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, 17 insertions, 8 deletions

diff --git a/src/gui/painting/qtextureglyphcache.cpp b/src/gui/painting/qtextureglyphcache.cpp
index 3728060caf..dfb937ed84 100644
--- a/src/gui/painting/qtextureglyphcache.cpp
+++ b/src/gui/painting/qtextureglyphcache.cpp
@@ -64,7 +64,7 @@ int QTextureGlyphCache::calculateSubPixelPositionCount(glyph_t glyph) const
     QImage images[NumSubpixelPositions];
     int numImages = 0;
     for (int i = 0; i < NumSubpixelPositions; ++i) {
-        QImage img = textureMapForGlyph(glyph, QFixed::fromReal(i / 12.0));
+        QImage img = textureMapForGlyph(glyph, QFixedPoint(QFixed::fromReal(i / 12.0), 0));
 
         if (numImages == 0) {
             QPainterPath path;
@@ -92,8 +92,11 @@ int QTextureGlyphCache::calculateSubPixelPositionCount(glyph_t glyph) const
     return numImages;
 }
 
-bool QTextureGlyphCache::populate(QFontEngine *fontEngine, int numGlyphs, const glyph_t *glyphs,
-                                                const QFixedPoint *positions)
+bool QTextureGlyphCache::populate(QFontEngine *fontEngine,
+                                  int numGlyphs,
+                                  const glyph_t *glyphs,
+                                  const QFixedPoint *positions,
+                                  QPainter::RenderHints renderHints)
 {
 #ifdef CACHE_DEBUG
     printf("Populating with %d glyphs\n", numGlyphs);
@@ -105,6 +108,8 @@ bool QTextureGlyphCache::populate(QFontEngine *fontEngine, int numGlyphs, const
     const int paddingDoubled = padding * 2;
 
     bool supportsSubPixelPositions = fontEngine->supportsSubPixelPositions();
+    bool verticalSubPixelPositions = fontEngine->supportsVerticalSubPixelPositions()
+            && (renderHints & QPainter::VerticalSubpixelPositioning) != 0;
     if (fontEngine->m_subPixelPositionCount == 0) {
         if (!supportsSubPixelPositions) {
             fontEngine->m_subPixelPositionCount = 1;
@@ -127,10 +132,12 @@ bool QTextureGlyphCache::populate(QFontEngine *fontEngine, int numGlyphs, const
     for (int i=0; i < numGlyphs; ++i) {
         const glyph_t glyph = glyphs[i];
 
-        QFixed subPixelPosition;
+        QFixedPoint subPixelPosition;
         if (supportsSubPixelPositions) {
-            QFixed x = positions != nullptr ? positions[i].x : QFixed();
-            subPixelPosition = fontEngine->subPixelPositionForX(x);
+            QFixedPoint pos = positions != nullptr ? positions[i] : QFixedPoint();
+            subPixelPosition = fontEngine->subPixelPositionFor(pos);
+            if (!verticalSubPixelPositions)
+                subPixelPosition.y = 0;
         }
 
         if (coords.contains(GlyphAndSubPixelPosition(glyph, subPixelPosition)))
@@ -263,7 +270,7 @@ void QTextureGlyphCache::fillInPendingGlyphs()
     m_pendingGlyphs.clear();
 }
 
-QImage QTextureGlyphCache::textureMapForGlyph(glyph_t g, QFixed subPixelPosition) const
+QImage QTextureGlyphCache::textureMapForGlyph(glyph_t g, const QFixedPoint &subPixelPosition) const
 {
     switch (m_format) {
     case QFontEngine::Format_A32:
@@ -317,7 +324,9 @@ void QImageTextureGlyphCache::createTextureData(int width, int height)
     m_image.fill(0);
 }
 
-void QImageTextureGlyphCache::fillTexture(const Coord &c, glyph_t g, QFixed subPixelPosition)
+void QImageTextureGlyphCache::fillTexture(const Coord &c,
+                                          glyph_t g,
+                                          const QFixedPoint &subPixelPosition)
 {
     QImage mask = textureMapForGlyph(g, subPixelPosition);