Add support for super and subscript HTML formatting in text renderer

This allows for either:

- <sup>superscript</sup> / <sub>subscript</sub> components in text,
where the text will be vertically super or subscript aligned
and automatically sized to 2/3rd of the parent font size. Users
can also set a fixed font size for the super/sub script by
including css rules, e.g. <sup style="font-size:33pt">super</sup>

- "vertical-align: super" or "vertical-align: sub" CSS formatting
rules in any other HTML element

Sponsored by OSGEO UK

python/core/auto_generated/textrenderer/qgstextdocumentmetrics.sip.in

@@ -49,6 +49,19 @@ Returns ``True`` if the metrics could not be calculated because the text format
     QSizeF documentSize( Qgis::TextLayoutMode mode, Qgis::TextOrientation orientation ) const;
 %Docstring
 Returns the overall size of the document.
+%End
+
+    QRectF outerBounds( Qgis::TextLayoutMode mode, Qgis::TextOrientation orientation ) const;
+%Docstring
+Returns the outer bounds of the document, which is the :py:func:`~QgsTextDocumentMetrics.documentSize` adjusted to account
+for any text elements which fall outside of the usual document margins (such as super or
+sub script elements)
+
+.. warning::
+
+   Currently this is only supported for the Qgis.TextLayoutMode.Labeling mode.
+
+.. versionadded:: 3.30
 %End
 
     double blockWidth( int blockIndex ) const;
@@ -70,6 +83,14 @@ Returns the offset from the top of the document to the text baseline for the giv
 %Docstring
 Returns the horizontal advance of the fragment at the specified block and fragment index.
 
+.. versionadded:: 3.30
+%End
+
+    double fragmentVerticalOffset( int blockIndex, int fragmentIndex, Qgis::TextLayoutMode mode ) const;
+%Docstring
+Returns the vertical offset from a text block's baseline which should be applied
+to the fragment at the specified index within that block.
+
 .. versionadded:: 3.30
 %End
 

src/core/labeling/qgspallabeling.cpp

@@ -1460,7 +1460,7 @@ bool QgsPalLayerSettings::checkMinimumSizeMM( const QgsRenderContext &ct, const
   return QgsPalLabeling::checkMinimumSizeMM( ct, geom, minSize );
 }
 
-void QgsPalLayerSettings::calculateLabelSize( const QFontMetricsF *fm, const QString &text, double &labelX, double &labelY, const QgsFeature *f, QgsRenderContext *context, double *rotatedLabelX, double *rotatedLabelY, QgsTextDocument *document, QgsTextDocumentMetrics *documentMetrics )
+void QgsPalLayerSettings::calculateLabelSize( const QFontMetricsF *fm, const QString &text, double &labelX, double &labelY, const QgsFeature *f, QgsRenderContext *context, double *rotatedLabelX, double *rotatedLabelY, QgsTextDocument *document, QgsTextDocumentMetrics *documentMetrics, QRectF *outerBounds )
 {
   if ( !fm || !f )
   {
@@ -1714,6 +1714,16 @@ void QgsPalLayerSettings::calculateLabelSize( const QFontMetricsF *fm, const QSt
     *rotatedLabelY = rh * uPP;
   }
 #endif
+
+  if ( outerBounds && documentMetrics )
+  {
+    const QRectF outerBoundsPixels = documentMetrics->outerBounds( Qgis::TextLayoutMode::Labeling, orientation );
+
+    *outerBounds = QRectF( outerBoundsPixels.left() * uPP,
+                           outerBoundsPixels.top() * uPP,
+                           outerBoundsPixels.width() * uPP,
+                           outerBoundsPixels.height() * uPP );
+  }
 }
 
 void QgsPalLayerSettings::registerFeature( const QgsFeature &f, QgsRenderContext &context )
@@ -2018,15 +2028,16 @@ std::unique_ptr<QgsLabelFeature> QgsPalLayerSettings::registerFeatureWithDetails
 
