Print with the correct physical dimensions when specified

components/printing/renderer/print_web_view_helper.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/printing/renderer/print_web_view_helper.h"
 
 #include <stddef.h>
 #include <stdint.h>
 #include <string>
 #include <utility>
@@ skipping 59 matching lines @@
 
 enum PrintPreviewHelperEvents {
   PREVIEW_EVENT_REQUESTED,
   PREVIEW_EVENT_CACHE_HIT,  // Unused
   PREVIEW_EVENT_CREATE_DOCUMENT,
   PREVIEW_EVENT_NEW_SETTINGS,  // Unused
   PREVIEW_EVENT_MAX,
 };
 
 const double kMinDpi = 1.0;
+const float kPrintingMinimumShrinkFactor = 1.333f;

[Lei Zhang, 2016/03/29 01:00:46]

Can you reference third_party/WebKit/Source/core/page/PrintContext.cpp which has
the same constant?

 
 #if defined(ENABLE_PRINT_PREVIEW)
 bool g_is_preview_enabled = true;
 
 const char kPageLoadScriptFormat[] =
     "document.open(); document.write(%s); document.close();";
 
 const char kPageSetupScriptFormat[] = "setup(%s);";
 
 void ExecuteScript(blink::WebFrame* frame,
@@ skipping 580 matching lines @@
   }
   ComputeWebKitPrintParamsInDesiredDpi(print_params, &web_print_params_);
 }
 
 PrepareFrameAndViewForPrint::~PrepareFrameAndViewForPrint() {
   FinishPrinting();
 }
 
 void PrepareFrameAndViewForPrint::ResizeForPrinting() {
   // Layout page according to printer page size. Since WebKit shrinks the
-  // size of the page automatically (from 125% to 200%) we trick it to
-  // think the page is 125% larger so the size of the page is correct for
+  // size of the page automatically (from 133.3% to 200%) we trick it to
+  // think the page is 133.3% larger so the size of the page is correct for
   // minimum (default) scaling.
+  // The scaling factor 1.25 was originally chosen as a magic number that
+  // was 'about right'. However per crbug.com/273306 1.333 seems to be

[Lei Zhang, 2016/03/29 01:00:46]

https://crbug.com/...

+  // the number that produces output with the correct physical size for elements
+  // that are specified in cm, mm, pt etc.
   // This is important for sites that try to fill the page.
-  // The 1.25 value is |printingMinimumShrinkFactor|.
   gfx::Size print_layout_size(web_print_params_.printContentArea.width,
                               web_print_params_.printContentArea.height);
   print_layout_size.set_height(
-      static_cast<int>(static_cast<double>(print_layout_size.height()) * 1.25));
+      static_cast<int>(static_cast<double>(print_layout_size.height()) *
+                       kPrintingMinimumShrinkFactor));
 
   if (!frame())
     return;
   blink::WebView* web_view = frame_.view();
   // Backup size and offset.
   if (blink::WebFrame* web_frame = web_view->mainFrame())
     prev_scroll_offset_ = web_frame->scrollOffset();
   prev_view_size_ = web_view->size();
 
   web_view->resize(print_layout_size);
@@ skipping 1074 matching lines @@
 
   SkCanvas* canvas = metafile->GetVectorCanvasForNewPage(
       page_size, canvas_area, scale_factor);
   if (!canvas)
     return;
 
   MetafileSkiaWrapper::SetMetafileOnCanvas(*canvas, metafile);
 
 #if defined(ENABLE_PRINT_PREVIEW)
   if (params.params.display_header_footer) {
-    // TODO(thestig): Figure out why Linux needs this. The value may be
-    // |printingMinimumShrinkFactor|.
+    // TODO(thestig): Figure out why Linux needs this. It is almost certainly
+    // |printingMinimumShrinkFactor| from Blink.
 #if defined(OS_WIN)
     const float fudge_factor = 1;
 #else
-    const float fudge_factor = 1.25;
+    const float fudge_factor = kPrintingMinimumShrinkFactor;
 #endif
     // |page_number| is 0-based, so 1 is added.
     PrintHeaderAndFooter(canvas, params.page_number + 1,
                          print_preview_context_.total_page_count(), *frame,
                          scale_factor / fudge_factor, page_layout_in_points,
                          params.params);
   }
 #endif  // defined(ENABLE_PRINT_PREVIEW)
 
   float webkit_scale_factor =
@@ skipping 439 matching lines @@
       blink::WebConsoleMessage::LevelWarning, message));
   return false;
 }
 
 void PrintWebViewHelper::ScriptingThrottler::Reset() {
   // Reset counter on successful print.
   count_ = 0;
 }
 
 }  // namespace printing