[FEATURE][composer] HTML frames no longer cut through lines of text when splitting over multiple pages (sponsored by City of Uster, Switzerland)

Author: nyalldawson

src/core/composer/qgscomposerframe.h

@@ -43,7 +43,6 @@ class CORE_EXPORT QgsComposerFrame: public QgsComposerItem
 
     QgsComposerMultiFrame* multiFrame() const { return mMultiFrame; }
 
-
   private:
     QgsComposerFrame(); //forbidden
     QgsComposerMultiFrame* mMultiFrame;

src/core/composer/qgscomposerhtml.cpp

@@ -21,9 +21,10 @@
 #include <QPainter>
 #include <QWebFrame>
 #include <QWebPage>
+#include <QImage>
 
 QgsComposerHtml::QgsComposerHtml( QgsComposition* c, bool createUndoCommands ): QgsComposerMultiFrame( c, createUndoCommands ),
-    mWebPage( 0 ), mLoaded( false ), mHtmlUnitsToMM( 1.0 )
+    mWebPage( 0 ), mLoaded( false ), mHtmlUnitsToMM( 1.0 ), mRenderedPage( 0 )
 {
   mHtmlUnitsToMM = htmlUnitsToMM();
   mWebPage = new QWebPage();
@@ -34,13 +35,14 @@ QgsComposerHtml::QgsComposerHtml( QgsComposition* c, bool createUndoCommands ):
   }
 }
 
-QgsComposerHtml::QgsComposerHtml(): QgsComposerMultiFrame( 0, false ), mWebPage( 0 ), mLoaded( false ), mHtmlUnitsToMM( 1.0 )
+QgsComposerHtml::QgsComposerHtml(): QgsComposerMultiFrame( 0, false ), mWebPage( 0 ), mLoaded( false ), mHtmlUnitsToMM( 1.0 ), mRenderedPage( 0 )
 {
 }
 
 QgsComposerHtml::~QgsComposerHtml()
 {
   delete mWebPage;
+  delete mRenderedPage;
 }
 
 void QgsComposerHtml::setUrl( const QUrl& url )
@@ -67,6 +69,9 @@ void QgsComposerHtml::setUrl( const QUrl& url )
   mWebPage->mainFrame()->setScrollBarPolicy( Qt::Vertical, Qt::ScrollBarAlwaysOff );
   mSize.setWidth( contentsSize.width() / mHtmlUnitsToMM );
   mSize.setHeight( contentsSize.height() / mHtmlUnitsToMM );
+
+  renderCachedImage();
+
   recalculateFrameSizes();
   emit changed();
 }
@@ -77,6 +82,20 @@ void QgsComposerHtml::frameLoaded( bool ok )
   mLoaded = true;
 }
 
+void QgsComposerHtml::renderCachedImage()
+{
+  //render page to cache image
+  if ( mRenderedPage )
+  {
+    delete mRenderedPage;
+  }
+  mRenderedPage = new QImage( mWebPage->viewportSize(), QImage::Format_ARGB32 );
+  QPainter painter;
+  painter.begin( mRenderedPage );
+  mWebPage->mainFrame()->render( &painter );
+  painter.end();
+}
+
 QSizeF QgsComposerHtml::totalSize() const
 {
   return mSize;
@@ -121,6 +140,94 @@ void QgsComposerHtml::addFrame( QgsComposerFrame* frame, bool recalcFrameSizes )
   }
 }
 
