[3D Tiles] What is tileIndex ?

[jailln]

Context

I'm currently working on improving 3D Tiles extension management introduced in #1022 to give the possibility to have extensions defined in multiple 3D Tiles objects (i.e. not only in batch table as it is currently the case).

To this end, I need to start refactoring the 3D Tiles Provider (which I think is planned later this year in your roadmap). I'll come up with propositions in the next days maybe as an 'in progress' PR. I still have questions about the provider and the structure resulting from the parsing of the tileset and its tiles though.

Questions

• What is tileIndex ? From what I understand, it seems to be a list of tiles (i.e. a flattened tileset). However, the tiles still have children attributes (and hence the hierarchy between tiles is still present but not stored as originally in the tileset tree)
• What is the purpose of the extendTileset method ?

[gchoqueux]

Thank you for this refactoring proposal.

What is tileIndex ? From what I understand, it seems to be a list of tiles (i.e. a flattened tileset). However, the tiles still have children attributes (and hence the hierarchy between tiles is still present but not stored as originally in the tileset tree)

I don't know the code of 3DTiles yet. I still don't clearly understand tileIndex. It seems to be the structure of tiles that allows the process to perform culling, replacement and fecthing of tiles.

There is also the meshs structure of the tiles in the LayerGeometry.object3d. It must be possible to unify these two structures.

What is the purpose of the extendTileset method ?

This method is used when loading a new tile. It just adds this new tile to the tileIndex.

Otherwise, an important thing in the refactoring will be to do 3DTilesLayer and 3DTilesSource and remove 3DTileProvider and 3DTileProcessing.

[jailln]

Thank you for the clarifications! With these information and by having a deeper look in the code, it seems that:

• tileIndex is an Object containing a flattened tileset (in the 3D Tiles spec, a tileset is a set of nested tiles organized according to a tree; each tile referencing a content being e.g a b3dm file).

Otherwise, an important thing in the refactoring will be to do 3DTilesLayer and 3DTilesSource and remove 3DTileProvider and 3DTileProcessing.

It would indeed be great to have a 3DTilesLayer and 3DTilesSource. I don’t think I will go this far but I will go in this direction. I also think the implementation should match as much as possible the spec and its classes (i.e. it might be interesting to have Tileset, Tile, BoundingVolume, etc. classes in iTowns) so it is easier for developers and users to understand the data structure in iTowns and also it facilitates maintainability. WDYT ?

• iTowns internal structure for representing the tiles contents (based on THREEjs): When a b3dm tile content is loaded (by the b3dmParser and gltfParser), it is converted to a THREE.Object3D and added as a child to the object3d attribute of the layer. The structure of each b3dm tile content is as follow: it has one children (of type THREE.Scene), which also has one children (of type THREE.Mesh) which has a property geometry (of type THREE.BufferGeometry) which has a property attributes which has 3 properties of type THREE.BufferAttribute : _BATCHID, normal, position. cf. this structure in the 3dtiles_hierarchy example:

My usecase is to switch objects visibility based on their batchID and on user inputs. To achieve that, it would be great to have one THREE.Mesh object per batch (i.e. all the geometries corresponding to batchId = 0 in one mesh, all the ones with batchId = 1 in another mesh, etc.). I think this would also be interesting for styling features based on their batch id and without having to define new shaders. WDYT ?

I’m currently trying to implement this mesh subdivision in the B3dmParser but it’s not as easy as expected… Does anyone has inputs on how to achieve this ?

More generally, how do you modify styling properties of geographic features based on their batch id ? (ping @mbredif )

[gchoqueux]

Thank you for your analysis of the 3Dtiles code.

iTowns internal structure and tileSet

In summary, in itowns, the data structure of 3d Tiles consists of the tileSet and the object3Ds tree. It should have only the one structure of object3Ds.

3DTilesLayer and 3DTilesSource.

On our side, we could, in parallel of your development, develop the 3DTilesLayer and 3DTilesSource.