   QgsTextDocument doc;
   QgsTextDocumentMetrics documentMetrics;
+  QRectF outerBounds;
   if ( format().allowHtmlFormatting() && !labelText.isEmpty() )
   {
     doc = QgsTextDocument::fromHtml( QStringList() << labelText );
     // also applies the line split to doc and calculates document metrics!
-    calculateLabelSize( labelFontMetrics.get(), labelText, labelWidth, labelHeight, mCurFeat, &context, &rotatedLabelX, &rotatedLabelY, &doc, &documentMetrics );
+    calculateLabelSize( labelFontMetrics.get(), labelText, labelWidth, labelHeight, mCurFeat, &context, &rotatedLabelX, &rotatedLabelY, &doc, &documentMetrics, &outerBounds );
   }
   else
   {
-    calculateLabelSize( labelFontMetrics.get(), labelText, labelWidth, labelHeight, mCurFeat, &context, &rotatedLabelX, &rotatedLabelY, nullptr, nullptr );
+    calculateLabelSize( labelFontMetrics.get(), labelText, labelWidth, labelHeight, mCurFeat, &context, &rotatedLabelX, &rotatedLabelY, nullptr, nullptr, &outerBounds );
   }
 
   // maximum angle between curved label characters (hardcoded defaults used in QGIS <2.0)
@@ -2668,6 +2679,11 @@ std::unique_ptr<QgsLabelFeature> QgsPalLayerSettings::registerFeatureWithDetails
                                          obstacleGeometry.boundingBox().height() ) );
   }
 
+  if ( outerBounds.left() != 0 || outerBounds.top() != 0 || !qgsDoubleNear( outerBounds.width(), labelWidth ) || !qgsDoubleNear( outerBounds.height(), labelHeight ) )
+  {
+    labelFeature->setOuterBounds( outerBounds );
+  }
+
   //set label's visual margin so that top visual margin is the leading, and bottom margin is the font's descent
   //this makes labels align to the font's baseline or highest character
   double topMargin = std::max( 0.25 * labelFontMetrics->ascent(), 0.0 );

src/core/labeling/qgspallabeling.h

@@ -758,7 +758,7 @@ class CORE_EXPORT QgsPalLayerSettings
      */
 #ifndef SIP_RUN
     void calculateLabelSize( const QFontMetricsF *fm, const QString &text, double &labelX, double &labelY, const QgsFeature *f = nullptr, QgsRenderContext *context = nullptr, double *rotatedLabelX SIP_OUT = nullptr, double *rotatedLabelY SIP_OUT = nullptr,
-                             QgsTextDocument *document = nullptr, QgsTextDocumentMetrics *documentMetrics = nullptr );
+                             QgsTextDocument *document = nullptr, QgsTextDocumentMetrics *documentMetrics = nullptr, QRectF *outerBounds = nullptr );
 #else
     void calculateLabelSize( const QFontMetricsF *fm, const QString &text, double &labelX, double &labelY, const QgsFeature *f = nullptr, QgsRenderContext *context = nullptr, double *rotatedLabelX SIP_OUT = nullptr, double *rotatedLabelY SIP_OUT = nullptr );
 #endif

src/core/labeling/qgsvectorlayerlabelprovider.cpp

@@ -556,7 +556,7 @@ void QgsVectorLayerLabelProvider::drawLabelPrivate( pal::LabelPosition *label, Q
     painter->drawRect( rect );
 
 
-    painter->setPen( QColor( 0, 0, 0, 120 ) );
+    painter->setPen( QColor( 0, 0, 0, 60 ) );
     const QgsMargins &margins = label->getFeaturePart()->feature()->visualMargin();
     if ( margins.top() > 0 )
     {
@@ -584,7 +584,10 @@ void QgsVectorLayerLabelProvider::drawLabelPrivate( pal::LabelPosition *label, Q
                                      outerBoundsPt2.x() - outerBoundsPt1.x(),
                                      outerBoundsPt2.y() - outerBoundsPt1.y() );
 
-      painter->setPen( QColor( 255, 0, 255, 140 ) );
+      QPen pen( QColor( 255, 0, 255, 140 ) );
+      pen.setCosmetic( true );
+      pen.setWidth( 1 );
+      painter->setPen( pen );
       painter->drawRect( outerBoundsPixel );
     }
 
@@ -594,13 +597,39 @@ void QgsVectorLayerLabelProvider::drawLabelPrivate( pal::LabelPosition *label, Q
       const QgsTextDocument &document = textFeature->document();
       const int blockCount = document.size();
 
