Fix missing glyphs when using NativeRendering

When we look up glyphs with subpixel positions in the glyph cache,
we use the calculated subpixel position (from a set of predefined
subpixel positions) as key. In some very rare cases, we could end
up with different subpixel positions when looking up an on-screen
position than when we entered it into the cache, due to numerical
differences when doing the calculation.

The reason for this was that when entering the glyph into the
cache, we used the 16.6 fixed point representation, whereas when
looking up, we used the unmodified float. In some cases, the
converted fixed point approximation might snap to a different
predefined subpixel position than the floating point equivalent.

To avoid this, we reuse the converted fixed point positions when
looking up the glyphs in the cache.

[ChangeLog][Text] Fixed an issue where text using NativeRendering
would sometimes be missing glyphs.

Fixes: QTBUG-108713
Change-Id: Iecc264eb3d27e875c24257eaefcfb18a1a5fb5be
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Lars Knoll <lars@knoll.priv.no>
(cherry picked from commit 4bad329985b75090c68a70cceee7edadc172d7ab)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/src/quick/scenegraph/qsgdefaultglyphnode_p.cpp b/src/quick/scenegraph/qsgdefaultglyphnode_p.cpp
index 61d542fbcb..8d76aa56a2 100644
--- a/src/quick/scenegraph/qsgdefaultglyphnode_p.cpp
+++ b/src/quick/scenegraph/qsgdefaultglyphnode_p.cpp
@@ -842,9 +842,11 @@ void QSGTextMaskMaterial::populate(const QPointF &p,
     bool supportsSubPixelPositions = fontD->fontEngine->supportsSubPixelPositions();
     for (int i=0; i<glyphIndexes.size(); ++i) {
          QPointF glyphPosition = glyphPositions.at(i) + position;
+         QFixedPoint fixedPointPosition = fixedPointPositions.at(i);
+
          QFixed subPixelPosition;
          if (supportsSubPixelPositions)
-             subPixelPosition = fontD->fontEngine->subPixelPositionForX(QFixed::fromReal(glyphPosition.x() * glyphCacheScaleX));
+             subPixelPosition = fontD->fontEngine->subPixelPositionForX(QFixed::fromReal(fixedPointPosition.x.toReal() * glyphCacheScaleX));
 
          QTextureGlyphCache::GlyphAndSubPixelPosition glyph(glyphIndexes.at(i), subPixelPosition);
          const QTextureGlyphCache::Coord &c = cache->coords.value(glyph);