canvaskit-webgpu

Custom CanvasKit WASM build with Skia Graphite + Dawn (WebGPU) backend, replacing the default Ganesh + WebGL backend.

Built for OpenPencil.

Why

Google removed the Dawn/WebGPU backend from CanvasKit's standard builds. The Graphite rendering engine offers better performance on modern GPUs, but CanvasKit only ships with Ganesh/WebGL. This repo patches Skia to produce a working CanvasKit WASM with Graphite/WebGPU.

Usage

Download canvaskit.js + canvaskit.wasm from Releases, then:

const ck = await CanvasKitInit({
  locateFile: (file) => `/canvaskit-webgpu/${file}`
})

const adapter = await navigator.gpu.requestAdapter()
const device = await adapter.requestDevice()

const devCtx = ck.MakeGPUDeviceContext(device)
const canvasCtx = ck.MakeGPUCanvasContext(devCtx, canvas)

// Each frame: create surface → draw → flush
const surface = ck.MakeGPUCanvasSurface(canvasCtx, ck.ColorSpace.SRGB)
const skCanvas = surface.getCanvas()
// ... draw ...
surface.flush()

Note: Graphite requires a fresh surface per frame (wrapping the current swapchain texture). Don't reuse surfaces across frames. Don't dispose the surface until the next frame — the browser needs the texture alive to present it.

Build

Requires: git, Python 3, CMake, Ninja, ~20 min on M-series Mac.

./build.sh

Output: out/canvaskit.js + out/canvaskit.wasm

Set SKIA_DIR to reuse an existing Skia checkout:

SKIA_DIR=~/skia ./build.sh

What's patched

6 files in Skia, 1 in Dawn (see patches/):

File	Change
modules/canvaskit/BUILD.gn	Add CK_ENABLE_WEBGPU define, merge EXPORTED_RUNTIME_METHODS
modules/canvaskit/canvaskit_bindings.cpp	Graphite context/recorder, surface creation via WrapBackendTexture, flush via snap+submit, embind registration
modules/canvaskit/webgpu.js	Per-frame surface from swapchain texture, Path→PathBuilder compat shim
src/gpu/graphite/dawn/DawnBuffer.cpp	Fix SKGPU_LOG constexpr issue, missing paren, wrong variable name
src/gpu/graphite/dawn/DawnCaps.cpp	Guard Dawn extensions not available in Emscripten WebGPU headers
third_party/dawn/BUILD.gn	Skip native Dawn library for WASM builds
Dawn include/tint/tint.h	Add missing include for bindings.h

Pinned versions

Component	Commit
Skia	f886711f180d6987f0ec7dd6bf04f85e4c86c50d
Dawn	bc602f0c897dde1d9bae1449477b0877c17150f3

GN args

skia_enable_graphite = true
skia_enable_ganesh = false
skia_use_dawn = true
skia_use_webgpu = true

License

Skia and Dawn are licensed under BSD-3-Clause. Patches in this repo are MIT.