[FEATURE] Interactive curve editing for property overrides

This adds a new interactive "curve" to the assistant widgets.
It allows you to fine tune exactly how input values get
mapped to output sizes/colors/etc.

Think GIMP or Photoshop curves, but for your data...

python/gui/gui.sip

@@ -51,6 +51,7 @@
 %Include qgsconfigureshortcutsdialog.sip
 %Include qgscredentialdialog.sip
 %Include qgscustomdrophandler.sip
+%Include qgscurveeditorwidget.sip
 %Include qgsdetaileditemdata.sip
 %Include qgsdetaileditemdelegate.sip
 %Include qgsdetaileditemwidget.sip

python/gui/qgscurveeditorwidget.sip

@@ -0,0 +1,23 @@
+class QgsCurveEditorWidget : QWidget
+{
+%TypeHeaderCode
+#include <qgscurveeditorwidget.h>
+%End
+
+  public:
+
+    QgsCurveEditorWidget( QWidget* parent /TransferThis/ = 0, const QgsCurveTransform& curve = QgsCurveTransform() );
+
+    QgsCurveTransform curve() const;
+
+    void setCurve( const QgsCurveTransform& curve );
+
+  signals:
+
+    void changed();
+
+  protected:
+
+    virtual void keyPressEvent( QKeyEvent *event );
+
+};

src/gui/CMakeLists.txt

@@ -193,6 +193,7 @@ SET(QGIS_GUI_SRCS
   qgscredentialdialog.cpp
   qgscursors.cpp
   qgscustomdrophandler.cpp
+  qgscurveeditorwidget.cpp
   qgsdatumtransformdialog.cpp
   qgsdetaileditemdata.cpp
   qgsdetaileditemdelegate.cpp
@@ -345,6 +346,7 @@ SET(QGIS_GUI_MOC_HDRS
   qgscompoundcolorwidget.h
   qgsconfigureshortcutsdialog.h
   qgscredentialdialog.h
+  qgscurveeditorwidget.h
   qgsdatumtransformdialog.h
   qgsdetaileditemdelegate.h
   qgsdetaileditemwidget.h