+bool candidateSort( const QPair<int, int> &c1, const QPair<int, int> &c2 )
+{
+  if ( c1.second < c2.second )
+    return true;
+  else if ( c1.second > c2.second )
+    return false;
+  else if ( c1.first > c2.first )
+    return true;
+  else
+    return false;
+}
+
+double QgsComposerHtml::findNearbyPageBreak( double yPos )
+{
+  if ( !mWebPage || !mRenderedPage )
+  {
+    return yPos;
+  }
+
+  //convert yPos to pixels
+  int idealPos = yPos * htmlUnitsToMM();
+
+  //if ideal break pos is past end of page, there's nothing we need to do
+  if ( idealPos >= mRenderedPage->height() )
+  {
+    return yPos;
+  }
+
+  int maxSearchDistance = 200;
+
+  //loop through all lines just before ideal break location, up to max distance
+  //of maxSearchDistance
+  int changes = 0;
+  QRgb currentColor;
+  QRgb pixelColor;
+  QList< QPair<int, int> > candidates;
+  for ( int candidateRow = idealPos; candidateRow >= idealPos - maxSearchDistance; --candidateRow )
+  {
+    changes = 0;
+    currentColor = qRgba( 0, 0, 0, 0 );
+    //check all pixels in this line
+    for ( int col = 0; col < mRenderedPage->width(); ++col )
+    {
+      //count how many times the pixels change color in this row
+      //eventually, we select a row to break at with the minimum number of color changes
+      //since this is likely a line break, or gap between table cells, etc
+      //but very unlikely to be midway through a text line or picture
+      pixelColor = mRenderedPage->pixel( col, candidateRow );
+      if ( pixelColor != currentColor )
+      {
+        //color has changed
+        currentColor = pixelColor;
+        changes++;
+      }
+    }
+    candidates.append( qMakePair( candidateRow, changes ) );
+  }
+
+  //sort candidate rows by number of changes ascending, row number descending
+  qSort( candidates.begin(), candidates.end(), candidateSort );
+  //first candidate is now the largest row with smallest number of changes
+
+  //ok, now take the mid point of the best candidate position
+  //we do this so that the spacing between text lines is likely to be split in half
+  //otherwise the html will be broken immediately above a line of text, which
+  //looks a little messy
+  int maxCandidateRow = candidates[0].first;
+  int minCandidateRow = maxCandidateRow + 1;
+  int minCandidateChanges = candidates[0].second;
+
+  QList< QPair<int, int> >::iterator it;
+  for ( it = candidates.begin(); it != candidates.end(); ++it )
+  {
+    if (( *it ).second != minCandidateChanges || ( *it ).first != minCandidateRow - 1 )
+    {
+      //no longer in a consecutive block of rows of minimum pixel colour changes
+      //so return the row mid-way through the block
+      //first converting back to mm
+      return ( minCandidateRow + ( maxCandidateRow - minCandidateRow ) / 2 ) / htmlUnitsToMM();
+    }
+    minCandidateRow = ( *it ).first;
+  }
+
+  //above loop didn't work for some reason
+  //return first candidate converted to mm
+  return candidates[0].first / htmlUnitsToMM();
+}
+
 bool QgsComposerHtml::writeXML( QDomElement& elem, QDomDocument & doc, bool ignoreFrames ) const
 {
   QDomElement htmlElem = doc.createElement( "ComposerHtml" );

src/core/composer/qgscomposerhtml.h

@@ -20,6 +20,7 @@
 #include <QUrl>
 
 class QWebPage;
+class QImage;
 
 class CORE_EXPORT QgsComposerHtml: public QgsComposerMultiFrame
 {
@@ -40,6 +41,9 @@ class CORE_EXPORT QgsComposerHtml: public QgsComposerMultiFrame
 
     void addFrame( QgsComposerFrame* frame, bool recalcFrameSizes = true );
 
+    //overriden to break frames without dividing lines of text
+    double findNearbyPageBreak( double yPos );
+
   private slots:
     void frameLoaded( bool ok );
 
@@ -49,8 +53,12 @@ class CORE_EXPORT QgsComposerHtml: public QgsComposerMultiFrame
     bool mLoaded;
     QSizeF mSize; //total size in mm
     double mHtmlUnitsToMM;
+    QImage* mRenderedPage;
 
     double htmlUnitsToMM(); //calculate scale factor
+
+    //renders a snapshot of the page to a cached image
+    void renderCachedImage();
 };
 
 #endif // QGSCOMPOSERHTML_H