meshoptimizer.js
This folder contains JavaScript/WebAssembly modules that can be used to access parts of functionality of meshoptimizer library. While normally these would be used internally by glTF loaders, processors and other Web optimization tools, they can also be used directly if needed.
Structure
Each component comes in two variants:

meshopt_component.js
uses a UMDstyle module declaration and can be used by a wide variety of JavaScript module loaders, including node.js require(), AMD, Common.JS, and can also be loaded into the web page directly via a<script>
tag which exposes the module as a global variable 
meshopt_component.module.js
uses ES6 module exports and can be imported from another ES6 module
In either case the export name is MeshoptComponent and is an object that has two fields:

supported
is a boolean that can be checked to see if the component is supported by the current execution environment; it will generally befalse
when WebAssembly is not supported or enabled. To use these components on browsers without WebAssembly a polyfill library is recommended. 
ready
is a Promise that is resolved when WebAssembly compilation and initialization finishes; any functions are unsafe to call before that happens.
In addition to that, each component exposes a set of specific functions documented below.
Decoder
MeshoptDecoder
(meshopt_decoder.js
) implements high performance decompression of attribute and index buffers encoded using meshopt compression. This can be used to decompress glTF buffers encoded with EXT_meshopt_compression
extension or for custom geometry compression pipelines. The module contains two implementations, scalar and SIMD, with the best performing implementation selected automatically. When SIMD is available, the decoders run at 13 GB/s on modern desktop computers.
To decode a buffer, one of the decoding functions should be called:
decodeVertexBuffer: (target: Uint8Array, count: number, size: number, source: Uint8Array, filter?: string) => void;
decodeIndexBuffer: (target: Uint8Array, count: number, size: number, source: Uint8Array) => void;
decodeIndexSequence: (target: Uint8Array, count: number, size: number, source: Uint8Array) => void;
The source
should contain the data encoded using meshopt codecs; count
represents the number of elements (attributes or indices); size
represents the size of each element and should be divisible by 4 for decodeVertexBuffer
and equal to 2 or 4 for the index decoders. target
must be count * size
bytes.
Given a valid encoded buffer and the correct input parameters, these functions always succeed; they fail if the input data is malformed.
When decoding attribute (vertex) data, additionally one of the decoding filters can be applied to further postprocess the decoded data. filter
must be equal to "OCTAHEDRAL"
, "QUATERNION"
or "EXPONENTIAL"
to activate this extra step. The description of filters can be found in the specification for EXT_meshopt_compression.
To simplify the decoding further, a wrapper function is provided that automatically calls the correct version of the decoding based on mode
 which should be "ATTRIBUTES"
, "TRIANGLES"
or "INDICES"
. The difference in terminology is due to the fact that the JavaScript API uses the terms established in the glTF extension, whereas the function names match that of the meshoptimizer C++ API.
decodeGltfBuffer: (target: Uint8Array, count: number, size: number, source: Uint8Array, mode: string, filter?: string) => void;
Encoder
MeshoptEncoder
(meshopt_encoder.js
) implements data preprocessing and compression of attribute and index buffers. It can be used to compress data that can be decompressed using the decoder module  note that the encoding process is more complicated and nuanced. It is typically split into three steps:
 Preprocess the mesh to improve index and vertex locality which increases compression ratio
 Quantize the data, either manually using integer or normalized integer format as a target, or using filter encoders
 Encode the data
Step 1 is optional but highly recommended for triangle meshes; it can be omitted when compressing data with a predefined order such as animation keyframes. Step 2 is the only lossy step in this process; without step 2, encoding will retain all semantics of the input exactly which can result in compressed data that is too large.
To reverse the process, decoder is used to reverse step 3 and (optionally) 2; the resulting data can typically be fed directly to the GPU. Note that the output of step 3 can also be further compressed in transport using a generalpurpose compression algorithm such as Deflate.
To preprocess the mesh, the following function should be called with the input index buffer:
reorderMesh: (indices: Uint32Array, triangles: boolean, optsize: boolean) => [Uint32Array, number];
The function optimizes the input array for locality of reference (make sure to pass triangles=true
for triangle lists, and false
otherwise). optsize
can choose whether the order should be optimal for transmission size (recommended for Web) or for GPU rendering performance. The function changes the indices
array in place and returns an additional remap array and the total number of unique vertices.
After this function returns, to maintain correct rendering the application should reorder all vertex streams  including morph targets if applicable  according to the remap array. For each original index, remap array contains the new location for that index, so the remapping pseudocode looks like this:
let newvertices = new VertexArray(unique); // unique is returned by reorderMesh
for (let i = 0; i < oldvertices.length; ++i)
newvertices[remap[i]] = oldvertices[i];
To quantize the attribute data (whether it represents a mesh component or something else like a rotation quaternion for a bone), typically some dataspecific analysis should be performed to determine the optimal quantization strategy. For linear data such as positions or texture coordinates remapping the input range to 0..1 and quantizing the resulting integer using fixedpoint encoding with a given number of bits stored in a 16bit or 8bit integer is recommended; however, this is not always best for compression ratio for data with complex crosscomponent dependencies.
To that end, three filter encoders are provided: octahedral (optimal for normal or tangent data), quaternion (optimal for unitlength quaternions) and exponential (optimal for compressing floatingpoint vectors). The last two are recommended for use for animation data, and exponential filter can additionally be used to quantize any floatingpoint vertex attribute for which integer quantization is not sufficiently precise.
encodeFilterOct: (source: Float32Array, count: number, stride: number, bits: number) => Uint8Array;
encodeFilterQuat: (source: Float32Array, count: number, stride: number, bits: number) => Uint8Array;
encodeFilterExp: (source: Float32Array, count: number, stride: number, bits: number) => Uint8Array;
All these functions take a source floating point buffer as an input, and perform a complex transformation that, when reversed by a decoder, results in an optimally quantized decompressed output. Because of this these functions assume specific configuration of input and output data:

