From ec311f303b40ed6457278ec157d49fb761144f36 Mon Sep 17 00:00:00 2001
From: Brendan Dahl <brendan.dahl@gmail.com>
Date: Fri, 29 Mar 2013 13:26:25 -0700
Subject: [PATCH] Use white backdrop when possible.

---
 src/api.js       |  6 +++---
 src/canvas.js    | 18 +++++++++++++++++-
 src/evaluator.js | 14 +++++++++++---
 test/driver.js   |  5 +----
 web/viewer.js    |  1 -
 5 files changed, 32 insertions(+), 12 deletions(-)

diff --git a/src/api.js b/src/api.js
index a5ea5b4cd..cc4852319 100644
--- a/src/api.js
+++ b/src/api.js
@@ -384,11 +384,11 @@ var PDFPageProxy = (function PDFPageProxyClosure() {
       var stats = this.stats;
       stats.time('Rendering');
 
-      gfx.beginDrawing(viewport);
+      var operatorList = this.operatorList;
+      gfx.beginDrawing(viewport, operatorList.transparency);
 
       var startIdx = 0;
-      var length = this.operatorList.fnArray.length;
-      var operatorList = this.operatorList;
+      var length = operatorList.fnArray.length;
       var stepper = null;
       if (PDFJS.pdfBug && 'StepperManager' in globalScope &&
           globalScope['StepperManager'].enabled) {
diff --git a/src/canvas.js b/src/canvas.js
index 5db39ebbb..88d58d64f 100644
--- a/src/canvas.js
+++ b/src/canvas.js
@@ -415,7 +415,23 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
       'shadingFill': true
     },
 
-    beginDrawing: function CanvasGraphics_beginDrawing(viewport) {
+    beginDrawing: function CanvasGraphics_beginDrawing(viewport, transparency) {
+      // For pdfs that use blend modes we have to clear the canvas else certain
+      // blend modes can look wrong since we'd be blending with a white
+      // backdrop. The problem with a transparent backdrop though is we then
+      // don't get sub pixel anti aliasing on text, so we fill with white if
+      // we can.
+      var width = this.ctx.canvas.width;
+      var height = this.ctx.canvas.height;
+      if (transparency) {
+        this.ctx.clearRect(0, 0, width, height);
+      } else {
+        this.ctx.save();
+        this.ctx.fillStyle = 'rgb(255, 255, 255)';
+        this.ctx.fillRect(0, 0, width, height);
+        this.ctx.restore();
+      }
+
       var transform = viewport.transform;
       this.ctx.save();
       this.ctx.transform.apply(this.ctx, transform);
diff --git a/src/evaluator.js b/src/evaluator.js
index 648762b81..77edf34b1 100644
--- a/src/evaluator.js
+++ b/src/evaluator.js
@@ -361,8 +361,11 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
           }, handler, xref, resources, image, inline);
       }
 
-      if (!queue)
-        queue = {};
+      if (!queue) {
+        queue = {
+          transparency: false
+        };
+      }
 
       if (!queue.argsArray) {
         queue.argsArray = [];
@@ -532,7 +535,6 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
                     case 'FL':
                     case 'CA':
                     case 'ca':
-                    case 'BM':
                       gsStateObj.push([key, value]);
                       break;
                     case 'Font':
@@ -542,6 +544,12 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
                         value[1]
                       ]);
                       break;
+                    case 'BM':
+                      if (!isName(value) || value.name !== 'Normal') {
+                        queue.transparency = true;
+                      }
+                      gsStateObj.push([key, value]);
+                      break;
                     case 'SMask':
                       // We support the default so don't trigger the TODO.
                       if (!isName(value) || value.name != 'None')
diff --git a/test/driver.js b/test/driver.js
index bb56877ca..e20cc3919 100644
--- a/test/driver.js
+++ b/test/driver.js
@@ -394,10 +394,7 @@ function info(message) {
 }
 
 function clear(ctx) {
-  ctx.save();
-  ctx.fillStyle = 'rgb(255, 255, 255)';
-  ctx.fillRect(0, 0, canvas.width, canvas.height);
-  ctx.restore();
+  ctx.clearRect(0, 0, canvas.width, canvas.height);
 }
 
 /* Auto-scroll if the scrollbar is near the bottom, otherwise do nothing. */
diff --git a/web/viewer.js b/web/viewer.js
index 232f8fbe5..a68b69cd9 100644
--- a/web/viewer.js
+++ b/web/viewer.js
@@ -2142,7 +2142,6 @@ var PageView = function pageView(container, pdfPage, id, scale,
     }
 
     var ctx = canvas.getContext('2d');
-    ctx.clearRect(0, 0, canvas.width, canvas.height);
     // TODO(mack): use data attributes to store these
     ctx._scaleX = outputScale.sx;
     ctx._scaleY = outputScale.sy;