src/gui/qgscurveeditorwidget.cpp

@@ -0,0 +1,312 @@
+/***************************************************************************
+    qgscurveeditorwidget.cpp
+    ------------------------
+    begin                : February 2017
+    copyright            : (C) 2017 by Nyall Dawson
+    email                : nyall dot dawson at gmail dot com
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+
+
+#include "qgscurveeditorwidget.h"
+
+#include <qmath.h>
+#include <QPainter>
+#include <QVBoxLayout>
+#include <QMouseEvent>
+
+// QWT Charting widget
+#include <qwt_global.h>
+#include <qwt_plot_canvas.h>
+#include <qwt_plot.h>
+#include <qwt_plot_curve.h>
+#include <qwt_plot_grid.h>
+#include <qwt_plot_marker.h>
+#include <qwt_plot_picker.h>
+#include <qwt_picker_machine.h>
+#include <qwt_plot_layout.h>
+#include <qwt_symbol.h>
+#include <qwt_legend.h>
+
+QgsCurveEditorWidget::QgsCurveEditorWidget( QWidget* parent, const QgsCurveTransform& transform )
+    : QWidget( parent )
+    , mCurve( transform )
+    , mCurrentPlotMarkerIndex( -1 )
+{
+  mPlot = new QwtPlot();
+  mPlot->setMinimumSize( QSize( 0, 100 ) );
+  mPlot->setAxisScale( QwtPlot::yLeft, 0, 1 );
+  mPlot->setAxisScale( QwtPlot::yRight, 0, 1 );
+  mPlot->setAxisScale( QwtPlot::xBottom, 0, 1 );
+  mPlot->setAxisScale( QwtPlot::xTop, 0, 1 );
+
+  QVBoxLayout* vlayout = new QVBoxLayout();
+  vlayout->addWidget( mPlot );
+  setLayout( vlayout );
+
+  // hide the ugly canvas frame
+  mPlot->setFrameStyle( QFrame::NoFrame );
+#if defined(QWT_VERSION) && QWT_VERSION>=0x060000
+  QFrame* plotCanvasFrame = dynamic_cast<QFrame*>( mPlot->canvas() );
+  if ( plotCanvasFrame )
+    plotCanvasFrame->setFrameStyle( QFrame::NoFrame );
+#else
+  mPlot->canvas()->setFrameStyle( QFrame::NoFrame );
+#endif
+
+  mPlot->enableAxis( QwtPlot::yLeft, false );
+  mPlot->enableAxis( QwtPlot::xBottom, false );
+
+  // add a grid
+  QwtPlotGrid * grid = new QwtPlotGrid();
+  QwtScaleDiv gridDiv( 0.0, 1.0, QList<double>(), QList<double>(), QList<double>() << 0.2 << 0.4 << 0.6 << 0.8 );
+  grid->setXDiv( gridDiv );
+  grid->setYDiv( gridDiv );
+  grid->setPen( QPen( QColor( 0, 0, 0, 50 ) ) );
+  grid->attach( mPlot );
+
+  mPlotCurve = new QwtPlotCurve();
+  mPlotCurve->setTitle( QStringLiteral( "Curve" ) );
+  mPlotCurve->setPen( QPen( QColor( 30, 30, 30 ), 0.0 ) ),
+  mPlotCurve->setRenderHint( QwtPlotItem::RenderAntialiased, true );
+  mPlotCurve->attach( mPlot );
+
+  mPlotFilter = new QgsCurveEditorPlotEventFilter( mPlot );
+  connect( mPlotFilter, &QgsCurveEditorPlotEventFilter::mousePress, this, &QgsCurveEditorWidget::plotMousePress );
+  connect( mPlotFilter, &QgsCurveEditorPlotEventFilter::mouseRelease, this, &QgsCurveEditorWidget::plotMouseRelease );
+  connect( mPlotFilter, &QgsCurveEditorPlotEventFilter::mouseMove, this, &QgsCurveEditorWidget::plotMouseMove );
+
+  mPlotCurve->setVisible( true );
+  updatePlot();
+}
+
+void QgsCurveEditorWidget::setCurve( const QgsCurveTransform& curve )
+{
+  mCurve = curve;
+  updatePlot();
+  emit changed();
+}
+
+void QgsCurveEditorWidget::keyPressEvent( QKeyEvent* event )
+{
+  if ( event->key() == Qt::Key_Delete || event->key() == Qt::Key_Backspace )
+  {
+    QList< QgsPoint > cp = mCurve.controlPoints();
+    if ( mCurrentPlotMarkerIndex > 0 && mCurrentPlotMarkerIndex < cp.count() - 1 )
+    {
+      cp.removeAt( mCurrentPlotMarkerIndex );
+      mCurve.setControlPoints( cp );
+      updatePlot();
+      emit changed();
+    }
+  }
+}
+
+void QgsCurveEditorWidget::plotMousePress( QPointF point )
+{
+  mCurrentPlotMarkerIndex = findNearestControlPoint( point );
+  if ( mCurrentPlotMarkerIndex < 0 )
+  {
+    // add a new point
+    mCurve.addControlPoint( point.x(), point.y() );
+    mCurrentPlotMarkerIndex = findNearestControlPoint( point );
+    emit changed();
+  }
+  updatePlot();
+}
+
+
+int QgsCurveEditorWidget::findNearestControlPoint( QPointF point ) const
+{
+  double minDist = 3.0 / mPlot->width();
+  int currentPlotMarkerIndex = -1;
+
+  QList< QgsPoint > controlPoints = mCurve.controlPoints();
+
+  for ( int i = 0; i < controlPoints.count(); ++i )
+  {
+    QgsPoint currentPoint = controlPoints.at( i );
+    double currentDist;
+    currentDist = qPow( point.x() - currentPoint.x(), 2.0 ) + qPow( point.y() - currentPoint.y(), 2.0 );
+    if ( currentDist < minDist )
+    {
+      minDist = currentDist;
