Webgpu Syntax Canvas Context

WebGPU Canvas Context

A GPUCanvasContext is the bridge between a <canvas> element and the GPU. It produces the per-frame GPUTexture that a render pass draws into and that the browser then composites to the screen. This skill covers context creation, configuration, per-frame texture acquisition, and resize handling for WebGPU 1.0-stable (Chrome 113+, Safari 26+, Firefox 141+).

Quick Reference

GPUCanvasConfiguration fields passed to context.configure():

Core methods:

Decision Tree

Choosing alphaMode:

Does the canvas need to blend with HTML content behind it?
├── No, canvas is fully opaque background
│   └── alphaMode: "opaque" (default) — alpha channel is ignored on composite
└── Yes, canvas overlays page content (transparent regions show through)
    └── alphaMode: "premultiplied" — fragment RGB MUST already be
        multiplied by alpha; output alpha < 1 lets the page show through

Choosing how to handle resize:

Does the canvas have a fixed pixel size that never changes?
├── Yes
│   └── Set canvas.width / canvas.height once before configure(); no observer
└── No, the canvas is responsive / CSS-sized / fullscreen
    └── Use a ResizeObserver:
        observe the canvas, on callback set canvas.width / canvas.height
        from devicePixelContentBoxSize, clamp to
        device.limits.maxTextureDimension2D, then draw a frame

Core Patterns

ALWAYS get format from navigator.gpu.getPreferredCanvasFormat(). The preferred format varies per platform ("bgra8unorm" on most desktops, "rgba8unorm" on some mobile GPUs). Hardcoding "bgra8unorm" forces a per-frame format conversion on platforms that prefer "rgba8unorm".

const context = canvas.getContext("webgpu");
const format = navigator.gpu.getPreferredCanvasFormat();
context.configure({ device, format });

ALWAYS call context.configure() before context.getCurrentTexture(). Calling getCurrentTexture() on an unconfigured context throws InvalidStateError. The context returned by getContext("webgpu") has no device until configure() runs.

ALWAYS call getCurrentTexture() once per frame and use the result immediately. The texture is valid only for the frame it was obtained in. After the browser composites the canvas, the swap-chain rotates and the texture becomes invalid.

function frame() {
  const texture = context.getCurrentTexture();   // fresh every frame
  const view = texture.createView();             // fresh view every frame
  // ... build and submit a render pass that targets `view` ...
  requestAnimationFrame(frame);
}

NEVER cache getCurrentTexture() or its createView() result across frames. A stale texture or view points at a swap-chain slot that is no longer the presentable one, producing a black canvas or a validation error that the texture is already in use.

ALWAYS resize by setting canvas.width and canvas.height attributes, NEVER only CSS. The width/height attributes define the swap-chain texture resolution. CSS width/height only stretch the rendered result. Setting CSS alone renders at the old low resolution and upscales, producing a blurry image.

const observer = new ResizeObserver((entries) => {
  for (const entry of entries) {
    const box = entry.devicePixelContentBoxSize[0];
    const max = device.limits.maxTextureDimension2D;
    canvas.width = Math.max(1, Math.min(box.inlineSize, max));
    canvas.height = Math.max(1, Math.min(box.blockSize, max));
  }
  frame();
});
observer.observe(canvas);

ALWAYS pass the same usage flags that downstream code needs. The default usage is RENDER_ATTACHMENT only. Setting usage to a different value REPLACES the default; to also copy the canvas (for a screenshot) pass GPUTextureUsage.RENDER_ATTACHMENT | GPUTextureUsage.COPY_SRC explicitly.

Common Anti-Patterns

Caching getCurrentTexture() across frames. The swap-chain rotates textures every frame. A cached texture or view targets a slot that is no longer presentable, so the canvas stays black or validation fails. Fix: call getCurrentTexture() fresh each frame.

Setting only CSS size, not width/height attributes. CSS sizing stretches a fixed-resolution swap-chain. Result: a blurry upscale on high-DPI or resized displays. Fix: write the device-pixel size into canvas.width and canvas.height.

Calling getCurrentTexture() before configure(). An unconfigured context has no swap-chain, so the call throws InvalidStateError. Fix: always configure() first.

Critical Warnings

• NEVER cache a canvas texture or its view beyond the current frame.
• NEVER call getCurrentTexture() before context.configure() has run.
• NEVER hardcode "bgra8unorm"; ALWAYS use navigator.gpu.getPreferredCanvasFormat().
• NEVER resize a WebGPU canvas with CSS alone; ALWAYS set canvas.width/canvas.height.
• NEVER set canvas.width/canvas.height above device.limits.maxTextureDimension2D; clamp first or texture allocation fails validation.
• NEVER assume the pipeline can target the canvas with any format; the render pipeline fragment.targets[0].format MUST equal the format passed to configure().

Reference Files

• references/methods.md — getContext, GPUCanvasConfiguration fields, configure, getCurrentTexture, unconfigure, getPreferredCanvasFormat signatures.
• references/examples.md — verified code: context setup, per-frame texture acquisition, ResizeObserver resize handling.
• references/anti-patterns.md — canvas-context mistakes with WHY-it-fails analysis.

Related skills:

• webgpu-core-architecture — navigator.gpu, adapter and device acquisition.
• webgpu-syntax-textures — GPUTexture, createView, texture formats and usage.
• webgpu-impl-render-targets — using the canvas texture as a render-pass attachment.
• webgpu-core-cross-browser — preferred-format and feature differences per browser.