Patterned Qwt Legend Line and Plot Line look different if thickness is greater than 1

[philw]

We do have a fix to the problem we were having with Qwt 6.1.3 (built with Qt 5.5.1) on Windows. This with the standard QwtLegend implementation. The fix is: commenting out the call to QPen::setCapStyle (Qt::FlatCap) in method QwtPlotCurve::legendIcon().

Uwe, and anyone else who knows about this. Is this fix (omitting call to setCapStyle) the right idea? Or am I missing something?

The two following screenshots show the discrepancy. Both have a line width of 2. Dotted, and Dashed:

Plot1-Prob-Dot2.png
Plot2-Prob-Dash2.png

http://cadswes2.colorado.edu/~philw/...-Prob-Dot2.png
http://cadswes2.colorado.edu/~philw/...Prob-Dash2.png

Here's a closer look at the problem. The "fixed" version matches what is shown on the plot:

LegendProb.jpg

http://cadswes2.colorado.edu/~philw/...LegendProb.png

The following show a fix -- effectively commenting out this line in QwtPlotCurve::legendIcon():

• pn.setCapStyle( Qt::FlatCap );

Plot3-Fix-Dot2.png
Plot4-Fix-Dash2.png

http://cadswes2.colorado.edu/~philw/...3-Fix-Dot2.png
http://cadswes2.colorado.edu/~philw/...-Fix-Dash2.png

Instead of actually modifying QwtPlotCurve::legendIcon(), we lifted that implementation out of that class, and implemented a modified version in our QwtPlotCurve subclass.

// virtual from QwtPlotCurve
QwtGraphic SlotCurve::legendIcon (int index, // (ignore, only one)
                                  const QSizeF& iconSize) const
{
   // call base class method
   //-- QwtGraphic graphic = QwtPlotCurve::legendIcon (index, iconSize);
 
   // Note [Phil, 3-2017, RW 7.1, Qwt 6.1.3, Qt 5.5.1, Gnats 5864]:
   // The following code is adapted from QwtPlotCurve::legendIcon().
   // See the fix for Gnats 5864, below ("Legend and plot line look
   // different if thickness is greater than one").
 
   Q_UNUSED( index );
 
   if ( iconSize.isEmpty() )
      return QwtGraphic();
 
   QwtGraphic graphic;
   graphic.setDefaultSize( iconSize );
   graphic.setRenderHint( QwtGraphic::RenderPensUnscaled, true );
 
   QPainter painter( &graphic );
   painter.setRenderHint( QPainter::Antialiasing,
      testRenderHint( QwtPlotItem::RenderAntialiased ) );
 
   bool doShowLine  = testLegendAttribute (QwtPlotCurve::LegendShowLine);
   bool doShowSymb  = testLegendAttribute (QwtPlotCurve::LegendShowSymbol);
   bool doShowBrush = testLegendAttribute (QwtPlotCurve::LegendShowBrush);
   bool noAttribs   = !(doShowLine || doShowSymb || doShowBrush);
 
   const QPen curvePen = pen();
   const QwtSymbol* curveSymb = symbol();
 
   if ( noAttribs || doShowBrush)
   {
      QBrush curveBrush = brush();
 
      if ( curveBrush.style() == Qt::NoBrush && noAttribs )
      {
         if ( style() != QwtPlotCurve::NoCurve )
         {
            curveBrush = QBrush( curvePen.color() );
         }
         else if ( curveSymb && (curveSymb->style() != QwtSymbol::NoSymbol) )
         {
            curveBrush = QBrush( curveSymb->pen().color() );
         }
      }
 
      if ( curveBrush.style() != Qt::NoBrush )
      {
         QRectF r( 0, 0, iconSize.width(), iconSize.height() );
         painter.fillRect( r, curveBrush );
      }
   }
 
   if ( doShowLine )
   {
      if ( curvePen != Qt::NoPen )
      {
         // CADSWES FIX, Gnats 5864 ("Legend and plot line look different if
         // thickness is greater than one"). DON'T set the pen Cap Style.
         //
         //-- QPen pn = curvePen;
         //-- pn.setCapStyle( Qt::FlatCap );
         //-- painter.setPen( pn );
 
         painter.setPen( curvePen );
         const double y = 0.5 * iconSize.height();
         QwtPainter::drawLine( &painter, 0.0, y, iconSize.width(), y );
      }
   }
 
   if ( doShowSymb )
   {
      if ( curveSymb )
      {
         QRectF r( 0, 0, iconSize.width(), iconSize.height() );
         curveSymb->drawSymbol( &painter, r );
      }
   }
 
   return graphic;
}

// virtual from QwtPlotCurve
QwtGraphic SlotCurve::legendIcon (int index, // (ignore, only one)
                                  const QSizeF& iconSize) const
{
   // call base class method
   //-- QwtGraphic graphic = QwtPlotCurve::legendIcon (index, iconSize);