+      double prevBlockBaseline = rect.bottom() - rect.top();
+
       // draw block baselines
-      painter->setPen( QColor( 0, 0, 255, 220 ) );
       for ( int blockIndex = 0; blockIndex < blockCount; ++blockIndex )
       {
         const double blockBaseLine = metrics.baselineOffset( blockIndex, Qgis::TextLayoutMode::Labeling );
-        painter->drawLine( QPointF( rect.left(), rect.top()  + blockBaseLine ),
-                           QPointF( rect.right(), rect.top() + blockBaseLine ) );
+
+        const QgsTextBlock &block = document.at( blockIndex );
+        const int fragmentCount = block.size();
+        double left = 0;
+        for ( int fragmentIndex = 0; fragmentIndex < fragmentCount; ++fragmentIndex )
+        {
+          const double fragmentVerticalOffset = metrics.fragmentVerticalOffset( blockIndex, fragmentIndex, Qgis::TextLayoutMode::Labeling );
+          const double right = left + metrics.fragmentHorizontalAdvance( blockIndex, fragmentIndex, Qgis::TextLayoutMode::Labeling );
+
+          if ( fragmentIndex > 0 )
+          {
+            QPen pen( QColor( 0, 0, 255, 220 ) );
+            pen.setStyle( Qt::PenStyle::DashLine );
+
+            painter->setPen( pen );
+
+            painter->drawLine( QPointF( rect.left() + left, rect.top() + blockBaseLine + fragmentVerticalOffset ),
+                               QPointF( rect.left() + left, rect.top() + prevBlockBaseline ) );
+
+          }
+
+          painter->setPen( QColor( 0, 0, 255, 220 ) );
+          painter->drawLine( QPointF( rect.left() + left, rect.top()  + blockBaseLine + fragmentVerticalOffset ),
+                             QPointF( rect.left() + right, rect.top() + blockBaseLine + fragmentVerticalOffset ) );
+          left = right;
+        }
+        prevBlockBaseline = blockBaseLine;
       }
     }
 

src/core/textrenderer/qgstextdocumentmetrics.cpp

@@ -23,6 +23,13 @@
 
 #include <QFontMetricsF>
 
+// to match QTextEngine handling of superscript/subscript font sizes
+constexpr double SUPERSCRIPT_SUBSCRIPT_FONT_SIZE_SCALING_FACTOR = 2.0 / 3.0;
+
+// to match Qt behavior in QTextLine::draw
+constexpr double SUPERSCRIPT_VERTICAL_BASELINE_ADJUSTMENT_FACTOR = 0.5;
+constexpr double SUBSCRIPT_VERTICAL_BASELINE_ADJUSTMENT_FACTOR = 1.0 / 6.0;
+
 QgsTextDocumentMetrics QgsTextDocumentMetrics::calculateMetrics( const QgsTextDocument &document, const QgsTextFormat &format, const QgsRenderContext &context, double scaleFactor )
 {
   QgsTextDocumentMetrics res;
@@ -48,14 +55,25 @@ QgsTextDocumentMetrics QgsTextDocumentMetrics::calculateMetrics( const QgsTextDo
 
   QVector < double > blockVerticalLineSpacing;
 
+  double outerXMin = 0;
+  double outerXMax = 0;
+  double outerYMinLabel = 0;
+  double outerYMaxLabel = 0;
+
   for ( int blockIndex = 0; blockIndex < blockSize; blockIndex++ )
   {
     const QgsTextBlock &block = document.at( blockIndex );
 
     double blockWidth = 0;
+    double blockXMax = 0;
+    double blockYMaxAdjustLabel = 0;
+
     double blockHeightUsingAscentDescent = 0;
     double blockHeightUsingLineSpacing = 0;
     double blockHeightVerticalOrientation = 0;
+
+    double blockHeightUsingAscentAccountingForVerticalOffset = 0;
+
     const int fragmentSize = block.size();
 
     double maxBlockAscent = 0;
@@ -64,18 +82,89 @@ QgsTextDocumentMetrics QgsTextDocumentMetrics::calculateMetrics( const QgsTextDo
     double maxBlockLeading = 0;
     double maxBlockMaxWidth = 0;
 
+    QList< double > fragmentVerticalOffsets;
+    fragmentVerticalOffsets.reserve( fragmentSize );
+
     QList< QFont > fragmentFonts;
     fragmentFonts.reserve( fragmentSize );
     QList< double >fragmentHorizontalAdvance;
     fragmentHorizontalAdvance.reserve( fragmentSize );
+
+    QFont previousNonSuperSubScriptFont;
+
     for ( int fragmentIndex = 0; fragmentIndex < fragmentSize; ++fragmentIndex )
     {
       const QgsTextFragment &fragment = block.at( fragmentIndex );
       const QgsTextCharacterFormat &fragmentFormat = fragment.characterFormat();
 
