Use a more compact typed array to pass the image mask from the worker to the main thread.

src/core/evaluator.js

@@ -262,8 +262,7 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
         var inverseDecode = !!decode && decode[0] > 0;
 
         operatorList.addOp(OPS.paintImageMaskXObject,
-          [PDFImage.createMask(imgArray, width, height,
-                                            inverseDecode)]
+          [PDFImage.createMask(imgArray, width, height, inverseDecode)]
         );
         return;
       }

src/core/image.js

@@ -209,26 +209,21 @@ var PDFImage = (function PDFImageClosure() {
 
   PDFImage.createMask = function PDFImage_createMask(imgArray, width, height,
                                                      inverseDecode) {
-    var buffer = new Uint8Array(width * height * 4);
-    var imgArrayPos = 0;
-    var i, j, mask, buf;
-    // removing making non-masked pixels transparent
-    var bufferPos = 3; // alpha component offset
-    for (i = 0; i < height; i++) {
-      mask = 0;
-      for (j = 0; j < width; j++) {
-        if (!mask) {
-          buf = imgArray[imgArrayPos++];
-          mask = 128;
-        }
-        if (!(buf & mask) !== inverseDecode) {
-          buffer[bufferPos] = 255;
-        }
-        bufferPos += 4;
-        mask >>= 1;
+    // Copy imgArray into a typed array (inverting if necessary) so it can be
+    // transferred to the main thread.
+    var length = ((width + 7) >> 3) * height;
+    var data = new Uint8Array(length);
+    if (inverseDecode) {
+      for (var i = 0; i < length; i++) {
+        data[i] = ~imgArray[i];
+      }
+    } else {
+      for (var i = 0; i < length; i++) {
+        data[i] = imgArray[i];
       }
     }
-    return {data: buffer, width: width, height: height};
+
+    return {data: data, width: width, height: height};
   };
 
   PDFImage.prototype = {

src/display/canvas.js

@@ -431,6 +431,34 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
     ctx.putImageData(tmpImgData, 0, 0);
   }
 
+  function putBinaryImageMask(ctx, imgData) {
+    var width = imgData.width, height = imgData.height;
+    var tmpImgData = ctx.createImageData(width, height);
+    var data = imgData.data;
+    var tmpImgDataPixels = tmpImgData.data;
+    var dataPos = 0;
+
+    // Expand the mask so it can be used by the canvas.  Any required inversion
+    // has already been handled.
+    var tmpPos = 3; // alpha component offset
+    for (var i = 0; i < height; i++) {
+      var mask = 0;
+      for (var j = 0; j < width; j++) {
+        if (!mask) {
+          var elem = data[dataPos++];
+          mask = 128;
+        }
+        if (!(elem & mask)) {
+          tmpImgDataPixels[tmpPos] = 255;
+        }
+        tmpPos += 4;
+        mask >>= 1;
+      }
+    }
+
+    ctx.putImageData(tmpImgData, 0, 0);
+  }
+
   function copyCtxState(sourceCtx, destCtx) {
     var properties = ['strokeStyle', 'fillStyle', 'fillRule', 'globalAlpha',
                       'lineWidth', 'lineCap', 'lineJoin', 'miterLimit',
@@ -1611,7 +1639,7 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
       var maskCtx = maskCanvas.context;
       maskCtx.save();
 
-      putBinaryImageData(maskCtx, img);
+      putBinaryImageMask(maskCtx, img);
 
       maskCtx.globalCompositeOperation = 'source-in';
 
@@ -1638,7 +1666,7 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
         var maskCtx = maskCanvas.context;
         maskCtx.save();
 
-        putBinaryImageData(maskCtx, image);
+        putBinaryImageMask(maskCtx, image);
 
         maskCtx.globalCompositeOperation = 'source-in';