[Fix] Support fetching images when using worker engine (#574)

The previous implementation relied on the HTMLImageElement constuctor
which is not available in worker contexts.

examples/vision-model/src/vision_model.ts

@@ -9,6 +9,8 @@ function setLabel(id: string, text: string) {
   label.innerText = text;
 }
 
+const USE_WEB_WORKER = false;
+
 const proxyUrl = "https://cors-anywhere.herokuapp.com/";
 const url_https_street = "https://www.ilankelman.org/stopsigns/australia.jpg";
 const url_https_tree = "https://www.ilankelman.org/sunset.jpg";
@@ -23,16 +25,25 @@ async function main() {
     setLabel("init-label", report.text);
   };
   const selectedModel = "Phi-3.5-vision-instruct-q4f16_1-MLC";
-  const engine: webllm.MLCEngineInterface = await webllm.CreateMLCEngine(
-    selectedModel,
-    {
-      initProgressCallback: initProgressCallback,
-      logLevel: "INFO", // specify the log level
-    },
-    {
-      context_window_size: 6144,
-    },
-  );
+
+  const engineConfig: webllm.MLCEngineConfig = {
+    initProgressCallback: initProgressCallback,
+    logLevel: "INFO", // specify the log level
+  };
+  const chatOpts = {
+    context_window_size: 6144,
+  };
+
+  const engine: webllm.MLCEngineInterface = USE_WEB_WORKER
+    ? await webllm.CreateWebWorkerMLCEngine(
+        new Worker(new URL("./worker.ts", import.meta.url), {
+          type: "module",
+        }),
+        selectedModel,
+        engineConfig,
+        chatOpts,
+      )
+    : await webllm.CreateMLCEngine(selectedModel, engineConfig, chatOpts);
 
   // 1. Single image input (with choices)
   const messages: webllm.ChatCompletionMessageParam[] = [

src/support.ts

@@ -411,28 +411,18 @@ export const IMAGE_EMBED_SIZE = 1921;
 /**
  * Given a url, get the image data. The url can either start with `http` or `data:image`.
  */
-export function getImageDataFromURL(url: string): Promise<ImageData> {
-  return new Promise((resolve, reject) => {
-    // Converts img to any, and later `as CanvasImageSource`, otherwise build complains
-    const img: any = new Image();
-    img.crossOrigin = "anonymous"; // Important for CORS
-    img.onload = () => {
-      const canvas = document.createElement("canvas");
-      const ctx = canvas.getContext("2d");
-      if (!ctx) {
-        reject(new Error("Could not get 2d context"));
-        return;
-      }
-      canvas.width = img.width;
-      canvas.height = img.height;
-      ctx.drawImage(img as CanvasImageSource, 0, 0);
+export async function getImageDataFromURL(url: string): Promise<ImageData> {
+  const response = await fetch(url, { mode: "cors" });
+  const img = await createImageBitmap(await response.blob());
+  const canvas = new OffscreenCanvas(img.width, img.height);
+  const ctx = canvas.getContext("2d");
+  if (!ctx) {
+    throw new Error("Could not get 2d context");
+  }
+  ctx.drawImage(img, 0, 0);
 
-      const imageData = ctx.getImageData(0, 0, img.width, img.height);
-      resolve(imageData);
-    };
-    img.onerror = () => reject(new Error("Failed to load image"));
-    img.src = url;
-  });
+  const imageData = ctx.getImageData(0, 0, img.width, img.height);
+  return imageData;
 }
 
 /**