   // Note [Phil, 3-2017, RW 7.1, Qwt 6.1.3, Qt 5.5.1, Gnats 5864]:
   // The following code is adapted from QwtPlotCurve::legendIcon().
   // See the fix for Gnats 5864, below ("Legend and plot line look
   // different if thickness is greater than one").

   Q_UNUSED( index );

   if ( iconSize.isEmpty() )
      return QwtGraphic();

   QwtGraphic graphic;
   graphic.setDefaultSize( iconSize );
   graphic.setRenderHint( QwtGraphic::RenderPensUnscaled, true );

   QPainter painter( &graphic );
   painter.setRenderHint( QPainter::Antialiasing,
      testRenderHint( QwtPlotItem::RenderAntialiased ) );

   bool doShowLine  = testLegendAttribute (QwtPlotCurve::LegendShowLine);
   bool doShowSymb  = testLegendAttribute (QwtPlotCurve::LegendShowSymbol);
   bool doShowBrush = testLegendAttribute (QwtPlotCurve::LegendShowBrush);
   bool noAttribs   = !(doShowLine || doShowSymb || doShowBrush);

   const QPen curvePen = pen();
   const QwtSymbol* curveSymb = symbol();

   if ( noAttribs || doShowBrush)
   {
      QBrush curveBrush = brush();

      if ( curveBrush.style() == Qt::NoBrush && noAttribs )
      {
         if ( style() != QwtPlotCurve::NoCurve )
         {
            curveBrush = QBrush( curvePen.color() );
         }
         else if ( curveSymb && (curveSymb->style() != QwtSymbol::NoSymbol) )
         {
            curveBrush = QBrush( curveSymb->pen().color() );
         }
      }

      if ( curveBrush.style() != Qt::NoBrush )
      {
         QRectF r( 0, 0, iconSize.width(), iconSize.height() );
         painter.fillRect( r, curveBrush );
      }
   }

   if ( doShowLine )
   {
      if ( curvePen != Qt::NoPen )
      {
         // CADSWES FIX, Gnats 5864 ("Legend and plot line look different if
         // thickness is greater than one"). DON'T set the pen Cap Style.
         //
         //-- QPen pn = curvePen;
         //-- pn.setCapStyle( Qt::FlatCap );
         //-- painter.setPen( pn );

         painter.setPen( curvePen );
         const double y = 0.5 * iconSize.height();
         QwtPainter::drawLine( &painter, 0.0, y, iconSize.width(), y );
      }
   }

   if ( doShowSymb )
   {
      if ( curveSymb )
      {
         QRectF r( 0, 0, iconSize.width(), iconSize.height() );
         curveSymb->drawSymbol( &painter, r );
      }
   }

   return graphic;
}

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[Uwe]

If you prefer to keep the cap style from the curve sure. To be honest I can't remember the reason behind explicitly setting FlatCap - maybe because of trying to get rid of the extra pixels at the ends, that make drawing the line inaccurate.

Uwe

[philw]

Oh wow. Uwe, that's interesting.

We didn't intend to set a Qt::PenCapStyle on our QwtPlotCurves' pens. We do want our curves to be as precise as possible, and would want to stick with Qt::FlatCap. We aren't explicitly doing anything with cap style, but I am understanding that a defaulted constructed QPen -- or one constructed from a color or pen style (e.g. Dotted) -- has a cap style of Qt::SquareCap. (We must be assigning a QPen like that to our QwtPlotCurves, but I haven't found that in our code yet).

I DO see that when I force our QwtPlotCurves to have the Qt::FlatCap cap style -- see below -- the QwtLegend does show pen styles (e.g. Dotted) consistently with the plotted curve (with my modified code omitting the setting of the legend line's cap style -- see my prior post). So that explicit setting of Qt::FlatCap in QwtPlotCurve::legendIcon() I found seems to be unnecessary (at least now, in Qwt 6.1.3) [using Qt 5.5.1].

QPen curvePen = pen();
curvePen.setCapStyle (Qt::FlatCap);
setPen (curvePen);

QPen curvePen = pen();
curvePen.setCapStyle (Qt::FlatCap);
setPen (curvePen);

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