encodeFilterOct
takes each 4 floats from the source array (for a total ofcount
4vectors), treats them as a unit vector (XYZ) and fourth component from 1..1 (W), and encodes them intostride
bytes in a way that, when decoded, the result is stored as a normalized signed 4vector.stride
must be 4 (in which case the roundtrip result is 4 8bit normalized values) or 8 (in which case the roundtrip result is 4 16bit normalized values). This encoding is recommended for normals (with stride=4 for medium quality and 8 for high quality output) and tangents (with stride=4 providing enough quality in all cases; note that 4th component is preserved in case it stores coordinate spaced winding).bits
represents the desired precision of each component and must be in[1..8]
range ifstride=4
and[1..16]
range ifstride=8
. 
encodeFilterQuat
takes each 4 floats from the source array (for a total ofcount
4vectrors), treats them as a unit quaternion, and encodes them intostride
bytes in a way that, when decoded, the result is stored as a normalized signed 4vector representing the same rotation as the source quaternion.stride
must be 8 (the roundtrip result is 4 16bit normalized values).bits
represents the desired precision of each component and must be in[4..16]
range, although using less than 910 bits is likely going to lead to significant deviation in rotations. 
encodeFilterExp
takes each K floats from the source array (whereK=stride/4
, for a total ofcount
Kvectors), and encodes them intostride
bytes in a way that, when decoded, the result is stored as K singleprecision floating point values. This may seem redundant but it allows to trade some precision for a higher compression ratio due to reduced precision of stored components, controlled bybits
which must be in[1..24]
range, and a shared exponent encoding used by the function.
Note that in all cases using the highest bits
value allowed by the output stride
won't change the size of the output array (which is always going to be count * stride
bytes), but it will reduce compression efficiency, as such the lowest acceptable bits
value is recommended to use. When multiple parts of the data require different levels of precision, encode filters can be called multiple times and the output of the same filter called with the same stride
can be concatenated even if bits
are different.
After data is quantized using filter encoding or manual quantization, the result should be compressed using one of the following functions that mirror the interface of the decoding functions described above:
encodeVertexBuffer: (source: Uint8Array, count: number, size: number) => Uint8Array;
encodeIndexBuffer: (source: Uint8Array, count: number, size: number) => Uint8Array;
encodeIndexSequence: (source: Uint8Array, count: number, size: number) => Uint8Array;
encodeGltfBuffer: (source: Uint8Array, count: number, size: number, mode: string) => Uint8Array;
size
is the size of each component in bytes; it must be divisible by 4 for attribute/vertex encoding and must be equal to 2 or 4 for index encoding; additionally, index buffer encoding assumes triangle lists as an input and as such count must be divisible by 3.
Note that the source is specified as byte arrays; for example, to quantize a position stream encoded using 16bit integers with 5 vertices, source
must have length of 5 * 8 = 40
bytes (8 bytes for each position  3*2 bytes of data and 2 bytes of padding to conform to alignment requirements), count
must be 5 and size
must be 8. When padding data to the alignment boundary make sure to use 0 as padding bytes for optimal compression.
When interleaved vertex data is compressed, encodeVertexBuffer
can be called with the full size of a single interleaved vertex; however, when compressing deinterleaved data, note that encodeVertexBuffer
should be called on each component individually if the strides of different streams are different.
License
This library is available to anybody free of charge, under the terms of MIT License (see LICENSE.md).