meshoptimizer
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0.20.0 • Public • Published

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. The modules are available as an NPM package but can also be redistributed individually on a file-by-file basis.

Structure

Each component comes in two variants:

  • meshopt_component.js uses a UMD-style 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 be false 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 1-3 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 post-process 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;

Note that all functions above run synchronously; sometimes decoding large buffers takes time, so this library provides support for asynchronous decoding using WebWorkers via the following API; useWorkers must be called once at startup to create the desired number of workers:

useWorkers: (count: number) => void;
decodeGltfBufferAsync: (count: number, size: number, source: Uint8Array, mode: string, filter?: string) => Promise<Uint8Array>;

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:

  1. Pre-process the mesh to improve index and vertex locality which increases compression ratio
  2. Quantize the data, either manually using integer or normalized integer format as a target, or using filter encoders
  3. 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 general-purpose compression algorithm such as Deflate.

To pre-process 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];

When the input is a point cloud and not a triangle mesh, it is recommended to reorder the points using a specialized function that performs spatial sorting that can result in significant improvements in compression ratio by the subsequent processing:

reorderPoints: (positions: Float32Array, positions_stride: number) => Uint32Array;

This function returns a remap array just like reorderMesh, so the vertices need to be reordered accordingly for every vertex stream - the positions input is not modified. Note that it assumes no index buffer is provided, as it is redundant for point clouds.

To quantize the attribute data (whether it represents a mesh component or something else like a rotation quaternion for a bone), typically some data-specific 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 fixed-point encoding with a given number of bits stored in a 16-bit or 8-bit integer is recommended; however, this is not always best for compression ratio for data with complex cross-component dependencies.

To that end, three filter encoders are provided: octahedral (optimal for normal or tangent data), quaternion (optimal for unit-length quaternions) and exponential (optimal for compressing floating-point vectors). The last two are recommended for use for animation data, and exponential filter can additionally be used to quantize any floating-point 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 of count 4-vectors), treats them as a unit vector (XYZ) and fourth component from -1..1 (W), and encodes them into stride bytes in a way that, when decoded, the result is stored as a normalized signed 4-vector. stride must be 4 (in which case the round-trip result is 4 8-bit normalized values) or 8 (in which case the round-trip result is 4 16-bit 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 4-th 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 if stride=4 and [1..16] range if stride=8.

  • encodeFilterQuat takes each 4 floats from the source array (for a total of count 4-vectrors), treats them as a unit quaternion, and encodes them into stride bytes in a way that, when decoded, the result is stored as a normalized signed 4-vector representing the same rotation as the source quaternion. stride must be 8 (the round-trip result is 4 16-bit normalized values). bits represents the desired precision of each component and must be in [4..16] range, although using less than 9-10 bits is likely going to lead to significant deviation in rotations.

  • encodeFilterExp takes each K floats from the source array (where K=stride/4, for a total of count K-vectors), and encodes them into stride bytes in a way that, when decoded, the result is stored as K single-precision 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 by bits 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 16-bit 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.

Simplifier

MeshoptSimplifier (meshopt_simplifier.js) implements mesh simplification, producing a mesh with fewer triangles/points that resembles the original mesh in its appearance. The simplification algorithms are lossy and may result in significant change in appearance, but can often be used without visible visual degradation on high poly input meshes or for level of detail variants far away.

To simplify the mesh, the following function needs to be called first:

simplify(indices: Uint32Array, vertex_positions: Float32Array, vertex_positions_stride: number, target_index_count: number, target_error: number, flags?: [Flags]) => [Uint32Array, number];

Given an input triangle mesh represented by an index buffer and a position buffer, the algorithm tries to simplify the mesh down to the target index count while maintaining the appearance. For meshes with inconsistent topology or many seams, such as faceted meshes, it can result in simplifier getting "stuck" and not being able to simplify the mesh fully. Therefore it's critical that identical vertices are "welded" together, that is, the input vertex buffer does not contain duplicates. Additionally, it may be possible to preprocess the index buffer to discard any vertex attributes that aren't critical and can be rebuilt later.

Target error is an approximate measure of the deviation from the original mesh using distance normalized to [0..1] range (e.g. 1e-2f means that simplifier will try to maintain the error to be below 1% of the mesh extents). Note that the simplifier attempts to produce the requested number of indices at minimal error, but because of topological restrictions and error limit it is not guaranteed to reach the target index count and can stop earlier.

The algorithm uses position data stored in a strided array; vertex_positions_stride represents the distance between subsequent positions in Float32 units and should typically be set to 3. If the input position data is quantized, it's necessary to dequantize it so that the algorithm can estimate the position error correctly. While the algorithm doesn't use other attributes like normals/texture coordinates, it automatically recognizes and preserves attribute discontinuities based on index data. Because of this, for the algorithm to function well, the mesh vertices should be unique (de-duplicated).

Upon completion, the function returns the new index buffer as well as the resulting appearance error. The index buffer can be used to render the simplified mesh with the same vertex buffer(s) as the original one, including non-positional attributes. For example, simplify can be called multiple times with different target counts/errors, and the application can select the appropriate index buffer to render for the mesh at runtime to implement level of detail.

To control behavior of the algorithm more precisely, flags may specify an array of strings that enable various additional options:

  • "LockBorder" locks the vertices that lie on the topological border of the mesh in place such that they don't move during simplification. This can be valuable to simplify independent chunks of a mesh, for example terrain, to ensure that individual levels of detail can be stitched together later without gaps.

When the resulting mesh is stored, it might be desireable to remove the redundant vertices from the attribute buffers instead of simply using the original vertex data with the smaller index buffer. For that purpose, the simplifier module provides the compactMesh function, which is similar to reorderMesh function that the encoder provides, but doesn't perform extra optimizations and merely prepares a new vertex order that can be used to create new, smaller, vertex buffers:

compactMesh: (indices: Uint32Array) => [Uint32Array, number];

The simplification algorithm uses relative errors for input and output; to convert these errors to absolute units, they need to be multiplied by the scaling factor which depends on the mesh geometry and can be computed by calling the following function with the position data:

getScale: (vertex_positions: Float32Array, vertex_positions_stride: number) => number;

License

This library is available to anybody free of charge, under the terms of MIT License (see LICENSE.md).

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