Workaround qt bug which causes word spacing to shrink to nothing

when very large font sizes are rendered Works around https://bugreports.qt.io/browse/QTBUG-98778 (cherry picked from commit fa5c53e)
qgis · Dec 7, 2021 · 5297a96 · 5297a96
1 parent 5d82289
commit 5297a96
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Showing 4 changed files with 49 additions and 12 deletions.
diff --git a/src/core/textrenderer/qgstextrenderer.cpp b/src/core/textrenderer/qgstextrenderer.cpp
@@ -294,7 +294,7 @@ double QgsTextRenderer::drawBuffer( QgsRenderContext &context, const QgsTextRend
 
   const double penSize = context.convertToPainterUnits( buffer.size(), buffer.sizeUnit(), buffer.sizeMapUnitScale() );
 
-  const double scaleFactor = ( context.flags() & Qgis::RenderContextFlag::ApplyScalingWorkaroundForTextRendering ) ? FONT_WORKAROUND_SCALE : 1.0;
+  const double scaleFactor = calculateScaleFactorForFormat( context, format );
 
   std::optional< QgsScopedRenderContextReferenceScaleOverride > referenceScaleOverride;
   if ( mode == Label )
@@ -451,7 +451,7 @@ void QgsTextRenderer::drawMask( QgsRenderContext &context, const QgsTextRenderer
   QPainterPath path;
   path.setFillRule( Qt::WindingFill );
 
-  const double scaleFactor = ( context.flags() & Qgis::RenderContextFlag::ApplyScalingWorkaroundForTextRendering ) ? FONT_WORKAROUND_SCALE : 1.0;
+  const double scaleFactor = calculateScaleFactorForFormat( context, format );
 
   // TODO: vertical text mode was ignored when masking feature was added.
   // Hopefully Oslandia come back and fix this? Hint hint...
@@ -542,7 +542,7 @@ double QgsTextRenderer::textWidth( const QgsRenderContext &context, const QgsTex
 double QgsTextRenderer::textWidth( const QgsRenderContext &context, const QgsTextFormat &format, const QgsTextDocument &document )
 {
   //calculate max width of text lines
-  const double scaleFactor = ( context.flags() & Qgis::RenderContextFlag::ApplyScalingWorkaroundForTextRendering ) ? FONT_WORKAROUND_SCALE : 1.0;
+  const double scaleFactor = calculateScaleFactorForFormat( context, format );
 
   bool isNullSize = false;
   const QFont baseFont = format.scaledFont( context, scaleFactor, &isNullSize );
@@ -613,7 +613,7 @@ double QgsTextRenderer::textHeight( const QgsRenderContext &context, const QgsTe
 
 double QgsTextRenderer::textHeight( const QgsRenderContext &context, const QgsTextFormat &format, QChar character, bool includeEffects )
 {
-  const double scaleFactor = ( context.flags() & Qgis::RenderContextFlag::ApplyScalingWorkaroundForTextRendering ) ? FONT_WORKAROUND_SCALE : 1.0;
+  const double scaleFactor = calculateScaleFactorForFormat( context, format );
   bool isNullSize = false;
   const QFont baseFont = format.scaledFont( context, scaleFactor, &isNullSize );
   if ( isNullSize )
@@ -654,7 +654,7 @@ double QgsTextRenderer::textHeight( const QgsRenderContext &context, const QgsTe
   document.applyCapitalization( format.capitalization() );
 
   //calculate max height of text lines
-  const double scaleFactor = ( context.flags() & Qgis::RenderContextFlag::ApplyScalingWorkaroundForTextRendering ) ? FONT_WORKAROUND_SCALE : 1.0;
+  const double scaleFactor = calculateScaleFactorForFormat( context, format );
 
   bool isNullSize = false;
   const QFont baseFont = format.scaledFont( context, scaleFactor, &isNullSize );
@@ -781,7 +781,7 @@ void QgsTextRenderer::drawBackground( QgsRenderContext &context, QgsTextRenderer
     component.rotationOffset = background.rotation();
   }
 
-  const double scaleFactor = ( context.flags() & Qgis::RenderContextFlag::ApplyScalingWorkaroundForTextRendering ) ? FONT_WORKAROUND_SCALE : 1;
+  const double scaleFactor = calculateScaleFactorForFormat( context, format );
 
   if ( mode != Label )
   {
@@ -1307,7 +1307,7 @@ void QgsTextRenderer::drawTextInternal( TextPart drawType,
   std::unique_ptr< QFontMetricsF > tmpMetrics;
   if ( !fontMetrics )
   {
-    fontScale = ( context.flags() & Qgis::RenderContextFlag::ApplyScalingWorkaroundForTextRendering ) ? FONT_WORKAROUND_SCALE : 1.0;
+    fontScale = calculateScaleFactorForFormat( context, format );
 
     std::optional< QgsScopedRenderContextReferenceScaleOverride > referenceScaleOverride;
     if ( mode == Label )
@@ -1969,3 +1969,23 @@ void QgsTextRenderer::drawTextInternalVertical( QgsRenderContext &context, const
   }
 }
 
