winzell, Facebook, [zell@fb.com](mailto:garyhsu@microsoft.com) +* Pär Winzell, Facebook, [zell@fb.com](mailto:zell@fb.com) * Michael Bunnell, Facebook ## Status