Cesium3Dtile - TransformationTraits - height issue ?

[SmartAnalog]

Hi,

I am trying to adjust the location of cesium 3Dtiles using the TransformationTraits option.
It seems that the height parameter is not working correctly, or, also, I am not using it correctly.
Below is an extract of my catalog:
{
"name": "TestOffset",
"type":"3d-tiles",
"description":"To be done",
"url":"pathToData",
"showShadowUi": false,
"hideSource":true,
"options": {
"maximumScreenSpaceError": 1,
"maximumNumberOfLoadedTiles": 2000
},
"origin": {
"latitude": 0,
"longitude": 0,
"height": 10
}
The 3D model seems to be well placed at the (0,0) point but it is floating into outer space. As written in the documentation, the height should be in meter and representing the height above the earth ellipsoid.

Are these parameters working with 3DTiles ?

[micholm]

Hi @SmartAnalog, I can confirm these parameters do work with3d-tiles, and your json looks to be valid. If your model is very far from the ellipsoid, can you confirm if the origin of the model is somewhere close to the model itself? i.e. (0,0,0) should be the centroid of the model ideally if you're using local coordinates and the origin parameters to adjust the position. What can happen is if the origin (0,0,0) is far away, it's impossible to visually place the model because of the large offset.

Is there any way you can georeference the model before converting into tiles?

[SmartAnalog]

Hi Micholm,
Thanks for the answer and the help provided. I made more tries yesterday and it seems that none of the parameters (lat,long,height) seem to work with my data.
My 3D model is created by Itwin Modeler and is already georeferenced. I wanted to use the "origin" parameter to adjust the position of my 3D model so it can fit well with the existing topography. And no, the position (0,0,10) is very far away from the true origin of the model. It was just to test these parameters with a large offset, jsut to be sure that these parameters work with my data.
I will try to input coordinates closest to my 3D Model origin.

[SmartAnalog]

I extract the origin of my model and convert the coordinates into latitude/longitude coordinates (EPSG:4326).
But I have the same issue. The model is porjected into space.
So my question is: does these parameters work with 3Dtiles already georeferenced ?

[micholm]

Hi @SmartAnalog, Yes the parameters do work with georeferenced 3d-tiles, they effectively override it. It's hard to say what the issue is without seeing the data, however, it seems like the origin coordinate of the 3d-tiles might not be correct, for example, it might be using the world centroid which means (0,0,0) is the centre of the earth ellipsoid.

Without seeing the data, what i'd recommend is:

• in ITwin, try to determine how far your model is from the global origin (i found a brief discussion here but might be worth digging into);
• depending on what kind of model it is, try creating a free Cesium Ion account and generate the tiles there (if possible with your data source). You'll be able to download the tiles once theyre ready and add them to Terria in the same way. This might be easier to debug what is going on.

Do the tiles generated by ITwin sit correctly on the map without using the override parameters?

[SmartAnalog]

Hi Micholm,

Thanks again for the help provided.
I will try to answer to your questions:

• Yes the tiles generated are well placed in Terria without the "origin" parameters.

• I didn't really understand the origin issue of my files. Itwin modeler produced 3D models from pictures (drones and/or by feet). It can export its results in many formats and many cartographic projection. But to produce Cesium 3Dtiles, the only choice is ECEF coordinates system. The results is a folder (and subfolfer) containing all the tiles (in *.b3dm format) and .json file with the coordinates (cartesian - EPSG:4978) of the tile's bounding box. I did not see where to input the origin coordinate of the 3DTiles in the process to produce Cesium 3DTiles. It is possible for other format as Obj or Osgb.

• I already have a Cesium Ion account (free). But I did not find an option to generate tiles from georeferenced pictures. I uploaded my 3Dtiles into anyway : CesiumIon. I don't know if CesiumIon makes some modification on my files ?

• If you want to have a look on the 3D Tiles, I would be pleased to share it.

[micholm]

Ah I see. It's a photogrammetric model.

Ok so your model uses ECEF coordinates, unfortunately the origin: {} parameters won't be useful for you while the model is in this system, for the reasons mentioned above. It's only really useful if your model is using a local coordinate system.

What you can try is to export as an OBJ and then use Cesium Ion to generate the 3d-tiles. You should then be able to manually move the 3d tiles in cesium ion and you can then either download the 3d tiles, or use the ionAssetId and ionAccessToken traits for your 3d-tiles terria catalog item.

for more info on ECEF for 3d tiles, check out 6.6 here: https://docs.ogc.org/cs/22-025r4/22-025r4.html#:~:text=z%2E-,3D,context

What problem were you trying to solve by lifting the model? It might be there's another way to do it.

[SmartAnalog]

Thanks again for these explanation.
I will try your workflow but for some of my 3D models, I think it will be an issue to export as OBJ. There are already pretty heavy in 3Dtiles format (10/15 Go), so I assume that in OBJ the the file size will explose. And I just have a free account on CesiumIon and i am limited to 5 Go.

The main objective of these manipulations was to offset the 3D Model, especially in height. Some of my 3D model are below the integrated topography of TerriaMap. I know it is possible to paly with the option "Terrain hide underground feature". Unfortunatly, even if this option, it is not possible to zoom very close to the 3D Model as the viewer still "hitting" the topography.

Maybe it will be a nice feature to add into Terria ? As this example: https://sandcastle.cesium.com/?id=3d-tiles-adjust-height