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, 8 insertions, 7 deletions

diff --git a/src/gui/text/coretext/qfontengine_coretext_p.h b/src/gui/text/coretext/qfontengine_coretext_p.h
index 1239072677..361153b37a 100644
--- a/src/gui/text/coretext/qfontengine_coretext_p.h
+++ b/src/gui/text/coretext/qfontengine_coretext_p.h
@@ -91,7 +91,8 @@ public:
     bool canRender(const QChar *string, int len) const override;
 
     int synthesized() const override { return synthesisFlags; }
-    bool supportsSubPixelPositions() const override { return true; }
+    bool supportsHorizontalSubPixelPositions() const override { return true; }
+    bool supportsVerticalSubPixelPositions() const override { return false; }
 
     QFixed lineThickness() const override;
     QFixed underlinePosition() const override;
@@ -101,11 +102,11 @@ public:
     FaceId faceId() const override;
     bool getSfntTableData(uint /*tag*/, uchar * /*buffer*/, uint * /*length*/) const override;
     void getUnscaledGlyph(glyph_t glyph, QPainterPath *path, glyph_metrics_t *metrics) override;
-    QImage alphaMapForGlyph(glyph_t, QFixed subPixelPosition) override;
-    QImage alphaMapForGlyph(glyph_t glyph, QFixed subPixelPosition, const QTransform &t) override;
-    QImage alphaRGBMapForGlyph(glyph_t, QFixed subPixelPosition, const QTransform &t) override;
-    glyph_metrics_t alphaMapBoundingBox(glyph_t glyph, QFixed, const QTransform &matrix, GlyphFormat) override;
-    QImage bitmapForGlyph(glyph_t, QFixed subPixelPosition, const QTransform &t, const QColor &color) override;
+    QImage alphaMapForGlyph(glyph_t, const QFixedPoint &subPixelPosition) override;
+    QImage alphaMapForGlyph(glyph_t glyph, const QFixedPoint &subPixelPosition, const QTransform &t) override;
+    QImage alphaRGBMapForGlyph(glyph_t, const QFixedPoint &subPixelPosition, const QTransform &t) override;
+    glyph_metrics_t alphaMapBoundingBox(glyph_t glyph, const QFixedPoint &, const QTransform &matrix, GlyphFormat) override;
+    QImage bitmapForGlyph(glyph_t, const QFixedPoint &subPixelPosition, const QTransform &t, const QColor &color) override;
     QFixed emSquareSize() const override;
     void doKerning(QGlyphLayout *g, ShaperFlags flags) const override;
 
@@ -132,7 +133,7 @@ public:
 protected:
     QCoreTextFontEngine(const QFontDef &def);
     void init();
-    QImage imageForGlyph(glyph_t glyph, QFixed subPixelPosition, const QTransform &m, const QColor &color = QColor());
+    QImage imageForGlyph(glyph_t glyph, const QFixedPoint &subPixelPosition, const QTransform &m, const QColor &color = QColor());
     void loadAdvancesForGlyphs(QVarLengthArray<CGGlyph> &cgGlyphs, QGlyphLayout *glyphs) const;
     bool hasColorGlyphs() const;
     bool shouldAntialias() const;