+      double fragmentHeightForVerticallyOffsetText = 0;
+      double fragmentYMaxAdjust = 0;
+
       QFont updatedFont = font;
       fragmentFormat.updateFontForFormat( updatedFont, context, scaleFactor );
-      const QFontMetricsF fm( updatedFont );
+
+      QFontMetricsF fm( updatedFont );
+
+      if ( fragmentIndex == 0 )
+        previousNonSuperSubScriptFont = updatedFont;
+
+      double fragmentVerticalOffset = 0;
+      if ( fragmentFormat.hasVerticalAlignmentSet() )
+      {
+        switch ( fragmentFormat.verticalAlignment() )
+        {
+          case Qgis::TextCharacterVerticalAlignment::Normal:
+            previousNonSuperSubScriptFont = updatedFont;
+            break;
+
+          case Qgis::TextCharacterVerticalAlignment::SuperScript:
+          {
+            const QFontMetricsF previousFM( previousNonSuperSubScriptFont );
+
+            if ( fragmentFormat.fontPointSize() < 0 )
+            {
+              // if fragment has no explicit font size set, then we scale the inherited font size to 60% of base font size
+              // this allows for easier use of super/subscript in labels as "my text<sup>2</sup>" will automatically render
+              // the superscript in a smaller font size. BUT if the fragment format HAS a non -1 font size then it indicates
+              // that the document has an explicit font size for the super/subscript element, eg "my text<sup style="font-size: 6pt">2</sup>"
+              // which we should respect
+              updatedFont.setPixelSize( updatedFont.pixelSize() * SUPERSCRIPT_SUBSCRIPT_FONT_SIZE_SCALING_FACTOR );
+              fm = QFontMetricsF( updatedFont );
+            }
+
+            // to match Qt behavior in QTextLine::draw
+            fragmentVerticalOffset = -( previousFM.ascent() + previousFM.descent() ) * SUPERSCRIPT_VERTICAL_BASELINE_ADJUSTMENT_FACTOR / scaleFactor;
+
+            // note -- this should really be fm.ascent(), not fm.capHeight() -- but in practice the ascent of most fonts is too large
+            // and causes unnecessarily large bounding boxes of vertically offset text!
+            fragmentHeightForVerticallyOffsetText = -fragmentVerticalOffset + fm.capHeight() / scaleFactor;
+            break;
+          }
+
+          case Qgis::TextCharacterVerticalAlignment::SubScript:
+          {
+            const QFontMetricsF previousFM( previousNonSuperSubScriptFont );
+
+            if ( fragmentFormat.fontPointSize() < 0 )
+            {
+              // see above!!
+              updatedFont.setPixelSize( updatedFont.pixelSize() * SUPERSCRIPT_SUBSCRIPT_FONT_SIZE_SCALING_FACTOR );
+              fm = QFontMetricsF( updatedFont );
+            }
+
+            // to match Qt behavior in QTextLine::draw
+            fragmentVerticalOffset = ( previousFM.ascent() + previousFM.descent() ) * SUBSCRIPT_VERTICAL_BASELINE_ADJUSTMENT_FACTOR / scaleFactor;
+
+            fragmentYMaxAdjust = fragmentVerticalOffset + fm.descent() / scaleFactor;
+            break;
+          }
+        }
+      }
+      else
+      {
+        previousNonSuperSubScriptFont = updatedFont;
+      }
+      fragmentVerticalOffsets << fragmentVerticalOffset;
 
       const double fragmentWidth = fm.horizontalAdvance( fragment.text() ) / scaleFactor;
 
@@ -85,12 +174,19 @@ QgsTextDocumentMetrics QgsTextDocumentMetrics::calculateMetrics( const QgsTextDo
       const double fragmentHeightUsingLineSpacing = fm.lineSpacing() / scaleFactor;
 
       blockWidth += fragmentWidth;
+      blockXMax += fragmentWidth;
       blockHeightUsingAscentDescent = std::max( blockHeightUsingAscentDescent, fragmentHeightUsingAscentDescent );
+
       blockHeightUsingLineSpacing = std::max( blockHeightUsingLineSpacing, fragmentHeightUsingLineSpacing );
       maxBlockAscent = std::max( maxBlockAscent, fm.ascent() / scaleFactor );
+
+      blockHeightUsingAscentAccountingForVerticalOffset = std::max( std::max( maxBlockAscent, fragmentHeightForVerticallyOffsetText ), blockHeightUsingAscentAccountingForVerticalOffset );
+
       maxBlockDescent = std::max( maxBlockDescent, fm.descent() / scaleFactor );
       maxBlockMaxWidth = std::max( maxBlockMaxWidth, fm.maxWidth() / scaleFactor );
 
