# Error: WebGPU canvas selector with vtkWebAssemblyHardwareWindow fails for the official Geometry Viewer Emscripten Example

**URL:** https://discourse.vtk.org/t/error-webgpu-canvas-selector-with-vtkwebassemblyhardwarewindow-fails-for-the-official-geometry-viewer-emscripten-example/16313
**Category:** Web
**Tags:** vtkwasm
**Created:** [March 4, 2026, 10:44am UTC](https://discourse.vtk.org/t/error-webgpu-canvas-selector-with-vtkwebassemblyhardwarewindow-fails-for-the-official-geometry-viewer-emscripten-example/16313 "2026-03-04T10:44:05Z")
**Posts on this page:** 5
**Page:** 1

<div class="post-metadata">

### Author: ![RaMmer](https://discourse.vtk.org/letter_avatar_proxy/v4/letter/r/d07c76/32.png) [@RaMmer](https://discourse.vtk.org/u/RaMmer)
#### Post date: [March 4, 2026, 10:44am UTC](https://discourse.vtk.org/t/error-webgpu-canvas-selector-with-vtkwebassemblyhardwarewindow-fails-for-the-official-geometry-viewer-emscripten-example/16313/1 "2026-03-04T10:44:05Z")

</div>

Hi,

I have noticed that the `WebGPU` canvas selector for the official example [Geometry Viewer](https://gitlab.kitware.com/vtk/vtk/-/tree/master/Examples/Emscripten/Cxx/GeometryViewer?ref_type=heads) fails as it defaults to the default canvas selection id `canvas` instead of `vtk-3d-canvas`.

These are the commands I have used to build with the latest docker image as well as the official 9.6.0 tag.

```bash
docker run --rm -it `
  -v"$PWD":/work kitware/vtk-wasm-sdk:latest `
  emcmake cmake -GNinja -S /work -B /work/build -DVTK_DIR=/VTK-install/Release/wasm32/lib/cmake/vtk

# Build
docker run --rm -it `
  -v"$PWD":/work kitware/vtk-wasm-sdk:latest `
  cmake --build /work/build

```

The build will be successful however the `WebGPU` backend will fail for the latest images.

I have noticed a new class `vtkWebAssemblyHardwareWindow ` is adopted in the latest commits and the class is changed from `vtkWebAssemblyWebGPURenderWidow` for canvas selection.

```cpp
  if (auto wasmHardwareWindow =
       vtkWebAssemblyHardwareWindow::SafeDownCast(this->P->Window->GetHardwareWindow()))
  {
    wasmHardwareWindow->SetCanvasSelector("#vtk-3d-canvas");
  } 

```

The cast is failing and the canvas selection is not set for the `WebGPU` backend.

---

<div class="post-metadata">

### Author: ![jaswantp](https://discourse.vtk.org/user_avatar/discourse.vtk.org/jaswantp/32/10046_2.png) [@jaswantp](https://discourse.vtk.org/u/jaswantp)
#### Post date: [March 4, 2026, 2:48pm UTC](https://discourse.vtk.org/t/error-webgpu-canvas-selector-with-vtkwebassemblyhardwarewindow-fails-for-the-official-geometry-viewer-emscripten-example/16313/2 "2026-03-04T14:48:37Z")

</div>

Hi @RaMmer

Thank you for posting this here. Let me look into it and get back.

---

<div class="post-metadata">

### Author: ![jaswantp](https://discourse.vtk.org/user_avatar/discourse.vtk.org/jaswantp/32/10046_2.png) [@jaswantp](https://discourse.vtk.org/u/jaswantp)
#### Post date: [March 4, 2026, 3:34pm UTC](https://discourse.vtk.org/t/error-webgpu-canvas-selector-with-vtkwebassemblyhardwarewindow-fails-for-the-official-geometry-viewer-emscripten-example/16313/3 "2026-03-04T15:34:54Z")

</div>

Here are my findings:

check out VTK v9.6.0, build GeometryViewer with vtk-wasm-sdk:9.6.0 image - OK  
check out VTK master, build GeometryViewer with vtk-wasm-sdk:latest image - not OK

```auto
Uncaught TypeError: Cannot read properties of null (reading 'getContext')
    at _wgpuInstanceCreateSurface (http://localhost:8001/GeometryViewer.js:11105:24)
    at GeometryViewer.wasm.vtkWebGPURenderWindow::CreateSurface() (http://localhost:8001/GeometryViewer.wasm:wasm-function[10569]:0x31ae8b)
    at GeometryViewer.wasm.vtkWebGPURenderWindow::Initialize() (http://localhost:8001/GeometryViewer.wasm:wasm-function[10570]:0x31af74)
    at GeometryViewer.wasm.vtkWebGPURenderWindow::Start() (http://localhost:8001/GeometryViewer.wasm:wasm-function[10605]:0x325768)
    at GeometryViewer.wasm.vtkWebAssemblyRenderWindowInteractor::Initialize() (http://localhost:8001/GeometryViewer.wasm:wasm-function[10953]:0x33e937)
    at GeometryViewer.wasm.vtkRenderWindow::Render() (http://localhost:8001/GeometryViewer.wasm:wasm-function[14323]:0x48806a)
    at GeometryViewer.wasm.vtkWebGPURenderWindow::Render() (http://localhost:8001/GeometryViewer.wasm:wasm-function[10581]:0x31fabe)
    at GeometryViewer.wasm.GeometryViewer::Render() (http://localhost:8001/GeometryViewer.wasm:wasm-function[467]:0x2db23)
    at GeometryViewer.wasm.emscripten::internal::MethodInvoker<emscripten::internal::rvp::default_tag, void (GeometryViewer::*)(), void, GeometryViewer*>::invoke(void (GeometryViewer::* const&)(), GeometryViewer*) (http://localhost:8001/GeometryViewer.wasm:wasm-function[736]:0x361d2)
    at async main (http://localhost:8001/index.js:340:3)

```

---

<div class="post-metadata">

### Author: ![jaswantp](https://discourse.vtk.org/user_avatar/discourse.vtk.org/jaswantp/32/10046_2.png) [@jaswantp](https://discourse.vtk.org/u/jaswantp)
#### Post date: [March 4, 2026, 5:46pm UTC](https://discourse.vtk.org/t/error-webgpu-canvas-selector-with-vtkwebassemblyhardwarewindow-fails-for-the-official-geometry-viewer-emscripten-example/16313/4 "2026-03-04T17:46:47Z")

</div>

I have a fix in [https://gitlab.kitware.com/vtk/vtk/-/merge\_requests/12985](https://gitlab.kitware.com/vtk/vtk/-/merge_requests/12985)

The example is now fixed in `master` branch. Please sync your VTK clone, and try again with the `vtk-wasm-sdk:latest` image.

---

<div class="post-metadata">

### Author: ![RaMmer](https://discourse.vtk.org/letter_avatar_proxy/v4/letter/r/d07c76/32.png) [@RaMmer](https://discourse.vtk.org/u/RaMmer)
#### Post date: [March 5, 2026, 4:21am UTC](https://discourse.vtk.org/t/error-webgpu-canvas-selector-with-vtkwebassemblyhardwarewindow-fails-for-the-official-geometry-viewer-emscripten-example/16313/5 "2026-03-05T04:21:13Z")

</div>

Thank You!