How to compute elevation image for a given bounding box ?

[valentinMachado]

Hello,

I am trying to convert a bounding box into an elevation image, like I would like to give as parameter a bounding box and in return a heightmap image representing the elevation of this area see also => VCityTeam/UD-Viz#852.

I've look here => https://github.com/iTowns/itowns/blob/master/packages/Main/src/Utils/DEMUtils.js#L319 but this is only working for a given point (I guess I could make a double for loop to reconstruct the elevation but I would like to know the resolution of the image in order to optimize this computation), any help are welcome.

Thanks !

[jailln]

Hi @valentinMachado,
I think you can get the tiles intersected by your bounding box, retrieve their associated elevation images and reconstruct an image from these. To get the tiles intersected by your bounding box, one approach is to get the tiles intersected by the four points of your bbox and get all tiles in between. You can use picking and/or methods from View, TiledGeometryLayer and DEMUtils I think. I think you can speed up the computation by computing the current displayed zoom level based on your projection system (some methods in View may help to do so).

I don't know if it'll be problematic for your use case but one issue you may find along the way is that you will only work with already loaded tiles (and not with the most resolved ones provided by your server). So the zoom level at which you draw your bounding box will have an impact on your result. We miss an API to load the most resolved tiles under a geometry (point, line, bbox, depending on the usecase). I encountered this issue when implementing an elevation measure tool under a point.

[valentinMachado]

Hi @jailln thanks for your response, I tried as you said by trying with a DEMUtils way and it could have work but while doing it I realised that I was going to do several drawImage with offset and scale in a canvas 2D context to recompose the heightmap then I was going to transform it in the resolution Iam working with. In my opinion it's like redoing all the system (which is a bit complicated) so instead I tried to modify the LayeredMaterial shader applied on the ground and got the result I wanted (the computation cost is a render pass and should be optimized enough). But in order to do that I need LayeredMaterial to be exposed in the API, here is a PR #2517 (EDIT: actually do a shallow copy is more easy here I have deleted this PR).

I don't know if it'll be problematic for your use case but one issue you may find along the way is that you will only work with already loaded tiles (and not with the most resolved ones provided by your server). So the zoom level at which you draw your bounding box will have an impact on your result.

Right but for now it's okay that the heightmap is based on what the user see.

We miss an API to load the most resolved tiles under a geometry (point, line, bbox, depending on the usecase). I encountered this issue when implementing an elevation measure tool under a point.

The API exists for 3DTiles (I don't know for other TileLayer) I've already done it here
https://github.com/VCityTeam/UD-Imuv/blob/88bc683359be750c9a67f044e055c6a506356783/src/browser/externalScript/CityMockUp.js#L55

with layer.update = itowns.process3dTilesNode you can override function used to request and refine 3DTiles.

[jailln]

Nice, well done!