Optimized genericComposeSMask

Declaring the composition and backgdrop functions outside of genericComposeSMask
is more efficient.

src/display/canvas.js

@@ -601,49 +601,54 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
     }
   }
 
+  function composeSMaskBackdrop(bytes, r0, g0, b0) {
+    var length = bytes.length;
+    for (var i = 3; i < length; i += 4) {
+      var alpha = bytes[i];
+      if (alpha === 0) {
+        bytes[i - 3] = r0;
+        bytes[i - 2] = g0;
+        bytes[i - 1] = b0;
+      } else if (alpha < 255) {
+        var alpha_ = 255 - alpha;
+        bytes[i - 3] = (bytes[i - 3] * alpha + r0 * alpha_) >> 8;
+        bytes[i - 2] = (bytes[i - 2] * alpha + g0 * alpha_) >> 8;
+        bytes[i - 1] = (bytes[i - 1] * alpha + b0 * alpha_) >> 8;
+      }
+    }
+  }
+
+  function composeSMaskAlpha(maskData, layerData) {
+    var length = maskData.length;
+    var scale = 1 / 255;
+    for (var i = 3; i < length; i += 4) {
+      var alpha = maskData[i];
+      layerData[i] = (layerData[i] * alpha * scale) | 0;
+    }
+  }
+
+  function composeSMaskLuminosity(maskData, layerData) {
+    var length = maskData.length;
+    for (var i = 3; i < length; i += 4) {
+      var y = ((maskData[i - 3] * 77) +     // * 0.3 / 255 * 0x10000
+               (maskData[i - 2] * 152) +    // * 0.59 ....
+               (maskData[i - 1] * 28)) | 0; // * 0.11 ....
+      layerData[i] = (layerData[i] * y) >> 16;
+    }
+  }
+
   function genericComposeSMask(maskCtx, layerCtx, width, height,
                                subtype, backdrop) {
-    var addBackdropFn;
-    if (backdrop) {
-      addBackdropFn = function (r0, g0, b0, bytes) {
-        var length = bytes.length;
-        for (var i = 3; i < length; i += 4) {
-          var alpha = bytes[i] / 255;
-          if (alpha === 0) {
-            bytes[i - 3] = r0;
-            bytes[i - 2] = g0;
-            bytes[i - 1] = b0;
-          } else if (alpha < 1) {
-            var alpha_ = 1 - alpha;
-            bytes[i - 3] = (bytes[i - 3] * alpha + r0 * alpha_) | 0;
-            bytes[i - 2] = (bytes[i - 2] * alpha + g0 * alpha_) | 0;
-            bytes[i - 1] = (bytes[i - 1] * alpha + b0 * alpha_) | 0;
-          }
-        }
-      }.bind(null, backdrop[0], backdrop[1], backdrop[2]);
-    } else {
-      addBackdropFn = function () {};
-    }
+    var hasBackdrop = backdrop !== undefined;
+    var r0 = hasBackdrop ? backdrop[0] : 0;
+    var g0 = hasBackdrop ? backdrop[1] : 0;
+    var b0 = hasBackdrop ? backdrop[2] : 0;
 
     var composeFn;
     if (subtype === 'Luminosity') {
-      composeFn = function (maskDataBytes, layerDataBytes) {
-        var length = maskDataBytes.length;
-        for (var i = 3; i < length; i += 4) {
-          var y = ((maskDataBytes[i - 3] * 77) +     // * 0.3 / 255 * 0x10000
-                   (maskDataBytes[i - 2] * 152) +    // * 0.59 ....
-                   (maskDataBytes[i - 1] * 28)) | 0; // * 0.11 ....
-          layerDataBytes[i] = (layerDataBytes[i] * y) >> 16;
-        }
-      };
+      composeFn = composeSMaskLuminosity;
     } else {
-      composeFn = function (maskDataBytes, layerDataBytes) {
-        var length = maskDataBytes.length;
-        for (var i = 3; i < length; i += 4) {
-          var alpha = maskDataBytes[i];
-          layerDataBytes[i] = (layerDataBytes[i] * alpha / 255) | 0;
-        }
-      };
+      composeFn = composeSMaskAlpha;
     }
 
     // processing image in chunks to save memory
@@ -654,7 +659,9 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
       var maskData = maskCtx.getImageData(0, row, width, chunkHeight);
       var layerData = layerCtx.getImageData(0, row, width, chunkHeight);
 
-      addBackdropFn(maskData.data);
+      if (hasBackdrop) {
+        composeSMaskBackdrop(maskData.data, r0, g0, b0);
+      }
       composeFn(maskData.data, layerData.data);
 
       maskCtx.putImageData(layerData, 0, row);