From aa79c6155c80ac1c58ab8fd0e568bbd52d2fdf46 Mon Sep 17 00:00:00 2001
From: Luna <279187109+lunadogbot@users.noreply.github.com>
Date: Wed, 29 Apr 2026 11:47:34 +0000
Subject: [PATCH 1/2] docs: document OffscreenCanvas (2.8)

Adds an OffscreenCanvas section to the Web platform APIs reference,
describing the new API available in Deno 2.8. Covers a typical
2D-drawing + convertToBlob() example, common use cases (server-side
image generation, off-thread drawing, thumbnails), and notes that
WebGL/WebGPU contexts on OffscreenCanvas are not yet supported.

Refs denoland/deno#29357
---
 runtime/reference/web_platform_apis.md | 28 ++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/runtime/reference/web_platform_apis.md b/runtime/reference/web_platform_apis.md
index 5f5c7e15d..b0840d360 100644
--- a/runtime/reference/web_platform_apis.md
+++ b/runtime/reference/web_platform_apis.md
@@ -489,6 +489,34 @@ const worker = new Worker(import.meta.resolve("./worker.js"), {
 });
 ```
 
+## OffscreenCanvas
+
+Starting in Deno 2.8, the
+[`OffscreenCanvas`](https://developer.mozilla.org/en-US/docs/Web/API/OffscreenCanvas)
+API is available. `OffscreenCanvas` is a canvas that lives outside any DOM and
+can be used in any context — including Web Workers — for off-thread rendering
+and image generation.
+
+```ts
+const canvas = new OffscreenCanvas(256, 256);
+const ctx = canvas.getContext("2d");
+ctx.fillStyle = "tomato";
+ctx.fillRect(0, 0, 256, 256);
+
+// Encode to PNG / JPEG / WebP via convertToBlob()
+const blob = await canvas.convertToBlob({ type: "image/png" });
+await Deno.writeFile("./tile.png", new Uint8Array(await blob.arrayBuffer()));
+```
+
+Typical uses:
+
+- generating images server-side without spinning up a headless browser,
+- running 2D drawing code originally written for the browser inside a Web
+  Worker,
+- producing thumbnails or social-card images at request time.
+
+WebGL / WebGPU rendering contexts on `OffscreenCanvas` are not yet supported.
+
 ## Deviations of other APIs from spec
 
 ### Cache API

From 4fcc9716f4dea9fe639f92774d4819aab14931fd Mon Sep 17 00:00:00 2001
From: Luna <279187109+lunadogbot@users.noreply.github.com>
Date: Fri, 1 May 2026 12:19:22 +0000
Subject: [PATCH 2/2] docs(review): correct OffscreenCanvas context list and
 example
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Per @Hajime-san — Deno's OffscreenCanvas supports only \`bitmaprenderer\`
and \`webgpu\`; \`2d\` / \`webgl\` / \`webgl2\` return null. Replaces
the broken 2D drawing example with a bitmaprenderer image-encode flow
and rewrites the "limitations" line into an explicit "Supported
rendering contexts" subsection. WebGPU is supported, so it no longer
appears in the unsupported list.

Refs denoland/deno#29357
---
 runtime/reference/web_platform_apis.md | 49 ++++++++++++++++++--------
 1 file changed, 35 insertions(+), 14 deletions(-)

diff --git a/runtime/reference/web_platform_apis.md b/runtime/reference/web_platform_apis.md
index b0840d360..202b41db4 100644
--- a/runtime/reference/web_platform_apis.md
+++ b/runtime/reference/web_platform_apis.md
@@ -494,28 +494,49 @@ const worker = new Worker(import.meta.resolve("./worker.js"), {
 Starting in Deno 2.8, the
 [`OffscreenCanvas`](https://developer.mozilla.org/en-US/docs/Web/API/OffscreenCanvas)
 API is available. `OffscreenCanvas` is a canvas that lives outside any DOM and
-can be used in any context — including Web Workers — for off-thread rendering
-and image generation.
+can be used anywhere — including Web Workers — for off-thread rendering and
+image generation.
+
+### Supported rendering contexts
+
+`OffscreenCanvas#getContext` accepts two of the spec-defined context ids:
+
+- `"bitmaprenderer"` — returns an
+  [`ImageBitmapRenderingContext`](https://developer.mozilla.org/en-US/docs/Web/API/ImageBitmapRenderingContext)
+  for displaying an `ImageBitmap` produced via `createImageBitmap`.
+- `"webgpu"` — returns a
+  [`GPUCanvasContext`](https://developer.mozilla.org/en-US/docs/Web/API/GPUCanvasContext)
+  for rendering with WebGPU.
+
+Calling `getContext` with `"2d"`, `"webgl"`, or `"webgl2"` returns `null` —
+these contexts are not implemented in Deno.
+
+### Example: encoding an image to PNG
+
+Decode an image into an `ImageBitmap`, place it on an `OffscreenCanvas` via
+the `bitmaprenderer` context, and write the result to disk:
 
 ```ts
-const canvas = new OffscreenCanvas(256, 256);
-const ctx = canvas.getContext("2d");
-ctx.fillStyle = "tomato";
-ctx.fillRect(0, 0, 256, 256);
+const data = await Deno.readFile("./input.jpg");
+const bitmap = await createImageBitmap(new Blob([data]));
+
+const canvas = new OffscreenCanvas(bitmap.width, bitmap.height);
+const ctx = canvas.getContext("bitmaprenderer")!;
+ctx.transferFromImageBitmap(bitmap);
 
-// Encode to PNG / JPEG / WebP via convertToBlob()
 const blob = await canvas.convertToBlob({ type: "image/png" });
-await Deno.writeFile("./tile.png", new Uint8Array(await blob.arrayBuffer()));
+await Deno.writeFile(
+  "./output.png",
+  new Uint8Array(await blob.arrayBuffer()),
+);
 ```
 
 Typical uses:
 
-- generating images server-side without spinning up a headless browser,
-- running 2D drawing code originally written for the browser inside a Web
-  Worker,
-- producing thumbnails or social-card images at request time.
-
-WebGL / WebGPU rendering contexts on `OffscreenCanvas` are not yet supported.
+- producing thumbnails, format conversions, or social-card images at request
+  time without spinning up a headless browser,
+- running off-thread image work inside a Web Worker,
+- driving WebGPU rendering targets that don't need a window.
 
 ## Deviations of other APIs from spec