+      blockYMaxAdjustLabel = std::max( blockYMaxAdjustLabel, fragmentYMaxAdjust );
+
       if ( ( fm.lineSpacing() / scaleFactor ) > maxLineSpacing )
       {
         maxLineSpacing = fm.lineSpacing() / scaleFactor;
@@ -117,6 +213,9 @@ QgsTextDocumentMetrics QgsTextDocumentMetrics::calculateMetrics( const QgsTextDo
       // for standard text rendering. Line height is also slightly different.
       currentLabelBaseline = -res.mFirstLineAscentOffset;
 
+      if ( blockHeightUsingAscentAccountingForVerticalOffset > maxBlockAscent )
+        outerYMinLabel = maxBlockAscent - blockHeightUsingAscentAccountingForVerticalOffset;
+
       // standard rendering - designed to exactly replicate QPainter's drawText method
       currentRectBaseline = -res.mFirstLineAscentOffset + lineHeight - 1 /*baseline*/;
 
@@ -141,11 +240,19 @@ QgsTextDocumentMetrics QgsTextDocumentMetrics::calculateMetrics( const QgsTextDo
         res.mLastLineAscentOffset = 0.25 * maxBlockAscent;
     }
 
+    if ( blockIndex == blockSize - 1 )
+    {
+      if ( blockYMaxAdjustLabel > maxBlockDescent )
+        outerYMaxLabel = blockYMaxAdjustLabel - maxBlockDescent;
+    }
+
     blockVerticalLineSpacing << ( format.lineHeightUnit() == QgsUnitTypes::RenderPercentage ? ( maxBlockMaxWidth * format.lineHeight() ) : lineHeightPainterUnits );
 
     res.mBlockHeights << blockHeightUsingLineSpacing;
 
     width = std::max( width, blockWidth );
+    outerXMax = std::max( outerXMax, blockXMax );
+
     heightVerticalOrientation = std::max( heightVerticalOrientation, blockHeightVerticalOrientation );
     res.mBlockWidths << blockWidth;
     res.mFragmentFonts << fragmentFonts;
@@ -154,6 +261,9 @@ QgsTextDocumentMetrics QgsTextDocumentMetrics::calculateMetrics( const QgsTextDo
     res.mBaselineOffsetsRectMode << currentRectBaseline;
     res.mBlockMaxDescent << maxBlockDescent;
     res.mBlockMaxCharacterWidth << maxBlockMaxWidth;
+    res.mFragmentVerticalOffsetsLabelMode << fragmentVerticalOffsets;
+    res.mFragmentVerticalOffsetsRectMode << fragmentVerticalOffsets;
+    res.mFragmentVerticalOffsetsPointMode << fragmentVerticalOffsets;
     res.mFragmentHorizontalAdvance << fragmentHorizontalAdvance;
 
     if ( blockIndex > 0 )
@@ -169,6 +279,8 @@ QgsTextDocumentMetrics QgsTextDocumentMetrics::calculateMetrics( const QgsTextDo
   // adjust baselines
   if ( !res.mBaselineOffsetsLabelMode.isEmpty() )
   {
+    // outerYMinLabel += res.mBaselineOffsetsLabelMode[0];
+
     const double labelModeBaselineAdjust = res.mBaselineOffsetsLabelMode.constLast() + res.mLastLineAscentOffset;
     const double pointModeBaselineAdjust = res.mBaselineOffsetsPointMode.constLast();
     for ( int i = 0; i < blockSize; ++i )
@@ -204,6 +316,10 @@ QgsTextDocumentMetrics QgsTextDocumentMetrics::calculateMetrics( const QgsTextDo
     res.mDocumentSizeVerticalOrientation = QSizeF( widthVerticalOrientation, heightVerticalOrientation );
   }
 
+  res.mOuterBoundsLabelMode = QRectF( outerXMin, -outerYMaxLabel,
+                                      outerXMax - outerXMin,
+                                      heightLabelMode - outerYMinLabel + outerYMaxLabel );
+
   return res;
 }
 
@@ -232,6 +348,31 @@ QSizeF QgsTextDocumentMetrics::documentSize( Qgis::TextLayoutMode mode, Qgis::Te
   BUILTIN_UNREACHABLE
 }
 
