Don't limit label distances to postive values

Sometimes it's useful to have negative distances, as a 0 distance
can still be quite far from line features for some font families.

src/core/qgspallabeling.cpp

@@ -192,7 +192,7 @@ void QgsPalLayerSettings::initPropertyDefinitions()
     },
     { QgsPalLayerSettings::OffsetXY, QgsPropertyDefinition( "OffsetXY", QObject::tr( "Offset" ), QgsPropertyDefinition::Offset, origin ) },
     { QgsPalLayerSettings::OffsetUnits, QgsPropertyDefinition( "OffsetUnits", QObject::tr( "Offset units" ), QgsPropertyDefinition::RenderUnits, origin ) },
-    { QgsPalLayerSettings::LabelDistance, QgsPropertyDefinition( "LabelDistance", QObject::tr( "Label distance" ), QgsPropertyDefinition::DoublePositive, origin ) },
+    { QgsPalLayerSettings::LabelDistance, QgsPropertyDefinition( "LabelDistance", QObject::tr( "Label distance" ), QgsPropertyDefinition::Double, origin ) },
     { QgsPalLayerSettings::DistanceUnits, QgsPropertyDefinition( "DistanceUnits", QObject::tr( "Label distance units" ), QgsPropertyDefinition::RenderUnits, origin ) },
     { QgsPalLayerSettings::OffsetRotation, QgsPropertyDefinition( "OffsetRotation", QObject::tr( "Offset rotation" ), QgsPropertyDefinition::Rotation, origin ) },
     { QgsPalLayerSettings::CurvedCharAngleInOut, QgsPropertyDefinition( "CurvedCharAngleInOut", QgsPropertyDefinition::DataTypeString, QObject::tr( "Curved character angles" ), QObject::tr( "double coord [<b>in,out</b> as 20.0-60.0,20.0-95.0]" ), origin ) },
@@ -2069,7 +2069,7 @@ void QgsPalLayerSettings::registerFeature( const QgsFeature &f, QgsRenderContext
   // candidates are created just offset from a border and avoids candidates being incorrectly flagged as colliding with neighbours
   if ( placement == QgsPalLayerSettings::Line || placement == QgsPalLayerSettings::Curved || placement == QgsPalLayerSettings::PerimeterCurved )
   {
-    distance = std::max( distance, 1.0 );
+    distance = ( distance < 0 ? -1 : 1 ) * std::max( std::fabs( distance ), 1.0 );
   }
 
   if ( !qgsDoubleNear( distance, 0.0 ) )

src/gui/qgstextformatwidget.cpp

@@ -150,6 +150,7 @@ void QgsTextFormatWidget::initWidget()
   mFontLetterSpacingSpinBox->setClearValue( 0.0 );
   mFontWordSpacingSpinBox->setClearValue( 0.0 );
   mZIndexSpinBox->setClearValue( 0.0 );
+  mLineDistanceSpnBx->setClearValue( 0.0 );
 
   mObstacleTypeComboBox->addItem( tr( "Over the feature's interior" ), QgsPalLayerSettings::PolygonInterior );
   mObstacleTypeComboBox->addItem( tr( "Over the feature's boundary" ), QgsPalLayerSettings::PolygonBoundary );

src/ui/qgstextformatwidgetbase.ui

@@ -4142,6 +4142,9 @@ font-style: italic;</string>
                            <property name="decimals">
                             <number>4</number>
                            </property>
+                           <property name="minimum">
+                            <double>-999999999.000000000000000</double>
+                           </property>
                            <property name="maximum">
                             <double>999999999.000000000000000</double>
                            </property>

tests/src/core/testqgslabelingengine.cpp

@@ -57,6 +57,7 @@ class TestQgsLabelingEngine : public QObject
     void testMergingLinesWithForks();
     void testCurvedLabelsWithTinySegments();
     void testCurvedLabelCorrectLinePlacement();
+    void testCurvedLabelNegativeDistance();
     void testLabelBoundary();
     void testLabelBlockingRegion();
     void testLabelRotationWithReprojection();
@@ -1104,6 +1105,58 @@ void TestQgsLabelingEngine::testCurvedLabelCorrectLinePlacement()
   QVERIFY( imageCheck( QStringLiteral( "label_curved_label_below_1" ), img, 20 ) );
 }
 
+void TestQgsLabelingEngine::testCurvedLabelNegativeDistance()
+{
+  // test line label rendering with negative distance
+  QgsPalLayerSettings settings;
+  setDefaultLabelParams( settings );
+
+  QgsTextFormat format = settings.format();
+  format.setSize( 20 );
+  format.setColor( QColor( 0, 0, 0 ) );
+  settings.setFormat( format );
+
+  settings.fieldName = QStringLiteral( "'XXXXXXXXXXXXXXXXXXXXXXXXXX'" );
+  settings.isExpression = true;
+  settings.placement = QgsPalLayerSettings::Curved;
+  settings.labelPerPart = false;
+  settings.dist = -5;
+
+  std::unique_ptr< QgsVectorLayer> vl2( new QgsVectorLayer( QStringLiteral( "LineString?crs=epsg:3946&field=id:integer" ), QStringLiteral( "vl" ), QStringLiteral( "memory" ) ) );
+  vl2->setRenderer( new QgsNullSymbolRenderer() );
+
+  QgsFeature f;
+  f.setAttributes( QgsAttributes() << 1 );
+  f.setGeometry( QgsGeometry::fromWkt( QStringLiteral( "LineString (190000 5000010, 190100 5000000, 190200 5000000)" ) ) );
+  QVERIFY( vl2->dataProvider()->addFeature( f ) );
+
+  vl2->setLabeling( new QgsVectorLayerSimpleLabeling( settings ) );  // TODO: this should not be necessary!
+  vl2->setLabelsEnabled( true );
+
+  // make a fake render context
+  QSize size( 640, 480 );
+  QgsMapSettings mapSettings;
+  mapSettings.setDestinationCrs( vl2->crs() );
+
+  mapSettings.setOutputSize( size );
+  mapSettings.setExtent( f.geometry().boundingBox() );
+  mapSettings.setLayers( QList<QgsMapLayer *>() << vl2.get() );
+  mapSettings.setOutputDpi( 96 );
+
+  QgsLabelingEngineSettings engineSettings = mapSettings.labelingEngineSettings();
+  engineSettings.setFlag( QgsLabelingEngineSettings::UsePartialCandidates, false );
+  engineSettings.setFlag( QgsLabelingEngineSettings::DrawLabelRectOnly, true );
+  //engineSettings.setFlag( QgsLabelingEngineSettings::DrawCandidates, true );
+  mapSettings.setLabelingEngineSettings( engineSettings );
+
+  QgsMapRendererSequentialJob job( mapSettings );
+  job.start();
+  job.waitForFinished();
+
+  QImage img = job.renderedImage();
+  QVERIFY( imageCheck( QStringLiteral( "label_curved_negative_distance" ), img, 20 ) );
+}
+
 void TestQgsLabelingEngine::testLabelBoundary()
 {
   // test that no labels are drawn outside of the specified label boundary