From d54b978c0cba2bf75e145c9e1b00d2a9ab495d70 Mon Sep 17 00:00:00 2001
From: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>
Date: Mon, 19 Dec 2022 10:05:33 +0100
Subject: [PATCH 15/19] 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.

Pick-to: 5.15 6.2 6.4 6.5
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)
---
 src/quick/scenegraph/qsgdefaultglyphnode_p.cpp | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/quick/scenegraph/qsgdefaultglyphnode_p.cpp b/src/quick/scenegraph/qsgdefaultglyphnode_p.cpp
index f912da5799..fd128aa06e 100644
--- a/src/quick/scenegraph/qsgdefaultglyphnode_p.cpp
+++ b/src/quick/scenegraph/qsgdefaultglyphnode_p.cpp
@@ -839,9 +839,11 @@ 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()));
+             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);
-- 
2.40.0