Implement vertical subpixel positioning where available

For some use cases, vertical subpixel positioning may be
useful, as it allows you to vertically align text with
other painting primitives. This does impose an overhead,
so we make it opt-int with a render hint on the painter.

Note that this is only supported on Freetype currently.
It might be possible to support on older macOS versions,
prior to Mojave (which has disabled subpixel positioning
entirely), but since it would have limited usefulness
and Freetype is cross-platform anyway, I skipped that.

Note: This drive-by-fixes an issue with subpixel
positioning where glyphs would always be offset by 1/64,
because we added the aliasing offset *after* we had
determined the closest subpixel position. The idea of
this, as far as I can understand, is rather to snap to
nearest subpixel position upwards, not to add an offset
to all glyphs, so it should be added before finding the
correct position. It had a subtle visual effect when
animating the position. It might be that we could get
rid of it entirely, as I haven't been able to reproduce
any issues with that, but I have moved it instead, to
match what I believe the intention was.

[ChangeLog][QtGui][Text] Added render hint flag
QPainter::VerticalSubpixelPositioning which will position
text at subpixel positions vertically whenever supported.
In absence of this, text position will be rounded
vertically as before.

Fixes: QTBUG-35682
Change-Id: I8ce7a72a64e5a0924dac7c244e3e07c2938bfd09
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

1 files changed, 46 insertions, 19 deletions

diff --git a/src/gui/text/freetype/qfontengine_ft.cpp b/src/gui/text/freetype/qfontengine_ft.cpp
index 2937857f71..42985fcaa6 100644
--- a/src/gui/text/freetype/qfontengine_ft.cpp
+++ b/src/gui/text/freetype/qfontengine_ft.cpp
@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2021 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtGui module of the Qt Toolkit.
@@ -943,7 +943,7 @@ static inline void transformBoundingBox(int *left, int *top, int *right, int *bo
 }
 
 QFontEngineFT::Glyph *QFontEngineFT::loadGlyph(QGlyphSet *set, uint glyph,
-                                               QFixed subPixelPosition,
+                                               const QFixedPoint &subPixelPosition,
                                                GlyphFormat format,
                                                bool fetchMetricsOnly,
                                                bool disableOutlineDrawing) const
@@ -967,8 +967,8 @@ QFontEngineFT::Glyph *QFontEngineFT::loadGlyph(QGlyphSet *set, uint glyph,
     FT_Matrix matrix = freetype->matrix;
 
     FT_Vector v;
-    v.x = format == Format_Mono ? 0 : FT_Pos(subPixelPosition.value());
-    v.y = 0;
+    v.x = format == Format_Mono ? 0 : FT_Pos(subPixelPosition.x.value());
+    v.y = format == Format_Mono ? 0 : FT_Pos(-subPixelPosition.y.value());
     FT_Set_Transform(face, &matrix, &v);
 
     bool hsubpixel = false;
@@ -1644,7 +1644,11 @@ void QFontEngineFT::recalcAdvances(QGlyphLayout *glyphs, QFontEngine::ShaperFlag
         } else {
             if (!face)
                 face = lockFace();
-            g = loadGlyph(cacheEnabled ? &defaultGlyphSet : nullptr, glyphs->glyphs[i], 0, Format_None, true);
+            g = loadGlyph(cacheEnabled ? &defaultGlyphSet : nullptr,
+                          glyphs->glyphs[i],
+                          QFixedPoint(),
+                          Format_None,
+                          true);
             if (g)
                 glyphs->advances[i] = design ? QFixed::fromFixed(g->linearAdvance) : QFixed(g->advance);
             else
@@ -1685,7 +1689,11 @@ glyph_metrics_t QFontEngineFT::boundingBox(const QGlyphLayout &glyphs)
         if (!g) {
             if (!face)
                 face = lockFace();
-            g = loadGlyph(cacheEnabled ? &defaultGlyphSet : nullptr, glyphs.glyphs[i], 0, Format_None, true);
+            g = loadGlyph(cacheEnabled ? &defaultGlyphSet : nullptr,
+                          glyphs.glyphs[i],
+                          QFixedPoint(),
+                          Format_None,
+                          true);
         }
         if (g) {
             QFixed x = overall.xoff + glyphs.offsets[i].x + g->x;
@@ -1730,7 +1738,11 @@ glyph_metrics_t QFontEngineFT::boundingBox(glyph_t glyph)
     Glyph *g = cacheEnabled ? defaultGlyphSet.getGlyph(glyph) : nullptr;
     if (!g) {
         face = lockFace();
-        g = loadGlyph(cacheEnabled ? &defaultGlyphSet : nullptr, glyph, 0, Format_None, true);
+        g = loadGlyph(cacheEnabled ? &defaultGlyphSet : nullptr,
+                      glyph,
+                      QFixedPoint(),
+                      Format_None,
+                      true);
     }
     if (g) {
         overall.x = g->x;
@@ -1762,10 +1774,13 @@ glyph_metrics_t QFontEngineFT::boundingBox(glyph_t glyph)
 
 glyph_metrics_t QFontEngineFT::boundingBox(glyph_t glyph, const QTransform &matrix)
 {
-    return alphaMapBoundingBox(glyph, 0, matrix, QFontEngine::Format_None);
+    return alphaMapBoundingBox(glyph, QFixedPoint(), matrix, QFontEngine::Format_None);
 }
 