+QRectF QgsTextDocumentMetrics::outerBounds( Qgis::TextLayoutMode mode, Qgis::TextOrientation orientation ) const
+{
+  switch ( orientation )
+  {
+    case Qgis::TextOrientation::Horizontal:
+      switch ( mode )
+      {
+        case Qgis::TextLayoutMode::Rectangle:
+        case Qgis::TextLayoutMode::Point:
+          return QRectF();
+
+        case Qgis::TextLayoutMode::Labeling:
+          return mOuterBoundsLabelMode;
+      };
+      BUILTIN_UNREACHABLE
+
+    case Qgis::TextOrientation::Vertical:
+      return QRectF();
+    case Qgis::TextOrientation::RotationBased:
+      return QRectF(); // label mode only
+  }
+
+  BUILTIN_UNREACHABLE
+}
+
 double QgsTextDocumentMetrics::blockWidth( int blockIndex ) const
 {
   return mBlockWidths.value( blockIndex );
@@ -261,6 +402,20 @@ double QgsTextDocumentMetrics::fragmentHorizontalAdvance( int blockIndex, int fr
   return mFragmentHorizontalAdvance.value( blockIndex ).value( fragmentIndex );
 }
 
+double QgsTextDocumentMetrics::fragmentVerticalOffset( int blockIndex, int fragmentIndex, Qgis::TextLayoutMode mode ) const
+{
+  switch ( mode )
+  {
+    case Qgis::TextLayoutMode::Rectangle:
+      return mFragmentVerticalOffsetsRectMode.value( blockIndex ).value( fragmentIndex );
+    case Qgis::TextLayoutMode::Point:
+      return mFragmentVerticalOffsetsPointMode.value( blockIndex ).value( fragmentIndex );
+    case Qgis::TextLayoutMode::Labeling:
+      return mFragmentVerticalOffsetsLabelMode.value( blockIndex ).value( fragmentIndex );
+  }
+  BUILTIN_UNREACHABLE
+}
+
 double QgsTextDocumentMetrics::verticalOrientationXOffset( int blockIndex ) const
 {
   return mVerticalOrientationXOffsets.value( blockIndex );

src/core/textrenderer/qgstextdocumentmetrics.h

@@ -22,6 +22,7 @@
 
 #include <QVector>
 #include <QSizeF>
+#include <QRectF>
 
 class QgsTextDocument;
 class QgsRenderContext;
@@ -64,6 +65,17 @@ class CORE_EXPORT QgsTextDocumentMetrics
      */
     QSizeF documentSize( Qgis::TextLayoutMode mode, Qgis::TextOrientation orientation ) const;
 
+    /**
+     * Returns the outer bounds of the document, which is the documentSize() adjusted to account
+     * for any text elements which fall outside of the usual document margins (such as super or
+     * sub script elements)
+     *
+     * \warning Currently this is only supported for the Qgis::TextLayoutMode::Labeling mode.
+     *
+     * \since QGIS 3.30
+     */
+    QRectF outerBounds( Qgis::TextLayoutMode mode, Qgis::TextOrientation orientation ) const;
+
     /**
      * Returns the width of the block at the specified index.
      */
@@ -86,6 +98,14 @@ class CORE_EXPORT QgsTextDocumentMetrics
      */
     double fragmentHorizontalAdvance( int blockIndex, int fragmentIndex, Qgis::TextLayoutMode mode ) const;
 
+    /**
+     * Returns the vertical offset from a text block's baseline which should be applied
+     * to the fragment at the specified index within that block.
+     *
+     * \since QGIS 3.30
+     */
+    double fragmentVerticalOffset( int blockIndex, int fragmentIndex, Qgis::TextLayoutMode mode ) const;
+
     /**
      * Returns the vertical orientation x offset for the specified block.
      */
@@ -119,6 +139,8 @@ class CORE_EXPORT QgsTextDocumentMetrics
     QSizeF mDocumentSizePointRectMode;
     QSizeF mDocumentSizeVerticalOrientation;
 
+    QRectF mOuterBoundsLabelMode;
+
     QList < QList< QFont > > mFragmentFonts;
     QList< double > mBlockWidths;
     QList< double > mBlockHeights;
@@ -128,6 +150,10 @@ class CORE_EXPORT QgsTextDocumentMetrics
 
     QList< QList< double > > mFragmentHorizontalAdvance;
 
