Setting attributes.position to a rounded number

Hey,

I’m encountering a very odd situation with trying to round up values of the bufferGeometry.attribues.position.array of a gltf model, the function i’m using is as follows (where (c) is a mesh fed to the function)…

const reducePrecision = (c) => {

let pPos = c.geometry.attributes.position

for(var p = 0; p < pPos.array.length; p++){

let pO = pPos.array[p]
let pX = Math.round(pO * 1000)
let pF = pX / 1000

console.log(pF);

pPos.array[p] = pF

console.log(pPos.array[p]);
}

pPos.needsUpdate = true
}

the console.log(pF); returns the expected value rounded to the 3rd decimal place, however setting pPos.array[p] = pF and running console.log(pPos.array[p]); returns the original decimal number to roughly 16 places, for perspective setting pPos.array[p] = 1 returns all positions as expected to 1… i’m wondering why this may be happening and if there’s something internal to THREE.BufferGeometry that prevents rounding decimal places and if anyones encountered this before?

any guidance on this would be greatly appreciated.

By far, not all numbers’ exact decimal value can be represented in IEEE-754 standard.

For example

const n = Math.round(50.67587*1000)/1000; console.log(n);
50.676

I guess JavaScript is showing you a string here, cut to a certain number of digits, that looks like a number.

The actual closest representation of 50.676 is:

and so:

const f32 = new Float32Array(1); f32[0] = 50.676; console.log(f32[0])
50.67599868774414
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thanks for the answer @tfoller that’s cleared up the confusion and makes complete sense, i found another source that also explains the reason quite well and simply.

The hope was that rounding the decimal places would somewhat reduce file sizes when re-exporting gltf models due to simply containing less information per position but this information on the functionality of float32Arrays with IEEE-754 standard of course stifles the idea.

GLTFExporter doesn’t natively suport draco compression as discussed in the github thread here, yet there is the option of using @gltf-Transform as demonstrated further up the thread, having used the cli previously this works great and has lots of options, the main issue is that the project i’m working on uses three js without a bundler and as it looks like the library source code is writen in ts makes it a little less accessible in this case, i’m wondering @donmccurdy is there a precompiled distribution of gltf-Transform we can use with standard three.js environments using import maps or will this require a bundler to access the toolset?

glTF Transform’s builds are mirrored to various CDNs (jsdelivr, unpkg, esm.sh, …) automatically like other npm packages. I do recommend using a bundler, and I intentionally have not written docs about getting it to work with import maps (this gets more complex as dependencies have dependencies have dependencies…), but it is certainly possible to do. Something like download-esm might help.

Aside – it is possible to do something like your original goal here by using int16 or int8 vertex attributes. This restricts the vertex positions to the [-1, 1] range, and so you’d need to scale each mesh by some amount to compensate. This is what glTF Transform’s quantize() function does. It isn’t as much compression as Draco or Meshopt, but doesn’t require any decompression either.

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Thanks for the response, cdn always skips my mind I am however keen on testing your suggestion of using int16 or int8 vertex attributes to begin with… When you say these restrict the vertex positions to the [-1, 1] range will it mean having to scale the model so that the bounding box is simply a maximal size of -1 to 1 unit beforehand and if so will this change be preserved if scaling back up to the original real world size of the model in question?

Two ways you could do this. Suppose the original model has dimensions 10x10x10 and is centered at the origin, and no existing scales specified on the scene graph.

• (A) Per-scene quantization grid: Scale all vertex data down to by 1/10, then apply a 10x scale to the entire model.
• (B) Per-mesh quantization grid: Scale vertex data for each mesh to individually fit in a [-1,1] box. Then scale that individual mesh up. Scale for each mesh may be different, and the model’s root node doesn’t necessarily need to be scaled.

glTF Transform implements both, and defaults to (B). If you observe seams in the quantized mesh, it may help to switch to (A). Certain cases are more complex than I’ve described, notably THREE.SkinnedMesh, THREE.InstancedMesh, and volumetric properties of THREE.MeshPhysicalMaterial. The implementation below operates on glTF data and not three.js objects directly, but might be helpful as a reference:

While WebGL supports only 8-, 16-, and 32-bit attributes, it’s also possible to quantize to arbitary bit depth. For example, quantize to 12 bits and then store the result in 16-bit attributes. VRAM usage is still 16 bits, but if you’re hosting the GLB files with Gzip or Brotli compression, you’ll get some download size benefit from the unused 4 bits in each integer component.

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