-glyph_metrics_t QFontEngineFT::alphaMapBoundingBox(glyph_t glyph, QFixed subPixelPosition, const QTransform &matrix, QFontEngine::GlyphFormat format)
+glyph_metrics_t QFontEngineFT::alphaMapBoundingBox(glyph_t glyph,
+                                                   const QFixedPoint &subPixelPosition,
+                                                   const QTransform &matrix,
+                                                   QFontEngine::GlyphFormat format)
 {
     Glyph *g = loadGlyphFor(glyph, subPixelPosition, format, matrix, true);
 
@@ -1828,8 +1843,10 @@ static inline QImage alphaMapFromGlyphData(QFontEngineFT::Glyph *glyph, QFontEng
     return img;
 }
 
-QFontEngine::Glyph *QFontEngineFT::glyphData(glyph_t glyphIndex, QFixed subPixelPosition,
-                                             QFontEngine::GlyphFormat neededFormat, const QTransform &t)
+QFontEngine::Glyph *QFontEngineFT::glyphData(glyph_t glyphIndex,
+                                             const QFixedPoint &subPixelPosition,
+                                             QFontEngine::GlyphFormat neededFormat,
+                                             const QTransform &t)
 {
     Q_ASSERT(cacheEnabled);
 
@@ -1854,7 +1871,7 @@ static inline bool is2dRotation(const QTransform &t)
 }
 
 QFontEngineFT::Glyph *QFontEngineFT::loadGlyphFor(glyph_t g,
-                                                  QFixed subPixelPosition,
+                                                  const QFixedPoint &subPixelPosition,
                                                   GlyphFormat format,
                                                   const QTransform &t,
                                                   bool fetchBoundingBox,
@@ -1882,12 +1899,14 @@ QFontEngineFT::Glyph *QFontEngineFT::loadGlyphFor(glyph_t g,
     return glyph;
 }
 
-QImage QFontEngineFT::alphaMapForGlyph(glyph_t g, QFixed subPixelPosition)
+QImage QFontEngineFT::alphaMapForGlyph(glyph_t g, const QFixedPoint &subPixelPosition)
 {
     return alphaMapForGlyph(g, subPixelPosition, QTransform());
 }
 
-QImage QFontEngineFT::alphaMapForGlyph(glyph_t g, QFixed subPixelPosition, const QTransform &t)
+QImage QFontEngineFT::alphaMapForGlyph(glyph_t g,
+                                       const QFixedPoint &subPixelPosition,
+                                       const QTransform &t)
 {
     const GlyphFormat neededFormat = antialias ? Format_A8 : Format_Mono;
 
@@ -1902,7 +1921,9 @@ QImage QFontEngineFT::alphaMapForGlyph(glyph_t g, QFixed subPixelPosition, const
     return img;
 }
 
-QImage QFontEngineFT::alphaRGBMapForGlyph(glyph_t g, QFixed subPixelPosition, const QTransform &t)
+QImage QFontEngineFT::alphaRGBMapForGlyph(glyph_t g,
+                                          const QFixedPoint &subPixelPosition,
+                                          const QTransform &t)
 {
     if (t.type() > QTransform::TxRotate)
         return QFontEngine::alphaRGBMapForGlyph(g, subPixelPosition, t);
@@ -1923,7 +1944,10 @@ QImage QFontEngineFT::alphaRGBMapForGlyph(glyph_t g, QFixed subPixelPosition, co
     return QFontEngine::alphaRGBMapForGlyph(g, subPixelPosition, t);
 }
 
-QImage QFontEngineFT::bitmapForGlyph(glyph_t g, QFixed subPixelPosition, const QTransform &t, const QColor &color)
+QImage QFontEngineFT::bitmapForGlyph(glyph_t g,
+                                     const QFixedPoint &subPixelPosition,
+                                     const QTransform &t,
+                                     const QColor &color)
 {
     Q_UNUSED(color);
 
@@ -1952,7 +1976,7 @@ QImage QFontEngineFT::bitmapForGlyph(glyph_t g, QFixed subPixelPosition, const Q
 
 void QFontEngineFT::removeGlyphFromCache(glyph_t glyph)
 {
-    defaultGlyphSet.removeGlyphFromCache(glyph, 0);
+    defaultGlyphSet.removeGlyphFromCache(glyph, QFixedPoint());
 }
 
 int QFontEngineFT::glyphCount() const
@@ -2033,7 +2057,8 @@ void QFontEngineFT::QGlyphSet::clear()
     glyph_data.clear();
 }
 
-void QFontEngineFT::QGlyphSet::removeGlyphFromCache(glyph_t index, QFixed subPixelPosition)
+void QFontEngineFT::QGlyphSet::removeGlyphFromCache(glyph_t index,
+                                                    const QFixedPoint &subPixelPosition)
 {
     if (useFastGlyphData(index, subPixelPosition)) {
         if (fast_glyph_data[index]) {
@@ -2047,7 +2072,9 @@ void QFontEngineFT::QGlyphSet::removeGlyphFromCache(glyph_t index, QFixed subPix
     }
 }
 
-void QFontEngineFT::QGlyphSet::setGlyph(glyph_t index, QFixed subPixelPosition, Glyph *glyph)
+void QFontEngineFT::QGlyphSet::setGlyph(glyph_t index,
+                                        const QFixedPoint &subPixelPosition,
+                                        Glyph *glyph)
 {
     if (useFastGlyphData(index, subPixelPosition)) {
         if (!fast_glyph_data[index])