One mesh by batchID

My usecase is to switch objects visibility based on their batchID and on user inputs. To achieve that, it would be great to have one THREE.Mesh object per batch .

Warning, in the past, we generated one mesh for each buildings (with the Wfs source and feature2Mesh). But the problem, the performance decreases strongly because the count of webGL draw calls increases. Moreover, there's not interest to have a batchId buffer, in Three.Geometry, if it isn't used in the shader. It must be possible to use THREE's chunks shaders to build a style-supporting material based on the batchId.

[jailln]

iTowns internal structure and tileSet

Yes, we could merge these two structures (just a precision: the data structure consists of tileIndex and the object3Ds)

3DTilesLayer and 3DTilesSource

Great :) Could you elaborate what are the roles of 3DTilesLayer and of 3DTilesSource please ? (e.g. in comparison with Provider and Processing)

My current work is here. It is still a bit messy (especially in the commits) but you can see the changes here. The main ideas are:

• I added a custom 3d tiles extension (named temporal) which eventually won't be pushed to iTowns and will live in an other client based on iTowns (UDV). The code related to this extension is in the files examples/3dtiles_temporal.html and src/Provider/TemporalExtension.js
  • I created a file src/Provider/3dTilesClasses.js in which I added a Tileset class (holding the tileIndex) a BoundingVolume class and a BatchTable class. This file also holds the code needed for extensions management.
  • I changed a bit the mechanism of extensions:
    • No more global object holding the registered extensions. They are now registered using Layer.defineLayerProperty (see here)
    • Instead of registering a parser of the extension, now you register and object having a parser and other methods. I modified a bit the parsing logic to allow one extension being defined in multiple classes (e.g. temporal extension is defined in Tileset, BatchTable and BoundingVolume).
    • The benefit of registering an object instead of a parser is that now extensions can have an impact on some iTowns 3D Tiles processing-related function (e.g. $3dTilesCulling, fillHTMLWithPickingInfo). Example: BatchTableHierarchy and Temporal extensions are defined in the layer. $3dTilesCulling does its work and then calls the culling method of both these extensions. BatchTableHierarchyExtension.culling() does nothing and TemporalExtension.culling hides some of the tiles based on the attributes stored in the temporalExtension of the boundingVolume.

One mesh by batchID

Good point. You mean by using Layer.OverrideMaterial with a custom material and custom shaders based on shaders and materials of iTowns ?

[gchoqueux]

Great :) Could you elaborate what are the roles of 3DTilesLayer and of 3DTilesSource please ? (e.g. in comparison with Provider and Processing)

• 3DTilesLayer have data structure and processing.
• 3DTilesSource defines the parameters (base url, network option) and conditions (extent) to obtain the data.

Tileset class (holding the tileIndex) a BoundingVolume class and a BatchTable class

Developing these classes is a good idea. Could you give url to link this files?

Tileset class is a leaf of tree structure?

It will be necessary to be careful because there are already objects similar to BoundingVolume: like OBB and THREE.Box3.

I changed a bit the mechanism of extensions:

it should be discussed on https://gitter.im/iTowns

OverrideMaterial with a custom material and custom shaders based on shaders and materials of iTowns ?

Yes, but there would be a little development

[jailln]

Thanks for the clarification. The 3D Tiles classes I mentionned are here. For now they are in a 3dTilesClasses.js file but based on your response they should be in 3DTilesLayer.

Tileset class is an implementation of the tileset from the 3D Tiles spec, i.e. the tree structure. Same for BoundingVolume and BatchTable.

Regarding the extensions management I will open a discussion with an implementation example (maybe as a PR/issue instead of gitter so there is a trace of the design choices?) once I'm sure my proposition works.

Regarding the custom materials/shaders, is there an existing implementation of this (e.g. in another project) at your knowledge ?

[mgermerie]

The complete 3d-tiles support is planned in the short term road map. We are currently studying possibilities in terms of libraries to enhance support (as mentioned in https://github.com/iTowns/itowns/discussions/1917).