+      currentPlotMarkerIndex = i;
+    }
+  }
+  return currentPlotMarkerIndex;
+}
+
+
+void QgsCurveEditorWidget::plotMouseRelease( QPointF )
+{
+}
+
+void QgsCurveEditorWidget::plotMouseMove( QPointF point )
+{
+  if ( mCurrentPlotMarkerIndex < 0 )
+    return;
+
+  QList< QgsPoint > cp = mCurve.controlPoints();
+  bool removePoint = false;
+  if ( mCurrentPlotMarkerIndex == 0 )
+  {
+    point.setX( qMin( point.x(), cp.at( 1 ).x() - 0.01 ) );
+  }
+  else
+  {
+    removePoint = point.x() <= cp.at( mCurrentPlotMarkerIndex - 1 ).x();
+  }
+  if ( mCurrentPlotMarkerIndex == cp.count() - 1 )
+  {
+    point.setX( qMax( point.x(), cp.at( mCurrentPlotMarkerIndex - 1 ).x() + 0.01 ) );
+    removePoint = false;
+  }
+  else
+  {
+    removePoint = removePoint || point.x() >= cp.at( mCurrentPlotMarkerIndex + 1 ).x();
+  }
+
+  if ( removePoint )
+  {
+    cp.removeAt( mCurrentPlotMarkerIndex );
+    mCurrentPlotMarkerIndex = -1;
+  }
+  else
+  {
+    cp[ mCurrentPlotMarkerIndex ] = QgsPoint( point.x(), point.y() );
+  }
+  mCurve.setControlPoints( cp );
+  updatePlot();
+  emit changed();
+}
+
+void QgsCurveEditorWidget::addPlotMarker( double x, double y, bool isSelected )
+{
+  QColor borderColor( 0, 0, 0 );
+
+  QColor brushColor = isSelected ? borderColor : QColor( 255, 255, 255, 0 );
+
+  QwtPlotMarker *marker = new QwtPlotMarker();
+#if defined(QWT_VERSION) && QWT_VERSION>=0x060000
+  marker->setSymbol( new QwtSymbol( QwtSymbol::Ellipse,  QBrush( brushColor ), QPen( borderColor, isSelected ? 2 : 1 ), QSize( 6, 6 ) ) );
+#else
+  marker->setSymbol( QwtSymbol( QwtSymbol::Ellipse,  QBrush( brushColor ), QPen( borderColor, isSelected ? 2 : 1 ), QSize( 6, 6 ) ) );
+#endif
+  marker->setValue( x, y );
+  marker->attach( mPlot );
+  marker->setRenderHint( QwtPlotItem::RenderAntialiased, true );
+  mMarkers << marker;
+}
+
+void QgsCurveEditorWidget::updatePlot()
+{
+  // remove existing markers
+  Q_FOREACH ( QwtPlotMarker* marker, mMarkers )
+  {
+    marker->detach();
+    delete marker;
+  }
+  mMarkers.clear();
+
+  QPolygonF curvePoints;
+  QVector< double > x;
+
+  int i = 0;
+  Q_FOREACH ( const QgsPoint& point, mCurve.controlPoints() )
+  {
+    x << point.x();
+    addPlotMarker( point.x(), point.y(), mCurrentPlotMarkerIndex == i );
+    i++;
+  }
+
+  //add extra intermediate points
+
+  for ( double p = 0; p <= 1.0; p += 0.01 )
+  {
+    x << p;
+  }
+  std::sort( x.begin(), x.end() );
+  QVector< double > y = mCurve.y( x );
+
+  for ( int j = 0; j < x.count(); ++j )
+  {
+    curvePoints << QPointF( x.at( j ), y.at( j ) );
+  }
+
+#if defined(QWT_VERSION) && QWT_VERSION>=0x060000
+  mPlotCurve->setSamples( curvePoints );
+#else
+  mPlotCurve->setData( curvePoints );
+#endif
+  mPlot->replot();
+}
+
+
+/// @cond PRIVATE
+
+QgsCurveEditorPlotEventFilter::QgsCurveEditorPlotEventFilter( QwtPlot *plot )
+    : QObject( plot )
+    , mPlot( plot )
+{
+  mPlot->canvas()->installEventFilter( this );
+}
+
+bool QgsCurveEditorPlotEventFilter::eventFilter( QObject *object, QEvent *event )
+{
+  if ( !mPlot->isEnabled() )
+    return QObject::eventFilter( object, event );
+
+  switch ( event->type() )
+  {
+    case QEvent::MouseButtonPress:
+    {
+      const QMouseEvent* mouseEvent = static_cast<QMouseEvent* >( event );
+      if ( mouseEvent->button() == Qt::LeftButton )
+      {
+        emit mousePress( mapPoint( mouseEvent->pos() ) );
+      }
+      break;
+    }
+    case QEvent::MouseMove:
+    {
+      const QMouseEvent* mouseEvent = static_cast<QMouseEvent* >( event );
+      if ( mouseEvent->buttons() & Qt::LeftButton )
+      {
+        // only emit when button pressed
+        emit mouseMove( mapPoint( mouseEvent->pos() ) );
+      }
+      break;
+    }
+    case QEvent::MouseButtonRelease:
+    {
+      const QMouseEvent* mouseEvent = static_cast<QMouseEvent* >( event );
+      if ( mouseEvent->button() == Qt::LeftButton )
+      {
+        emit mouseRelease( mapPoint( mouseEvent->pos() ) );
+      }
+      break;
+    }
+    default:
+      break;
+  }
+
+  return QObject::eventFilter( object, event );
+}
+
+QPointF QgsCurveEditorPlotEventFilter::mapPoint( QPointF point ) const
+{
+  if ( !mPlot )
+    return QPointF();
+
+  return QPointF( mPlot->canvasMap( QwtPlot::xBottom ).invTransform( point.x() ),
+                  mPlot->canvasMap( QwtPlot::yLeft ).invTransform( point.y() ) );
+}
+
+///@endcond

