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

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:

Attachment 12411
Attachment 12412

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:

Attachment 12413

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

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

• pn.setCapStyle( Qt::FlatCap );

Attachment 12414
Attachment 12415

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.

Code:

---

// 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;
}

---

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

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].

Code:

---

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

---