Document the background transformer a bit better

Author: robintown

src/livekit/BlurBackgroundTransformer.ts

@@ -21,8 +21,16 @@ interface WasmFileset {
   wasmBinaryPath: string;
 }
 
-// n.b. this only includes the SIMD versions of the WASM files which have good support:
-// https://caniuse.com/?search=simd
+// The MediaPipe package, by default, ships some alternative versions of the
+// WASM files which avoid SIMD for compatibility with older browsers. But SIMD
+// in WASM is actually fine by our support policy, so we include just the SIMD
+// versions.
+// It's really not ideal that we have to reference these internal files from
+// MediaPipe and depend on node_modules having this specific structure. It's
+// easy to see this breaking if our dependencies changed and MediaPipe were
+// no longer hoisted, or if we switched to another dependency loader such as
+// Yarn PnP.
+// https://github.com/google-ai-edge/mediapipe/issues/5961
 const wasmFileset: WasmFileset = {
   wasmLoaderPath: new URL(
     "../../node_modules/@mediapipe/tasks-vision/wasm/vision_wasm_internal.js",
@@ -34,12 +42,20 @@ const wasmFileset: WasmFileset = {
   ).href,
 };
 
+/**
+ * Track processor that applies effects such as blurring to a user's background.
+ *
+ * This is just like LiveKit's prebuilt BackgroundTransformer except that it
+ * loads the segmentation models from our own bundle rather than as an external
+ * resource fetched from the public internet.
+ */
 export class BlurBackgroundTransformer extends BackgroundTransformer {
   public async init({
     outputCanvas,
     inputElement: inputVideo,
   }: VideoTransformerInitOptions): Promise<void> {
-    // call super.super.init()
+    // Call super.super.init() since we're totally replacing the init method of
+    // BackgroundTransformer here, rather than extending it
     await VideoTransformer.prototype.init.call(this, {
       outputCanvas,
       inputElement: inputVideo,