src/gui/qgscurveeditorwidget.h

@@ -0,0 +1,118 @@
+/***************************************************************************
+    qgscurveeditorwidget.h
+    ----------------------
+    begin                : February 2017
+    copyright            : (C) 2017 by Nyall Dawson
+    email                : nyall dot dawson at gmail dot com
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+
+#ifndef QGSCURVEEDITORWIDGET_H
+#define QGSCURVEEDITORWIDGET_H
+
+#include <QWidget>
+#include "qgis_gui.h"
+#include "qgspropertytransformer.h"
+
+class QwtPlot;
+class QwtPlotCurve;
+class QwtPlotMarker;
+class QgsCurveEditorPlotEventFilter;
+
+/** \ingroup gui
+ * \class QgsCurveEditorWidget
+ * A widget for manipulating QgsCurveTransform curves.
+ * \note added in QGIS 3.0
+ */
+class GUI_EXPORT QgsCurveEditorWidget : public QWidget
+{
+    Q_OBJECT
+
+  public:
+
+    /**
+     * Constructor for QgsCurveEditorWidget.
+     */
+    QgsCurveEditorWidget( QWidget* parent = nullptr, const QgsCurveTransform& curve = QgsCurveTransform() );
+
+    /**
+     * Returns a curve representing the current curve from the widget.
+     * @see setCurve()
+     */
+    QgsCurveTransform curve() const { return mCurve; }
+
+    /**
+     * Sets the \a curve to show in the widget.
+     * @see curve()
+     */
+    void setCurve( const QgsCurveTransform& curve );
+
+  signals:
+
+    //! Emitted when the widget curve changes
+    void changed();
+
+  protected:
+
+    virtual void keyPressEvent( QKeyEvent *event ) override ;
+
+  private slots:
+
+    void plotMousePress( QPointF point );
+    void plotMouseRelease( QPointF point );
+    void plotMouseMove( QPointF point );
+
+  private:
+
+    QgsCurveTransform mCurve;
+
+    QwtPlot* mPlot = nullptr;
+
+    QwtPlotCurve* mPlotCurve = nullptr;
+
+    QList< QwtPlotMarker* > mMarkers;
+    QgsCurveEditorPlotEventFilter* mPlotFilter = nullptr;
+    int mCurrentPlotMarkerIndex;
+
+    void updatePlot();
+    void addPlotMarker( double x, double y, bool isSelected = false );
+
+    int findNearestControlPoint( QPointF point ) const;
+};
+
+
+//
+// NOTE:
+// For private only, not part of stable api or exposed to Python bindings
+//
+/// @cond PRIVATE
+class GUI_EXPORT QgsCurveEditorPlotEventFilter: public QObject
+{
+    Q_OBJECT
+
+  public:
+
+    QgsCurveEditorPlotEventFilter( QwtPlot *plot );
+
+    virtual bool eventFilter( QObject* object, QEvent* event ) override;
+
+  signals:
+
+    void mousePress( QPointF );
+    void mouseRelease( QPointF );
+    void mouseMove( QPointF );
+
+  private:
+
+    QwtPlot* mPlot;
+    QPointF mapPoint( QPointF point ) const;
+};
+///@endcond
+
+#endif // QGSCURVEEDITORWIDGET_H

