Hide Regions of 3D Tiles or Terrain with Clipping Polygons

[SoftwareMechanic]

Description

Hello!

I was looking at this interesting feature in CesiumJS, and I think we need it also in this library 😄

basically we need a way to cut some part of the tiles out, in order to highlight our assets, and avoid artefacts from the tiles, when a model is aligned. especially with photorealistic ones.

I think somehow this is doable, but I don't know where to start for it's implementation.

Solution

do you think we should use a CSG library, or is there another 'native' way?

Alternatives

I'd like to know the possibilities we have in order to achieve this effect.
as a newbie, I'd say the solution is CGS where the base geometry is the union of tiles where our "clipping polygon" lives.
but I am not sure Cesium followed that way, maybe the solution is more simple.
I am looking inside their codebase but still have not understood exactly how it is managed on a geometry level.

Additional context

in the link, you can also see they start from the geojson, which now this library supports, hopefully that can help us.

the following picture, show you basically the reason for this enhancement, I have some models aligned to their geo position, but in some cases, you can see tiles artefacts which would be nice if deleted:

eventually I would like to help for this

[gkjohnson]

I was looking at this interesting feature in CesiumJS, and I think we need it also in this library 😄

A few people have mentioned a feature like this in the past. It's technically possible to implement your own version currently with a plugin but I think it's a useful enough feature to discuss adding a common plugin for. We'll start with a few ways to achieve this along with some pros and cons:

TileFlatteningPlugin

Some users have used the TileFlatteningPluging plugin to shift existing geometry "out of the way".

Pros

• Already implemented
• No holes (geometry is flattened to a surface)
• Raycasting will only hit the flattened surface

Cons

• Noise from existing geometry
• Resolution is limited by the underlying geometry
• Raycasting may not work if geometry is flattened outside the tile bounding box

Shader-based Polygon Clip

This is the approach that cesium uses and works by uploading the polygon to a shader as a series of lines packed into a data texture. Then you can determine whether each fragment is inside or outside the polygon by using the "ray crossing algorithm" and clip.

Pros

• Pixel perfect polygon clipping.

Cons

• Performance would not scale well with large polygons
• Likely difficult to support AA
• Clipping at the lat / lon jump at the IDL may need special handling
• Raycasting will still hit the original geometry surface

Texture-based Alpha Test

Rather than uploading the polygon to the shader it can be drawn to an alpha map and fragments can be discarded based on the alpha map. Similar to the geojson renderer approach.

Pros

• Can probably support AA.
• Fairly simple given existing plugins & overlays

Cons

• Possibly imperfect outline from underlying texture resolution
• Higher memory usage
• Raycasting will still hit the original geometry surface

CSG Approach

CSG could work but it gets a bit complex. One interesting thing about CSG is that you can choose to clip only a portion of the model (eg remove just the bridge and not the ground underneath). But it can generally be performance intensive. And because 3d tiles geometry is typically not water tight you'll still get "holes" in the geometry rather than a solid form the way you typically would with CSG. I'm not sure about other libraries but three-bvh-csg I wrote supports this kind of "hollow" clipping.

Pros

• AA supported
• Raycasting supported out of the box

Cons

• Performance intensive
• Complex to deal with adding and removing CSG shapes (eg resetting geometry)
• Requires triangulating and extending shapes to clip into 3d volumes

I'm open to other opinions but I'm thinking that the texture-based alpha test approach is probably the most straight forward. Though it remains to be seen what kinds of artifacts you may run into due to resolution difference between tiles.

[SoftwareMechanic]

Thank you a lot for the clarification about the options we have,

I am trying with the alpha map as you suggested, and the results seems nice to me. 😄
I just modified a bit the wrapOverlaysMaterial file, and the ImageOverlayPlugin.
and extended the GeoJsonOverlay.

I am not sure it's exactly the way you'd do it, I'll create a Pull Request.

Raycasting will still hit the original geometry surface

in the case of existing assets in the photorealistic tiles, it's not that bad.
it can be worse if we are representing an asset that will be built, but I think we can bear with that.

One interesting thing about CSG is that you can choose to clip only a portion of the model (eg remove just the bridge and not the ground underneath)

It could be nice to try CSG using with the Oriented Bounding Box of the models as cutting geometry, it could be the most precise way for managing this issue.

I think it would be nice to have also a clipInside: boolean variable to let the user decide to render what is inside of the geojson geometry, or outside.

here the current result:

[gkjohnson]

I am not sure it's exactly the way you'd do it, I'll create a Pull Request.

A PR would be great. We can talk about what the user-facing API looks like from there. One note is that if you use "alphaToCoverage" and opacity rather that alphaTest or discard we should be able to get antialiasing on the clipped outline.

Raycasting will still hit the original geometry surface

in the case of existing assets in the photorealistic tiles, it's not that bad.

I think users can filter results on their own based on the clipped region, as well, once getting the raycast results.