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EXT_mesh_gpu_instancing #1691

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May 28, 2020
Merged

EXT_mesh_gpu_instancing #1691

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76587d9
instancing extension
ultrafishotoy Oct 23, 2019
de031ed
cleanup
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overview statement
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tweak statements
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statement
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description
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simple example
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syntax fix
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2ed4903
teapots galore
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example image
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fix path
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332af44
update attribs to POSITION, ROTATION, and SCALE
ultrafishotoy Dec 2, 2019
133d28d
rename extension
ultrafishotoy Dec 2, 2019
3857fa7
updated POSITION attribute to TRANSLATION
ultrafishotoy Dec 31, 2019
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remove 'world space' language
ultrafishotoy Jan 5, 2020
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new sample files
ultrafishotoy Feb 19, 2020
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sample updated with rotation as a quaternion
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c70f30e
latest sample files
ultrafishotoy Mar 26, 2020
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oops
ultrafishotoy Mar 26, 2020
8d8f4a0
KHR_mesh_instancing -> EXT_mesh_gpu_instancing
Mar 28, 2020
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Merge pull request #1 from donmccurdy/ultrafishotoy-EXT_mesh_gpu_inst…
ultrafishotoy Mar 30, 2020
900ac06
moved to Vendor folder and renamed
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Merge branch 'KHR_instancing' of https://github.com/ultrafishotoy/glT…
ultrafishotoy Mar 31, 2020
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specify the types of the attributes
ultrafishotoy Apr 29, 2020
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clean up readme.md
ultrafishotoy May 27, 2020
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61 changes: 61 additions & 0 deletions ...ions/2.0/Khronos/KHR_instancing/extensions/2.0/Khronos/KHR_instancing/README.md
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# KHR\_instancing

## Khronos 3D Formats Working Group

* John Cooke
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* TODO

## Acknowledgments

* TODO

## Status

Experimental

## Dependencies

Written against the glTF 2.0 spec.

## Overview

This extension is specfically designed to enable GPU instancing which renders many copies of a single mesh at once using a small number of draw calls. The data consists of
a transform and an instance id. The transforms allow the mesh to be displayed at many different locations with different rotations and scales. The id can be used to alter
other parameters based on the run-time inplementation.

## Extending Nodes with per instance attributes

Instancing is defined by adding the `KHR_instancing` extension to any glTF mesh node. The attributes section contains accessor ids for new TRANSFORM4x3 and ID attribute buffers.
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For example, the following defines some instancing attributes to a mesh node in the graph. Instancing only applies to mesh nodes. Applying to nodes rather than meshes allows the
same mesh to be attached as normal to nodes while also being instanced.

```json
{
"nodes": [
{
"mesh": 0,
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One option we should consider is enabling the KHR_instancing extension to (optionally) override the node.mesh property. For example:

name: "teapot",
mesh: 0,
extensions: {
  KHR_instancing {
    mesh: 1,
    attributes: { ... }
  }
}

This simply provides exporters with some flexibility to control how/if fallback happens for viewers that don't recognize the extension:

  1. When the mesh override is omitted, clients that don't recognize the extension will render a single instance of the mesh with the node's transform.
  2. When the mesh override is provided, it should reference a single instance of the mesh. The fallback node.mesh value could then point to anything the exporter chooses – a merged mesh of all instances, a single point, or a textured plane saying "404 Not Found".
  3. When the KHR_instancing extension is marked as required, clients that don't recognize the extension are expected to fail fast without attempting to proceed.

"name": "teapot",
"extensions": {
"KHR_instancing": {
"attributes": {
"TRANSFORM4x3": 0,
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The spec should mention in which coordinate space those transforms are defined. Reasonable options are

  • local, like a regular mesh (full chain: instanced transforms, local node's TRS, all parents to the scene root);
  • scene, like skinned mesh (all local and parent node transforms are ignored).

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We're only using local space at otoy. Maybe that could be handled by alternate 'TRANSFORM' attributes that include the space? i.e. TRANSFORM_LOCAL, TRANSFORM_SCENE, with maybe TRANSFORM defaulting to LOCAL?

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TRANSFORM_SCENE behavior can be achieved with a node positioned at the scene root. Supporting only local (with explicitly described interaction between JSON-stored local TRS and accessor-stored instanced TRS) seems enough to cover both cases from engine perspective.

"ID": 1
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Are these IDs implementation-dependent, with no spec-defined usage? If so, they shouldn't be included here. glTF 2.0 has an established workflow for supporting app-specific attribute semantics - they just start with an underscore.

Otherwise, the spec should be more sound and include at least:

  • allowed datatypes (likely scalar uint8/16/32, uint32 is not available on WebGL 1.0);
  • uniqueness within a set of instances, within a scene, or within an asset (otherwise they should not be called IDs).

In shaders, GPU instances have built-in IDs. Should this attribute be somehow related to them?

},
}
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}
}
]
}
```

## Appendix

TODO

## Reference

TODO

### Theory, Documentation and Implementations

TODO
19 changes: 19 additions & 0 deletions ...HR_instancing/extensions/2.0/Khronos/KHR_instancing/schema/glTF.KHR_instaning.schema.json
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{
"$schema": "http://json-schema.org/draft-04/schema",
"title": "KHR_instancing glTF extension",
"type": "object",
"description": "glTF extension defines per mesh instance attributes for a node mesh.",
"allOf": [ { "$ref": "glTFProperty.schema.json" } ],
"properties": {
"attributes": {
"type": "object",
"description": "A dictionary object, where each key corresponds to mesh attribute semantic and each value is the index of the accessor containing attribute's data (current valid attribs are TRANSFORM4x3, ID).",
"minProperties": 1,
"additionalProperties": {
"$ref": "glTFid.schema.json"
}
},
"extensions": { },
"extras": { }
}
}