src/gui/qgspropertyassistantwidget.cpp

@@ -51,6 +51,12 @@ QgsPropertyAssistantWidget::QgsPropertyAssistantWidget( QWidget* parent ,
   {
     minValueSpinBox->setValue( initialState.transformer()->minValue() );
     maxValueSpinBox->setValue( initialState.transformer()->maxValue() );
+
+    if ( initialState.transformer()->curveTransform() )
+    {
+      mTransformCurveCheckBox->setChecked( true );
+      mCurveEditor->setCurve( *initialState.transformer()->curveTransform() );
+    }
   }
 
   connect( computeValuesButton, &QPushButton::clicked, this, &QgsPropertyAssistantWidget::computeValuesFromLayer );
@@ -108,10 +114,12 @@ QgsPropertyAssistantWidget::QgsPropertyAssistantWidget( QWidget* parent ,
     mOutputWidget->layout()->addWidget( mTransformerWidget );
     connect( mTransformerWidget, &QgsPropertyAbstractTransformerWidget::widgetChanged, this, &QgsPropertyAssistantWidget::widgetChanged );
   }
+  mTransformCurveCheckBox->setVisible( mTransformerWidget );
 
   connect( minValueSpinBox, static_cast < void ( QgsDoubleSpinBox::* )( double ) > ( &QgsDoubleSpinBox::valueChanged ), this, &QgsPropertyAssistantWidget::widgetChanged );
   connect( maxValueSpinBox, static_cast < void ( QgsDoubleSpinBox::* )( double ) > ( &QgsDoubleSpinBox::valueChanged ), this, &QgsPropertyAssistantWidget::widgetChanged );
   connect( mExpressionWidget, static_cast < void ( QgsFieldExpressionWidget::* )( const QString& ) > ( &QgsFieldExpressionWidget::fieldChanged ), this, &QgsPropertyAssistantWidget::widgetChanged );
+  connect( mCurveEditor, &QgsCurveEditorWidget::changed, this, &QgsPropertyAssistantWidget::widgetChanged );
   connect( this, &QgsPropertyAssistantWidget::widgetChanged, this, &QgsPropertyAssistantWidget::updatePreview );
   updatePreview();
 }
@@ -131,7 +139,18 @@ void QgsPropertyAssistantWidget::updateProperty( QgsProperty& property )
     property.setField( mExpressionWidget->currentField() );
 
   if ( mTransformerWidget )
-    property.setTransformer( mTransformerWidget->createTransformer( minValueSpinBox->value(), maxValueSpinBox->value() ) );
+  {
+    std::unique_ptr< QgsPropertyTransformer> t( mTransformerWidget->createTransformer( minValueSpinBox->value(), maxValueSpinBox->value() ) );
+    if ( mTransformCurveCheckBox->isChecked() )
+    {
+      t->setCurveTransform( new QgsCurveTransform( mCurveEditor->curve() ) );
+    }
+    else
+    {
+      t->setCurveTransform( nullptr );
+    }
+    property.setTransformer( t.release() );
+  }
 }
 
 void QgsPropertyAssistantWidget::setDockMode( bool dockMode )
@@ -180,8 +199,9 @@ void QgsPropertyAssistantWidget::updatePreview()
   QList<double> breaks = QgsSymbolLayerUtils::prettyBreaks( minValueSpinBox->value(),
                          maxValueSpinBox->value(), 8 );
 
+  QgsCurveTransform curve = mCurveEditor->curve();
   QList< QgsSymbolLegendNode* > nodes = mTransformerWidget->generatePreviews( breaks, mLayerTreeLayer, mSymbol.get(), minValueSpinBox->value(),
-                                        maxValueSpinBox->value() );
+                                        maxValueSpinBox->value(), mTransformCurveCheckBox->isChecked() ? &curve : nullptr );
 
   int widthMax = 0;
   int i = 0;
@@ -353,7 +373,7 @@ QgsSizeScaleTransformer* QgsPropertySizeAssistantWidget::createTransformer( doub
   return transformer;
 }
 
-QList< QgsSymbolLegendNode* > QgsPropertySizeAssistantWidget::generatePreviews( const QList<double>& breaks, QgsLayerTreeLayer* parent, const QgsSymbol* symbol, double minValue, double maxValue ) const
+QList< QgsSymbolLegendNode* > QgsPropertySizeAssistantWidget::generatePreviews( const QList<double>& breaks, QgsLayerTreeLayer* parent, const QgsSymbol* symbol, double minValue, double maxValue, QgsCurveTransform* curve ) const
 {
   QList< QgsSymbolLegendNode* > nodes;
 
