Fix distorted line pattern fill when line offset is large compared

with pattern distance

(cherry picked from commit e277b91)

Author: nyalldawson

src/core/symbology-ng/qgsfillsymbollayerv2.cpp

@@ -2569,6 +2569,15 @@ void QgsLinePatternFillSymbolLayer::applyPattern( const QgsSymbolV2RenderContext
   double outputPixelDist = distance * QgsSymbolLayerV2Utils::pixelSizeScaleFactor( ctx, mDistanceUnit, mDistanceMapUnitScale );
   double outputPixelOffset = mOffset * QgsSymbolLayerV2Utils::pixelSizeScaleFactor( ctx,  mOffsetUnit, mOffsetMapUnitScale );
 
+  // NOTE: this may need to be modified if we ever change from a forced rasterized/brush approach,
+  // because potentially we may want to allow vector based line pattern fills where the first line
+  // is offset by a large distance
+
+  // fix truncated pattern with larger offsets
+  outputPixelOffset = fmod( outputPixelOffset, outputPixelDist );
+  if ( outputPixelOffset > outputPixelDist / 2.0 )
+    outputPixelOffset -= outputPixelDist;
+
   // To get all patterns into image, we have to consider symbols size (estimateMaxBleed()).
   // For marker lines we have to get markers interval.
   double outputPixelBleed = 0;

tests/src/core/testqgslinefillsymbol.cpp

@@ -61,6 +61,7 @@ class TestQgsLineFillSymbol : public QObject
 
     void lineFillSymbol();
     void lineFillSymbolOffset();
+    void lineFillLargeOffset();
 
     void dataDefinedSubSymbol();
 
@@ -161,6 +162,17 @@ void TestQgsLineFillSymbol::lineFillSymbolOffset()
   mLineFill->setOffset( 0 );
 }
 
+void TestQgsLineFillSymbol::lineFillLargeOffset()
+{
+  // test line fill with large offset compared to line distance
+  mLineFill->setOffset( 8 );
+  QVERIFY( imageCheck( "symbol_linefill_large_posoffset" ) );
+
+  mLineFill->setOffset( -8 );
+  QVERIFY( imageCheck( "symbol_linefill_large_negoffset" ) );
+  mLineFill->setOffset( 0 );
+}
+
 void TestQgsLineFillSymbol::dataDefinedSubSymbol()
 {
   mReport += "<h2>Line fill symbol data defined sub symbol test</h2>\n";