+double QgsTextRenderer::calculateScaleFactorForFormat( const QgsRenderContext &context, const QgsTextFormat &format )
+{
+  if ( !( context.flags() & Qgis::RenderContextFlag::ApplyScalingWorkaroundForTextRendering ) )
+    return 1.0;
+
+  const double pixelSize = context.convertToPainterUnits( format.size(), format.sizeUnit(), format.sizeMapUnitScale() );
+
+  // THESE THRESHOLD MAY NEED TWEAKING!
+
+  // for small font sizes we need to apply a growth scaling workaround designed to stablise the rendering of small font sizes
+  if ( pixelSize < 50 )
+    return FONT_WORKAROUND_SCALE;
+  //... but for font sizes we might run into https://bugreports.qt.io/browse/QTBUG-98778, which messes up the spacing between words for large fonts!
+  // so instead we scale down the painter so that we render the text at 200 pixel size and let painter scaling handle making it the correct size
+  else if ( pixelSize > 200 )
+    return 200 / pixelSize;
+  else
+    return 1.0;
+}
+
diff --git a/src/core/textrenderer/qgstextrenderer.h b/src/core/textrenderer/qgstextrenderer.h
@@ -381,6 +381,8 @@ class CORE_EXPORT QgsTextRenderer
                                           VAlignment vAlignment,
                                           double rotation );
 
+    static double calculateScaleFactorForFormat( const QgsRenderContext &context, const QgsTextFormat &format );
+
     friend class QgsVectorLayerLabelProvider;
     friend class QgsLabelPreview;
 

diff --git a/tests/src/python/test_qgstextrenderer.py b/tests/src/python/test_qgstextrenderer.py
@@ -17,7 +17,8 @@
 from qgis.PyQt.QtCore import (Qt, QSizeF, QPointF, QRectF, QDir, QSize)
 from qgis.PyQt.QtGui import (QColor, QPainter, QFont, QImage, QBrush, QPen)
 from qgis.PyQt.QtXml import QDomDocument
-from qgis.core import (QgsTextBufferSettings,
+from qgis.core import (Qgis,
+                       QgsTextBufferSettings,
                        QgsTextMaskSettings,
                        QgsTextBackgroundSettings,
                        QgsTextShadowSettings,
@@ -1408,9 +1409,10 @@ def imageCheck(self, name, reference_image, image):
     def checkRender(self, format, name, part=None, angle=0, alignment=QgsTextRenderer.AlignLeft,
                     text=['test'],
                     rect=QRectF(100, 100, 50, 250),
-                    vAlignment=QgsTextRenderer.AlignTop):
+                    vAlignment=QgsTextRenderer.AlignTop,
+                    image_size=400):
 
-        image = QImage(400, 400, QImage.Format_RGB32)
+        image = QImage(image_size, image_size, QImage.Format_RGB32)
 
         painter = QPainter()
         ms = QgsMapSettings()
@@ -1462,8 +1464,9 @@ def checkRender(self, format, name, part=None, angle=0, alignment=QgsTextRendere
 
     def checkRenderPoint(self, format, name, part=None, angle=0, alignment=QgsTextRenderer.AlignLeft,
                          text=['test'],
-                         point=QPointF(100, 200)):
-        image = QImage(400, 400, QImage.Format_RGB32)
+                         point=QPointF(100, 200),
+                         image_size=400):
+        image = QImage(image_size, image_size, QImage.Format_RGB32)
 
         painter = QPainter()
         ms = QgsMapSettings()
@@ -1506,6 +1509,18 @@ def checkRenderPoint(self, format, name, part=None, angle=0, alignment=QgsTextRe
         painter.end()
         return self.imageCheck(name, name, image)
 
+    def testDrawMassiveFont(self):
+        """
+        Test that we aren't bitten by https://bugreports.qt.io/browse/QTBUG-98778
+
+        This test should pass when there's a correct WORD space between the 'a' and 't' characters, or fail when
+        the spacing between these characters is nill or close to a letter spacing
+        """
+        format = QgsTextFormat()
+        format.setFont(getTestFont('bold'))
+        format.setSize(1100)
+        assert self.checkRender(format, 'massive_font', rect=QRectF(-800, -600, 1000, 1000), text=['a t'], image_size=800)
+
     def testDrawBackgroundDisabled(self):
         format = QgsTextFormat()
         format.setFont(getTestFont('bold'))

diff --git a/tests/testdata/control_images/text_renderer/massive_font/massive_font.png b/tests/testdata/control_images/text_renderer/massive_font/massive_font.png