Add shadow rendering helper functions

Summary:
This patch makes other helper functions reuse more code by calling these shadow rendering functions instead of defining their own shadows.

The shadows are perfectly shaped, tested up to 8x scaling.
Without this patch, the radius of the shadows for checkboxes is slighly too wide and there is a gap between the inside edge of the radio button shadow and the outside edge of the radio button.
Shadow rendering performance has also been improved a bit.

Test Plan:
run `QT_SCALE_FACTOR=8 oxygen-demo5` and examine the shadows. You can try higher scaling levels, but that's overkill.

8x scaling

before: {F7824689, size=full} {F7824691, size=full}

after: {F7824692, size=full} {F7824693, size=full}

Reviewers: #breeze, hpereiradacosta, #plasma

Reviewed By: hpereiradacosta

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26094

kstyle/breezehelper.cpp

@@ -641,12 +641,9 @@ namespace Breeze
 
             frameRect.translate( 1, 1 );
 
-        } else if( shadow.isValid() ) {
+        } else {
 
-            const qreal shadowRadius = qMax( radius - 1, qreal( 0.0 ) );
-            painter->setPen( QPen( shadow, 2 ) );
-            painter->setBrush( Qt::NoBrush );
-            painter->drawRoundedRect( shadowRect( frameRect ), shadowRadius, shadowRadius );
+            renderRoundedRectShadow( painter, frameRect, shadow, radius );
 
         }
 
@@ -875,11 +872,7 @@ namespace Breeze
 
         } else {
 
-            painter->setPen( QPen( shadow, 1 ) );
-            painter->setBrush( Qt::NoBrush );
-
-            const qreal shadowRadius( radius + 0.5 );
-            painter->drawRoundedRect( shadowRect( frameRect ).adjusted( -0.5, -0.5, 0.5, 0.5 ), shadowRadius, shadowRadius );
+            renderRoundedRectShadow( painter, frameRect, shadow, radius );
 
         }
 
@@ -982,9 +975,7 @@ namespace Breeze
 
         } else {
 
-            painter->setPen( QPen( shadow, 1 ) );
-            painter->setBrush( Qt::NoBrush );
-            painter->drawEllipse( shadowRect( frameRect ).adjusted( -0.5, -0.5, 0.5, 0.5 ) );
+            renderEllipseShadow( painter, frameRect, shadow );
 
         }
 
@@ -1141,12 +1132,10 @@ namespace Breeze
         frameRect.adjust( 1, 1, -1, -1 );
 
         // shadow
-        if( shadow.isValid() && !sunken )
+        if( !sunken )
         {
 
-            painter->setPen( QPen( shadow, 2 ) );
-            painter->setBrush( Qt::NoBrush );
-            painter->drawEllipse( shadowRect( frameRect ) );
+            renderEllipseShadow( painter, frameRect, shadow );
 
         }
 
@@ -1408,7 +1397,61 @@ namespace Breeze
         painter->restore();
 
     }
+    
+    //______________________________________________________________________________
+    void Helper::renderRoundedRectShadow( QPainter* painter, const QRectF& rect, const QColor& color, qreal radius ) const
+    {
+        if( !color.isValid() ) return;
+        
+        painter->save();
+        
+        /* Clipping prevents shadows from being visible inside checkboxes.
+         * Clipping away unneeded parts here also improves performance by 40-60%
+         * versus using just an outline of a rectangle.
+         * Tested by looking at the paint analyser in GammaRay.
+         */
+        // Right side
+        QRegion clip( rect.right() - std::ceil( radius ), rect.top(), 
+                      std::ceil( radius ) + 1, rect.height() );
+        // Bottom side
+        clip = clip.united( QRegion( rect.left(), rect.bottom() - std::ceil( radius ), 
+                                     rect.width(), std::ceil( radius ) + 1 ) );
+
+        painter->setClipRegion( clip );
+        painter->setPen( color );
+        painter->setBrush( Qt::NoBrush );
+        painter->drawRoundedRect( rect.translated( 0.5, 0.5 ), radius, radius );
+        
+        painter->restore();
+    }
+    
+    //______________________________________________________________________________
+    void Helper::renderEllipseShadow( QPainter* painter, const QRectF& rect, const QColor& color ) const
+    {
+        if( !color.isValid() ) return;
+        
+        painter->save();
 
+        // Clipping does not improve performance here
+
+        // 0.5 is subtracted because of the pen
+        qreal radius = rect.width() / 2 - 0.5;
+        
+        /* The right side is offset by +0.5 for the visible part of the shadow.
+         * The other sides are offset by +0.5 or -0.5 because of the pen.
+         */
+        QRectF shadowRect = rect.adjusted( 0.5, 0.5, 0.5, -0.5 );
+        
+        painter->translate( rect.center() );
+        painter->rotate( 45 );
+        painter->translate( -rect.center() );
+        painter->setPen( color );
+        painter->setBrush( Qt::NoBrush );
+        painter->drawRoundedRect( shadowRect, radius, radius );
+        
+        painter->restore();
+    }
+    
     //______________________________________________________________________________
     bool Helper::isX11()
     {
@@ -1454,11 +1497,7 @@ namespace Breeze
         qreal adjustment = 0.5 * penWidth;
         return rect.adjusted( adjustment, adjustment, -adjustment, -adjustment );
     }
-    
-    //______________________________________________________________________________
-    QRectF Helper::shadowRect( const QRectF& rect ) const
-    { return rect.adjusted( 0.5, 0.5, -0.5, -0.5 ).translated( 0.5, 0.5 ); }
-    
+
     //______________________________________________________________________________
     QPainterPath Helper::roundedPath( const QRectF& rect, Corners corners, qreal radius ) const
     {

kstyle/breezehelper.h

@@ -251,6 +251,12 @@ namespace Breeze
         //* generic button (for mdi decorations, tabs and dock widgets)
         void renderDecorationButton( QPainter*, const QRect&, const QColor&, ButtonType, bool inverted ) const;
 
+        //* generic shadow for rounded rectangles
+        void renderRoundedRectShadow ( QPainter*, const QRectF&, const QColor&, qreal radius = Metrics::Frame_FrameRadius - 0.5 ) const;
+        
+        //* generic shadow for ellipses
+        void renderEllipseShadow( QPainter*, const QRectF&, const QColor& ) const;
+        
         //@}
 
         //*@name compositing utilities
@@ -319,9 +325,6 @@ namespace Breeze
         //* initialize
         void init();
 
-        //* return rectangle for widgets shadow, offset depending on light source
-        QRectF shadowRect( const QRectF& ) const;
-
         //* return rounded path in a given rect, with only selected corners rounded, and for a given radius
         QPainterPath roundedPath( const QRectF&, Corners, qreal ) const;