orca-wasm — OrcaSlicer WebAssembly engine

Standalone Emscripten build of OrcaSlicer v2.4.2. This repository contains the C++ bridge, compatibility patches, toolchain configuration, and release workflow for the host-ready engine.

Repository layout

orca/                 OrcaSlicer source checkout (submodule or shallow clone)
bridge/               exported C API and host boundary
cmake/                dependency discovery and WASM configuration
overrides/            source overrides for unavailable desktop libraries
patches/              idempotent OrcaSlicer compatibility patcher
wasm/                 Emscripten target and compatibility shims
scripts/              local build, smoke-test, and comparison tools
.github/workflows/    reproducible CI build and release workflow

Artifacts

File	Description
slicer.js / slicer.wasm	Single-threaded compatibility engine
slicer-mt.js / slicer-mt.wasm	Multithreaded engine for COOP/COEP hosts

The build does not produce slicer.data: the headless engine uses its virtual filesystem only for input and output files.

Local build

Install Emscripten 3.1.74 and the system tools used by CI (CMake, Ninja, Python 3, m4, texinfo, OpenSSL, ccache, and a C/C++ toolchain). Then run:

WASM_VARIANT=st ./scripts/build-local-wsl.sh
WASM_VARIANT=mt ./scripts/build-local-wsl.sh

The script builds the pinned OrcaSlicer dependencies, applies patches/apply.py, and writes the selected pair to artifacts/. Use EMSDK=/path/to/emsdk when the toolchain is not installed at /opt/emsdk. Its Emscripten cache is stored under build-wasm/emscripten-cache-$VARIANT; set WASM_EM_CACHE to choose a different writable cache directory.

The generated local script is derived from .github/workflows/build-wasm.yml. After changing that workflow, regenerate it with:

node scripts/gen-wsl-build-script.mjs

C API

The module exports the one-wasm-slicer-api 0.4 ABI. The old orc_* and 0.2 STL-specific symbols are not part of the canonical surface; hosts use the project-oriented API below:

onewasm_session_create / onewasm_session_destroy
onewasm_init / onewasm_init_profile / onewasm_apply_profile / onewasm_set_progress_callback
onewasm_cancel
onewasm_project_set_objects / onewasm_project_get_manifest
onewasm_project_prepare / onewasm_project_slice
onewasm_project_get_asset / onewasm_project_export
onewasm_obj_to_stl / onewasm_cad_to_stl
onewasm_get_capabilities
onewasm_free / onewasm_last_error

The Emscripten exports therefore use _onewasm_* names. The canonical header is vendored at bridge/onewasm_slicer_api.h and is synchronized with the private one-wasm-slicer-api repository at v0.4.0. The extra onewasm_read_3mf export is a geometry-only PoC import helper, outside the canonical project ABI. An Orca project .3mf is loaded as a native profile with onewasm_init_profile(session, "project.3mf", ...).

one-wasm-slicer-api compatibility

Target capability	OrcaWasm 0.4 status	Evidence
Session lifecycle	supported	onewasm_session_create/destroy
Native config initialization	supported	onewasm_init, format orca.native-json
Full native profile	supported	onewasm_init_profile, format project.3mf
Native profile fragments	supported	onewasm_apply_profile, formats orca.profile-json and orca.native-json
Progress callback	supported	onewasm_set_progress_callback plus worker progress messages
Neutral project manifest	supported	onewasm_project_set_objects/get_manifest
Project transforms	supported	row-major 4x4 affine matrices
Auto-orient / arrange	supported	onewasm_project_prepare
Single/multi/all-plate slice	supported	onewasm_project_slice with selected/all plate selection
Per-plate G-code/statistics	supported	result manifest plus onewasm_project_get_asset
OBJ / STEP to STL	supported	onewasm_obj_to_stl / onewasm_cad_to_stl
Native project export	supported with limits	onewasm_project_export, explicit preservation policy
G-code in native 3MF	unsupported	includeSliceArtifacts=true returns ONEWASM_ERR_UNSUPPORTED
Capability metadata	supported	onewasm_get_capabilities
Stable status and ownership	supported	0.3 status values and onewasm_free
Cooperative cancellation	supported	onewasm_cancel, native PrintBase::cancel(), -11 completion status

