[X3D-Public] LOD versus GeoLOD

doug sanden highaspirations at hotmail.com
Sat Nov 22 06:29:01 PST 2014

Depending on the browser/implementation, I suspect the LOD + Inline would normally keep the level data loaded in memory once viewed, or until it gets an explicit load=false or change of url. GeoLOD would/should unload a level when its out of range (or zoomed to finer level), so large/endless worlds can be explored in quad chunks with fixed memory. 
Unlike games, real geographic information tends not to repeat, so is 'bulky' in imagery, terrain and cartographic detail - needs memory management for large areas.

> Date: Sat, 22 Nov 2014 16:27:14 +1000
> From: tom_a_sparks at yahoo.com.au
> To: x3d-public at web3d.org
> Subject: Re: [X3D-Public] LOD versus GeoLOD
> On 22/11/14 06:27, Andreas Plesch wrote:
>> Hello,
>> I want to clarify my understanding of the GeoLOD node but I am
>> struggling with how it improves on the regular LOD node. Here is what I
>> currently understand. GeoLOD defines a single level of refinement by
>> providing up to four children nodes (by URLs) which will be substituted
>> for the main root node if the viewer gets closer than a range distance.
>> A progressive tree of refinements can be achieved by making the children
>> nodes themselves GeoLOD nodes. On the contrary, a LOD node defines
>> multiple levels of refinement by providing a child node for each level
>> which is then selected based on viewing distance. The idea is that a
>> single child node covers completely the presented area. The child nodes
>> are not specified by URL but are internal to the node. So there seem to
>> be clear differences between GeoLOD and LOD nodes. However, since the
>> LOD node definition is more general, it seems to me that GeoLOD node
>> could be substituted functionally by a LOD node in this way. Given this
>> GeoLOD node:
>> <GeoLOD range="100"
>> child1Url="child1url"
>> child2Url="child2url"
>> child3Url="child3url"
>> child4Url="child4url">
>> <Group DEF="rootNode">
>> <GeoElevationGrid />
>> </Group>
>> </GeoLOD>
>> an equivalent LOD node may look like this:
>> <LOD range="100">
>> <Group DEF="detail-tiles">
>> <inline url="child1url"/>
>> <inline url="child2url"/>
>> <inline url="child3url"/>
>> <inline url="child4url"/>
>> </Group>
>> <Group DEF="root-tile">
>> <GeoElevationGrid />
>> </Group>
>> </LOD>
>> Here the LOD node simply just uses one range level, (instead of
>> multiple), a group to define up to four child nodes and inline nodes to
>> get the equivalent of the GeoLOD childUrl fields. The inlined nodes
>> could include other LOD nodes in order to get a quadtree this way.
>> Inline nodes would need to be able to load contained inline nodes which
>> is something I am not sure x3dom.js at least currently supports although
>> it may. Also the GeoLOD specification language is more specific about
>> loading and unloading tiles. There are probably other differences which
>> I am not seeing, so any comments would be helpful.
>> Thanks, Andreas
> Geo LODing is known as Quad LODing, it is mostly used in virtual
> terrains [1] https://www.youtube.com/watch?v=LxZhWrSmrOY
> the LOD node is a discrete LODing
> both of them suffer from popping[2]
> [1] http://vterrain.org/
> [2] https://en.wikipedia.org/wiki/Popping_%28computer_graphics%29
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