@@ -376,6 +396,8 @@ QList< QgsSymbolLegendNode* > QgsPropertySizeAssistantWidget::generatePreviews(
     return nodes;
 
   std::unique_ptr< QgsSizeScaleTransformer > t( createTransformer( minValue, maxValue ) );
+  if ( curve )
+    t->setCurveTransform( new QgsCurveTransform( *curve ) );
 
   for ( int i = 0; i < breaks.length(); i++ )
   {
@@ -401,7 +423,7 @@ QList< QgsSymbolLegendNode* > QgsPropertySizeAssistantWidget::generatePreviews(
   return nodes;
 }
 
-QList<QgsSymbolLegendNode*> QgsPropertyAbstractTransformerWidget::generatePreviews( const QList<double>& , QgsLayerTreeLayer* , const QgsSymbol*, double, double ) const
+QList<QgsSymbolLegendNode*> QgsPropertyAbstractTransformerWidget::generatePreviews( const QList<double>& , QgsLayerTreeLayer* , const QgsSymbol*, double, double, QgsCurveTransform* ) const
 {
   return QList< QgsSymbolLegendNode* >();
 }
@@ -451,7 +473,7 @@ QgsColorRampTransformer* QgsPropertyColorAssistantWidget::createTransformer( dou
   return transformer;
 }
 
-QList<QgsSymbolLegendNode*> QgsPropertyColorAssistantWidget::generatePreviews( const QList<double>& breaks, QgsLayerTreeLayer* parent, const QgsSymbol* symbol, double minValue, double maxValue ) const
+QList<QgsSymbolLegendNode*> QgsPropertyColorAssistantWidget::generatePreviews( const QList<double>& breaks, QgsLayerTreeLayer* parent, const QgsSymbol* symbol, double minValue, double maxValue , QgsCurveTransform* curve ) const
 {
   QList< QgsSymbolLegendNode* > nodes;
 
@@ -467,6 +489,8 @@ QList<QgsSymbolLegendNode*> QgsPropertyColorAssistantWidget::generatePreviews( c
     return nodes;
 
   std::unique_ptr< QgsColorRampTransformer > t( createTransformer( minValue, maxValue ) );
+  if ( curve )
+    t->setCurveTransform( new QgsCurveTransform( *curve ) );
 
   for ( int i = 0; i < breaks.length(); i++ )
   {

src/gui/qgspropertyassistantwidget.h

@@ -48,7 +48,7 @@ class GUI_EXPORT QgsPropertyAbstractTransformerWidget : public QWidget
 
     virtual QgsPropertyTransformer* createTransformer( double minValue, double maxValue ) const = 0;
 
-    virtual QList< QgsSymbolLegendNode* > generatePreviews( const QList<double>& breaks, QgsLayerTreeLayer* parent, const QgsSymbol* symbol, double minValue, double maxValue ) const;
+    virtual QList< QgsSymbolLegendNode* > generatePreviews( const QList<double>& breaks, QgsLayerTreeLayer* parent, const QgsSymbol* symbol, double minValue, double maxValue, QgsCurveTransform* curve ) const;
 
   signals:
 
@@ -82,7 +82,7 @@ class GUI_EXPORT QgsPropertySizeAssistantWidget : public QgsPropertyAbstractTran
 
     virtual QgsSizeScaleTransformer* createTransformer( double minValue, double maxValue ) const override;
 
-    QList< QgsSymbolLegendNode* > generatePreviews( const QList<double>& breaks, QgsLayerTreeLayer* parent, const QgsSymbol* symbol, double minValue, double maxValue ) const override;
+    QList< QgsSymbolLegendNode* > generatePreviews( const QList<double>& breaks, QgsLayerTreeLayer* parent, const QgsSymbol* symbol, double minValue, double maxValue, QgsCurveTransform* curve ) const override;
 };
 
 class GUI_EXPORT QgsPropertyColorAssistantWidget : public QgsPropertyAbstractTransformerWidget, private Ui::PropertyColorAssistant
@@ -95,7 +95,7 @@ class GUI_EXPORT QgsPropertyColorAssistantWidget : public QgsPropertyAbstractTra
 
     virtual QgsColorRampTransformer* createTransformer( double minValue, double maxValue ) const override;
 