+    QList< QList< double > > mFragmentVerticalOffsetsLabelMode;
+    QList< QList< double > > mFragmentVerticalOffsetsPointMode;
+    QList< QList< double > > mFragmentVerticalOffsetsRectMode;
+
     QList< double > mVerticalOrientationXOffsets;
     QList< double > mBlockMaxDescent;
     QList< double > mBlockMaxCharacterWidth;

src/core/textrenderer/qgstextrenderer.cpp

@@ -364,7 +364,8 @@ double QgsTextRenderer::drawBuffer( QgsRenderContext &context, const QgsTextRend
         if ( component.extraWordSpacing || component.extraLetterSpacing )
           applyExtraSpacingForLineJustification( fragmentFont, component.extraWordSpacing, component.extraLetterSpacing );
 
-        path.addText( xOffset, 0, fragmentFont, fragment.text() );
+        const double yOffset = metrics.fragmentVerticalOffset( component.blockIndex, fragmentIndex, mode );
+        path.addText( xOffset, yOffset, fragmentFont, fragment.text() );
 
         xOffset += metrics.fragmentHorizontalAdvance( component.blockIndex, fragmentIndex, mode );
 
@@ -390,12 +391,15 @@ double QgsTextRenderer::drawBuffer( QgsRenderContext &context, const QgsTextRend
 
         const QFontMetricsF fragmentMetrics( fragmentFont );
 
+        const double fragmentYOffset = metrics.fragmentVerticalOffset( component.blockIndex, fragmentIndex, mode )
+                                       / 1;
+
         const QStringList parts = QgsPalLabeling::splitToGraphemes( fragment.text() );
         for ( const QString &part : parts )
         {
           double partXOffset = ( blockMaximumCharacterWidth - ( fragmentMetrics.horizontalAdvance( part ) / scaleFactor - letterSpacing ) ) / 2;
           partYOffset += fragmentMetrics.ascent() / scaleFactor;
-          path.addText( partXOffset, partYOffset, fragmentFont, part );
+          path.addText( partXOffset, partYOffset + fragmentYOffset, fragmentFont, part );
           partYOffset += letterSpacing;
         }
         partLastDescent = fragmentMetrics.descent() / scaleFactor;
@@ -525,7 +529,8 @@ void QgsTextRenderer::drawMask( QgsRenderContext &context, const QgsTextRenderer
   {
     const QFont fragmentFont = metrics.fragmentFont( component.blockIndex, fragmentIndex );
 
-    path.addText( xOffset, 0, fragmentFont, fragment.text() );
+    const double fragmentYOffset = metrics.fragmentVerticalOffset( component.blockIndex, fragmentIndex, mode );
+    path.addText( xOffset, fragmentYOffset, fragmentFont, fragment.text() );
 
     xOffset += metrics.fragmentHorizontalAdvance( component.blockIndex, fragmentIndex, mode );
     fragmentIndex++;
@@ -1621,7 +1626,9 @@ void QgsTextRenderer::drawTextInternalHorizontal( QgsRenderContext &context, con
           if ( extraWordSpace || extraLetterSpace )
             applyExtraSpacingForLineJustification( fragmentFont, extraWordSpace * fontScale, extraLetterSpace * fontScale );
 
-          path.addText( xOffset, 0, fragmentFont, fragment.text() );
+          const double yOffset = metrics.fragmentVerticalOffset( blockIndex, fragmentIndex, mode );
+
+          path.addText( xOffset, yOffset, fragmentFont, fragment.text() );
 
           QColor textColor = fragment.characterFormat().textColor().isValid() ? fragment.characterFormat().textColor() : format.color();
           textColor.setAlphaF( fragment.characterFormat().textColor().isValid() ? textColor.alphaF() * format.opacity() : format.opacity() );
@@ -1673,6 +1680,8 @@ void QgsTextRenderer::drawTextInternalHorizontal( QgsRenderContext &context, con
             if ( extraWordSpace || extraLetterSpace )
               applyExtraSpacingForLineJustification( fragmentFont, extraWordSpace * fontScale, extraLetterSpace * fontScale );
 
+            const double yOffset = metrics.fragmentVerticalOffset( blockIndex, fragmentIndex, mode );
+
             QColor textColor = fragment.characterFormat().textColor().isValid() ? fragment.characterFormat().textColor() : format.color();
             textColor.setAlphaF( fragment.characterFormat().textColor().isValid() ? textColor.alphaF() * format.opacity() : format.opacity() );
 
