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>

3 files changed, 22 insertions, 8 deletions

diff --git a/src/opengl/qopenglpaintengine.cpp b/src/opengl/qopenglpaintengine.cpp
index 3bb3496de5..9c8325ca11 100644
--- a/src/opengl/qopenglpaintengine.cpp
+++ b/src/opengl/qopenglpaintengine.cpp
@@ -1797,11 +1797,13 @@ void QOpenGL2PaintEngineExPrivate::drawCachedGlyphs(QFontEngine::GlyphFormat gly
     if (recreateVertexArrays) {
         cache->setPaintEnginePrivate(this);
         if (!cache->populate(fe, staticTextItem->numGlyphs,
-                             staticTextItem->glyphs, staticTextItem->glyphPositions)) {
+                             staticTextItem->glyphs, staticTextItem->glyphPositions,
+                             s->renderHints)) {
             // No space for glyphs in cache. We need to reset it and try again.
             cache->clear();
             cache->populate(fe, staticTextItem->numGlyphs,
-                            staticTextItem->glyphs, staticTextItem->glyphPositions);
+                            staticTextItem->glyphs, staticTextItem->glyphPositions,
+                            s->renderHints);
         }
 
         if (cache->hasPendingGlyphs()) {
@@ -1874,10 +1876,15 @@ void QOpenGL2PaintEngineExPrivate::drawCachedGlyphs(QFontEngine::GlyphFormat gly
         textureCoordinates->clear();
 
         bool supportsSubPixelPositions = fe->supportsSubPixelPositions();
+        bool verticalSubPixelPositions = fe->supportsVerticalSubPixelPositions()
+                && (s->renderHints & QPainter::VerticalSubpixelPositioning) != 0;
         for (int i=0; i<staticTextItem->numGlyphs; ++i) {
-            QFixed subPixelPosition;
-            if (supportsSubPixelPositions)
-                subPixelPosition = fe->subPixelPositionForX(staticTextItem->glyphPositions[i].x);
+            QFixedPoint subPixelPosition;
+            if (supportsSubPixelPositions) {
+                subPixelPosition = fe->subPixelPositionFor(staticTextItem->glyphPositions[i]);
+                if (!verticalSubPixelPositions)
+                    subPixelPosition.y = 0;
+            }
 
             QTextureGlyphCache::GlyphAndSubPixelPosition glyph(staticTextItem->glyphs[i], subPixelPosition);
 
@@ -1886,7 +1893,10 @@ void QOpenGL2PaintEngineExPrivate::drawCachedGlyphs(QFontEngine::GlyphFormat gly
                 continue;
 
             int x = qFloor(staticTextItem->glyphPositions[i].x.toReal() * cache->transform().m11()) + c.baseLineX - margin;
-            int y = qRound(staticTextItem->glyphPositions[i].y.toReal() * cache->transform().m22()) - c.baseLineY - margin;
+            int y = verticalSubPixelPositions
+                    ? qRound(staticTextItem->glyphPositions[i].y.toReal() * cache->transform().m22())
+                    : qFloor(staticTextItem->glyphPositions[i].y.toReal() * cache->transform().m22());
+            y -= c.baseLineY + margin;
 
             vertexCoordinates->addQuad(QRectF(x, y, c.w, c.h));
             textureCoordinates->addQuad(QRectF(c.x*dx, c.y*dy, c.w * dx, c.h * dy));
diff --git a/src/opengl/qopengltextureglyphcache.cpp b/src/opengl/qopengltextureglyphcache.cpp
index a38d4328cc..6c8971fddb 100644
--- a/src/opengl/qopengltextureglyphcache.cpp
+++ b/src/opengl/qopengltextureglyphcache.cpp
@@ -421,7 +421,9 @@ void QOpenGLTextureGlyphCache::resizeTextureData(int width, int height)
     }
 }
 
-void QOpenGLTextureGlyphCache::fillTexture(const Coord &c, glyph_t glyph, QFixed subPixelPosition)
+void QOpenGLTextureGlyphCache::fillTexture(const Coord &c,
+                                           glyph_t glyph,
+                                           const QFixedPoint &subPixelPosition)
 {
     QOpenGLContext *ctx = QOpenGLContext::currentContext();
     if (ctx == nullptr) {
diff --git a/src/opengl/qopengltextureglyphcache_p.h b/src/opengl/qopengltextureglyphcache_p.h
index 15ecd6209b..5b30881e55 100644
--- a/src/opengl/qopengltextureglyphcache_p.h
+++ b/src/opengl/qopengltextureglyphcache_p.h
@@ -115,7 +115,9 @@ public:
 
     virtual void createTextureData(int width, int height) override;
     virtual void resizeTextureData(int width, int height) override;
-    virtual void fillTexture(const Coord &c, glyph_t glyph, QFixed subPixelPosition) override;
+    virtual void fillTexture(const Coord &c,
+                             glyph_t glyph,
+                             const QFixedPoint &subPixelPosition) override;
     virtual int glyphPadding() const override;
     virtual int maxTextureWidth() const override;
     virtual int maxTextureHeight() const override;