Implementing a module for LOD

[sasha240100]

As you remember from terrain updating issue #9 we need to improve terrain generation. And there is one post similar to http://threejs.org/examples/#webgl_lod how we can improve fps while rendering large objects.

[thejmazz]

Would be nice if LOD optionally could turn loops for that component on/off. So in addition to geometry detail, don't need to waste time on more expensive loops.

[hirako2000]

I propose a smarter API than three.js way, e.g of usage:

const icosahedron = new Icosahedron({
     geometry: {
        radius: 100,
        detail: 5
    },
    module: [new LODModule(['detail']) // array should defaults to all instructions.
        .generate({
        levels: 5, // optional , default to some value, say 3.
        factor: 1/5, // optional (factor means for each step, substracted from the highest level, maybe a function instead?)
        floorLevels: true, // optional, to determine values should remain rounded, or get to decimals,
        range: [0, 100] //optional, perhaps default should be relative to camera near/far.
    })]
});

Such module would iterate through the first param, the array (here just detail). For each instruction it would iterate 5 minus 1 times (to end up with 5 levels of details), substract the highest level of detail by the factor on each iteration, flooring to rounded values for each level of detail.

[sasha240100]

@hirako2000 such LOD module is only for primitive geometries and should interact with their API

[thejmazz]

a common mesh simplification algorithm is the one described by this paper: Surface Simplification Using Quadric Error Metrics. there's various implementations of it on the internet. in C. in Go (I actually have a WIP port of this to JS on my laptop, never pushed). theres definitely a C++ implementation in some game engine out there. this is out of the scope of whs, but the API should probably be something similar to what @hirako2000 suggests, and let you return your own set of meshes.

see also this three thread.

I think this also might be one of the cases where wasm could come in handy, and the tradeoff of moving items from js memory to wasm memory (at least before shared array buffers) would be worth it since the task is so large (can easily be on the order of seconds).

[hirako2000]

So for non primitive you suggest using the already implemented SimplifyModifier in three?

[thejmazz]

For now yes, but to future proof for alternative mesh simplification algorithms, should accept a common format (array/object of meshes or something like that), basically have a way to pass in custom meshes for each LOD level (it should be possible to make a simple LOD switch from cube to sphere for example - not practical but a good example)