@@ -1681,7 +1690,7 @@ void QgsTextRenderer::drawTextInternalHorizontal( QgsRenderContext &context, con
             context.painter()->setRenderHint( QPainter::TextAntialiasing );
 
             context.painter()->scale( 1 / fontScale, 1 / fontScale );
-            context.painter()->drawText( xOffset, 0, fragment.text() );
+            context.painter()->drawText( xOffset, yOffset, fragment.text() );
             context.painter()->scale( fontScale, fontScale );
 
             xOffset += metrics.fragmentHorizontalAdvance( blockIndex, fragmentIndex, mode );

tests/src/python/test_qgstextrenderer.py

@@ -3310,6 +3310,75 @@ class PyQgsTextRenderer(unittest.TestCase):
             '<i>t</i><b style="font-size: 30pt">e</b><p><span style="color: red">s<span style="color: rgba(255,0,0,0.5); text-decoration: underline; font-size:80pt">t</span></span>'],
             point=QPointF(50, 200))
 
+    def testHtmlSuperSubscript(self):
+        format = QgsTextFormat()
+        format.setFont(getTestFont('bold'))
+        format.setSize(60)
+        format.setSizeUnit(QgsUnitTypes.RenderPoints)
+        format.setColor(QColor(255, 0, 0))
+        format.setAllowHtmlFormatting(True)
+        assert self.checkRenderPoint(format, 'text_html_supersubscript', None, text=[
+            '<sub>sub</sub>N<sup>sup</sup>'],
+            point=QPointF(50, 200))
+
+    def testHtmlSuperSubscriptFixedFontSize(self):
+        format = QgsTextFormat()
+        format.setFont(getTestFont('bold'))
+        format.setSize(60)
+        format.setSizeUnit(QgsUnitTypes.RenderPoints)
+        format.setColor(QColor(255, 0, 0))
+        format.setAllowHtmlFormatting(True)
+        assert self.checkRenderPoint(format, 'text_html_supersubscript_fixed_font_size', None, text=[
+            '<sub style="font-size:80pt">s<span style="font-size:30pt">u</span></sub>N<sup style="font-size:40pt">s<span style="font-size: 20pt">up</span></sup>'],
+            point=QPointF(50, 200))
+
+    def testHtmlSuperSubscriptBuffer(self):
+        format = QgsTextFormat()
+        format.setFont(getTestFont('bold'))
+        format.setSize(60)
+        format.setSizeUnit(QgsUnitTypes.RenderPoints)
+        format.setColor(QColor(255, 0, 0))
+        format.setAllowHtmlFormatting(True)
+        format.buffer().setEnabled(True)
+        format.buffer().setSize(5)
+        format.buffer().setColor(QColor(50, 150, 200))
+        assert self.checkRenderPoint(format, 'text_html_supersubscript_buffer', None, text=[
+            '<sub>sub</sub>N<sup>sup</sup>'],
+            point=QPointF(50, 200))
+
+    def testHtmlSuperSubscriptShadow(self):
+        format = QgsTextFormat()
+        format.setFont(getTestFont('bold'))
+        format.setSize(60)
+        format.setSizeUnit(QgsUnitTypes.RenderPoints)
+        format.setColor(QColor(0, 255, 0))
+        format.setAllowHtmlFormatting(True)
+        format.shadow().setEnabled(True)
+        format.shadow().setOffsetDistance(5)
+        format.shadow().setBlurRadius(0)
+        format.shadow().setColor(QColor(50, 150, 200))
+        assert self.checkRenderPoint(format, 'text_html_supersubscript_shadow', None, text=[
+            '<sub>sub</sub>N<sup>sup</sup>'],
+            point=QPointF(50, 200))
+
+    def testHtmlSuperSubscriptBufferShadow(self):
+        format = QgsTextFormat()
+        format.setFont(getTestFont('bold'))
+        format.setSize(60)
+        format.setSizeUnit(QgsUnitTypes.RenderPoints)
+        format.setColor(QColor(0, 255, 0))
+        format.setAllowHtmlFormatting(True)
+        format.buffer().setEnabled(True)
+        format.buffer().setSize(5)
+        format.buffer().setColor(QColor(200, 50, 150))
+        format.shadow().setEnabled(True)
+        format.shadow().setOffsetDistance(5)
+        format.shadow().setBlurRadius(0)
+        format.shadow().setColor(QColor(50, 150, 200))
+        assert self.checkRenderPoint(format, 'text_html_supersubscript_buffer_shadow', None, text=[
+            '<sub>sub</sub>N<sup>sup</sup>'],
+            point=QPointF(50, 200))
+
     def testTextRenderFormat(self):
         format = QgsTextFormat()
         format.setFont(getTestFont('bold'))