0.3 project adapter scope

The bridge implements the promoted 0.3 project adapter:

0.3 surface	Status	Scope
onewasm_project_set_objects	implemented	neutral 0.3 manifest plus owned concatenated STL blob
onewasm_project_get_manifest	implemented	returns the neutral project state
onewasm_project_prepare	implemented	native Orca arrange and auto-orient per plate
onewasm_project_slice	implemented	selected/all plates, sequential adapter orchestration, exact 0.3 affine transforms, per-plate result assets/statistics; serializable native meshes are materialized into the neutral blob
onewasm_project_get_asset	implemented	reads G-code and exported-project assets from the last successful project operation
onewasm_project_export	implemented with limits	clean native projects can be passed through losslessly; host/dirty projects are regenerated with require/best-effort/portable reporting; includeSliceArtifacts=true is still unsupported
init_profile("project.3mf", ...) as a full project load	implemented with limits	OrcaSlicer loads native configuration/project data and the bridge exposes a neutral manifest; serializable native meshes are copied for later prepare/slice/export

The canonical contract and schemas live in the private one-wasm-slicer-api repository.

Exception model and Memory64 decision

Both wasm32 artifacts use native WebAssembly exception handling. C++ builds compile and link with -fwasm-exceptions; C code that uses setjmp or longjmp explicitly uses -sSUPPORT_LONGJMP=wasm. The bundled zlib, libpng, and libjpeg ports are built with matching settings. OCCT signal conversion is disabled because it relies on setjmp/longjmp, and the oneTBB Emscripten profile no longer overrides native EH with -fexceptions. The workflow runs scripts/check-wasm-eh.sh before each engine build; it exercises mixed C/C++ exceptions and longjmp, pthread execution, and separate wasm32/wasm64 toolchain probes.

Memory64 is not shipped. The four small ST/MT wasm32/wasm64 probes pass with Emscripten 3.1.74 and Node.js 26.9.0. CI pins Node.js 26 for this check because the runner's Node 22 cannot instantiate the final wasm64 table encoding. The ST module measured 22,469 bytes for wasm32 and 22,531 bytes for wasm64; MT measured 46,880 and 48,834 bytes. These are probe sizes, not engine benchmarks. The 64-bit probe reports 8-byte pointers, so an array of 1,048,576 pointer slots uses 8 MiB instead of 4 MiB.

The current API 0.3 uses uint32_t byte lengths. The extension worker also writes and reads pointer outputs through HEAPU32 and i32, which assumes wasm32 pointers. wasm/CMakeLists.txt caps memory at 4 GiB as well. A Memory64 engine would require a coordinated API and worker ABI change and a larger-memory configuration; there is no measured workload here that needs those changes. Keep the current wasm32 artifact contract until a real model demonstrates the need and the API/worker path is updated.

Implementation references: Emscripten C++ exceptions, Emscripten setjmp/longjmp, and the WebAssembly Memory64 proposal.

CI and releases

The Build WASM workflow validates pull requests and builds both st and mt variants on the default branch. Each successful build runs the real engine smoke test before publishing immutable GitHub Release assets:

wasm-v2.4.2
wasm-v2.4.2-patchN
wasm-v2.4.2-patchN-multithreaded

A rebuild never overwrites an existing release. Consumers should resolve the highest patch number in the desired release family and use the JavaScript and WASM files from the same tag. This repository publishes engine releases only; the frontend deployment is handled separately by the JustSlice-PoC Cloudflare Workers project.

Licence and notices

OrcaSlicer and the linked libraries retain their upstream licences. See LICENSE and NOTICE.md for source and attribution details. The bridge and build infrastructure are original project code under the licence stated in LICENSE.