faster findFace worker postMessage

Use transferList to send imageData ArrayBuffer, which transfers ownership
instead of serializing / deserialzing

js/jsfeat/JsfeatFace.js

@@ -69,11 +69,15 @@ export default class JsfeatFace extends EventEmitter {
     }
 
     // console.time('findFace');
-    this.worker.postMessage({
+    // Send the underlying ArrayBuffer to worker
+    const imageDataBuffer = imageData.data.buffer;
+    const message = {
       w: w,
       h: h,
-      imageData: imageData
-    });
+      imageDataBuffer: imageDataBuffer,
+      imageDataLength: imageData.length
+    };
+    this.worker.postMessage(message, [message.imageDataBuffer]);
   }
 
 }

js/jsfeat/findFace.worker.js

@@ -1,28 +1,29 @@
 import jsfeat from './jsfeatWithFrontalface';
 
 onmessage = function (e) {
-  const imageData = e.data.imageData;
+  const buffer = e.data.imageDataBuffer;
+  const imageData = new Uint8Array(buffer, 0, e.data.imageDataLength);
   const w = e.data.w;
   const h = e.data.h;
 
-  const img_u8 = new jsfeat.matrix_t(w, h, jsfeat.U8_t | jsfeat.C1_t);
-  const ii_sum = new Int32Array((w + 1) * (h + 1));
-  const ii_sqsum = new Int32Array((w + 1) * (h + 1));
-  const ii_tilted = new Int32Array((w + 1) * (h + 1));
+  const imgU8 = new jsfeat.matrix_t(w, h, jsfeat.U8_t | jsfeat.C1_t);
+  const iiSum = new Int32Array((w + 1) * (h + 1));
+  const iiSqsum = new Int32Array((w + 1) * (h + 1));
+  const iiTilted = new Int32Array((w + 1) * (h + 1));
 
   const classifier = jsfeat.haar.frontalface;
 
   // Old findFace
-  jsfeat.imgproc.grayscale(imageData.data, img_u8.data);
-  jsfeat.imgproc.equalize_histogram(img_u8, img_u8);
-  jsfeat.imgproc.compute_integral_image(img_u8, ii_sum, ii_sqsum, null);
+  jsfeat.imgproc.grayscale(imageData, imgU8.data);
+  jsfeat.imgproc.equalize_histogram(imgU8, imgU8);
+  jsfeat.imgproc.compute_integral_image(imgU8, iiSum, iiSqsum, null);
   let rects = jsfeat.haar.detect_multi_scale(
-    ii_sum,
-    ii_sqsum,
-    ii_tilted,
+    iiSum,
+    iiSqsum,
+    iiTilted,
     null,
-    img_u8.cols,
-    img_u8.rows,
+    imgU8.cols,
+    imgU8.rows,
     classifier,
     1.15,
     2