Billboards on terrain - some problems

[duvifn]

Current implementation of billboard on terrain is well written and I have learned a lot from reading it.
However I found some problems while working on #4622:

1. I think the billboard height should be computed also if the level of the scene getting lower (zoom-out). Currently the height is computed only in one direction. ( if (tile.level > data.level) in this link).
   However, sometimes lower levels have higher terrain at a given point so the billboard disappears.

2. In the following code the isChildAvailable is invoked on the grandparent of the child, instead of on tile

var parentTile = tile.parent;
if ((defined(tileDataAvailable) && !tileDataAvailable) ||
    (defined(parentTile) && defined(parentTile.data) && defined(parentTile.data.terrainData) &&
    !parentTile.data.terrainData.isChildAvailable(parentTile.x, parentTile.y, child.x, child.y))) {
              data.removeFunc();
     }

(link).

3. If the tile is upsampled the data.terrainData.isChildAvailable in the lines above would always return false (see this line). So, in such case the billboard wouldn't be updated after some zooming.

I found that this is the reason to this behavior (the billboard should be clamped to the ground):

4. I'm not sure at this point but I think the billboard height must be recomputed if the original data of the tile was a result of an upsampling process, and the tile was later loaded.

5. Please consider depth offset in the BillboardCollectionVS.glsl. In previous versions there were the following lines:

#ifdef CLAMPED_TO_GROUND
    // move slightly closer to camera to avoid depth issues.
    positionEC.z *= 0.995;
    

It would be great if, in addition to eyeOffset paramter in billboard and label, there will be a depthOffset which will be expressed as a fraction of the camera distance to the vertex (0-1).

6. Currently if the terrain provider is replaced by another one, clamped billboards would disappear.
   Maybe that because the QuadtreePrimitive.prototype.invalidateAllTiles treat only levelZeroTiles's customData but I didn't enter into it

[pjcozzi]

Thanks for all the input, @duvifn!

@bagnell can you please review these?

[mramato]

I think we might have addressed all of these issues at one time or another and with @hpinkos' work in this are might make this entire issue OBE.

@hpinkos any thoughts here?

[hpinkos]

@mramato 2, 3, and 4 are all things we still potentially need to address. 2 in particular seems wrong to me but I'm not sure what the right thing to do is. @bagnell can you take a look at this?

1 is probably something we're not going to address. We made a decision to clamp to the highest resolution LOD and not reclamp if lower resolution tiles are loaded in when you zoom out from an area.

5 is OBE

6 was fixed at some point

[mramato]

Thanks, I tagged this as high priority since we'll want to address this as part of our terrain by default work.