-    QList< QgsSymbolLegendNode* > generatePreviews( const QList<double>& breaks, QgsLayerTreeLayer* parent, const QgsSymbol* symbol, double minValue, double maxValue ) const override;
+    QList< QgsSymbolLegendNode* > generatePreviews( const QList<double>& breaks, QgsLayerTreeLayer* parent, const QgsSymbol* symbol, double minValue, double maxValue, QgsCurveTransform* curve ) const override;
 };
 
 ///@endcond PRIVATE

src/ui/qgspropertyassistantwidgetbase.ui

@@ -6,22 +6,12 @@
    <rect>
     <x>0</x>
     <y>0</y>
-    <width>522</width>
-    <height>332</height>
+    <width>525</width>
+    <height>426</height>
    </rect>
   </property>
-  <layout class="QGridLayout" name="gridLayout_2" rowstretch="0,0,0">
-   <item row="0" column="1" rowspan="2">
-    <widget class="QTreeView" name="mLegendPreview">
-     <property name="minimumSize">
-      <size>
-       <width>200</width>
-       <height>0</height>
-      </size>
-     </property>
-    </widget>
-   </item>
-   <item row="1" column="0">
+  <layout class="QGridLayout" name="gridLayout_2" rowstretch="0,0,0,0">
+   <item row="2" column="0">
     <widget class="QGroupBox" name="groupBox_2">
      <property name="title">
       <string>Output</string>
@@ -60,6 +50,16 @@
      </layout>
     </widget>
    </item>
+   <item row="3" column="0">
+    <widget class="QFrame" name="mLegendVerticalFrame">
+     <property name="frameShape">
+      <enum>QFrame::NoFrame</enum>
+     </property>
+     <property name="frameShadow">
+      <enum>QFrame::Plain</enum>
+     </property>
+    </widget>
+   </item>
    <item row="0" column="0">
     <widget class="QGroupBox" name="groupBox">
      <property name="title">
@@ -153,14 +153,60 @@
      </layout>
     </widget>
    </item>
-   <item row="2" column="0">
-    <widget class="QFrame" name="mLegendVerticalFrame">
-     <property name="frameShape">
-      <enum>QFrame::NoFrame</enum>
+   <item row="0" column="1" rowspan="3">
+    <widget class="QTreeView" name="mLegendPreview">
+     <property name="minimumSize">
+      <size>
+       <width>200</width>
+       <height>0</height>
+      </size>
      </property>
-     <property name="frameShadow">
-      <enum>QFrame::Plain</enum>
+    </widget>
+   </item>
+   <item row="1" column="0">
+    <widget class="QgsCollapsibleGroupBoxBasic" name="mTransformCurveCheckBox">
+     <property name="title">
+      <string>Apply transform curve</string>
      </property>
+     <property name="checkable">
+      <bool>true</bool>
+     </property>
+     <property name="checked">
+      <bool>false</bool>
+     </property>
+     <property name="collapsed" stdset="0">
+      <bool>true</bool>
+     </property>
+     <property name="syncGroup" stdset="0">
+      <string notr="true">composeritem</string>
+     </property>
+     <layout class="QVBoxLayout" name="verticalLayout_2">
+      <property name="leftMargin">
+       <number>0</number>
+      </property>
+      <property name="topMargin">
+       <number>0</number>
+      </property>
+      <property name="rightMargin">
+       <number>0</number>
+      </property>
+      <property name="bottomMargin">
+       <number>0</number>
+      </property>
+      <item>
+       <widget class="QgsCurveEditorWidget" name="mCurveEditor" native="true">
+        <property name="minimumSize">
+         <size>
+          <width>0</width>
+          <height>100</height>
+         </size>
+        </property>
+        <property name="focusPolicy">
+         <enum>Qt::StrongFocus</enum>
+        </property>
+       </widget>
+      </item>
+     </layout>
     </widget>
    </item>
   </layout>
@@ -183,6 +229,18 @@
    <header>qgspanelwidget.h</header>
    <container>1</container>
   </customwidget>
+  <customwidget>
+   <class>QgsCollapsibleGroupBoxBasic</class>
+   <extends>QGroupBox</extends>
+   <header location="global">qgscollapsiblegroupbox.h</header>
+   <container>1</container>
+  </customwidget>
+  <customwidget>
+   <class>QgsCurveEditorWidget</class>
+   <extends>QWidget</extends>
+   <header location="global">qgscurveeditorwidget.h</header>
+   <container>1</container>
+  </customwidget>
  </customwidgets>
  <tabstops>
   <tabstop>mExpressionWidget</tabstop>