QQuickNinePatchImage: fix aliasing by respecting the smooth property

When scaling with fractional values, aliasing can occur. Use the same
approach as BorderImage and respect the smooth property of
NinePatchImage. This property comes from QQuickImageBase, which sets it
to true by default. We change QQuickNinePatchImage's default value for
it to false, but this can be overridden by setting
QT_QUICK_CONTROLS_IMAGINE_SMOOTH to 1.

As NinePatchImage is not public API, and users would have to set the
smooth property on every image (where some items contain multiple
NinePatchImages), it's better to have one place to set this for all
controls.

[ChangeLog][Controls] The Imagine style now supports smooth scaling for
9-patch images when the QT_QUICK_CONTROLS_IMAGINE_SMOOTH environment
variable is set to 1.

Fixes: QTBUG-107989
Pick-to: 6.2 6.4
Change-Id: I250a041f87c0270d67af191168f7bcfbf6237925
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

2 files changed, 46 insertions, 1 deletions

diff --git a/src/quickcontrols2/doc/src/includes/qquickimaginestyle.qdocinc b/src/quickcontrols2/doc/src/includes/qquickimaginestyle.qdocinc
index f6fe5a970a..411c6c1088 100644
--- a/src/quickcontrols2/doc/src/includes/qquickimaginestyle.qdocinc
+++ b/src/quickcontrols2/doc/src/includes/qquickimaginestyle.qdocinc
@@ -53,5 +53,8 @@
 
             \note Due to a technical limitation, the path should not be named
                   \e "imagine" if it is relative to the \c qtquickcontrols2.conf file.
+        \li \c QT_QUICK_CONTROLS_IMAGINE_SMOOTH
+        \li Set to \c 1 to enable smooth scaling for 9-patch images.
+            This environment variable was added in Qt 6.5.
 \endtable
 //! [env]
diff --git a/src/quickcontrols2impl/qquickninepatchimage.cpp b/src/quickcontrols2impl/qquickninepatchimage.cpp
index 48a892f812..3f8a9e03e9 100644
--- a/src/quickcontrols2impl/qquickninepatchimage.cpp
+++ b/src/quickcontrols2impl/qquickninepatchimage.cpp
@@ -54,6 +54,9 @@ QList<qreal> QQuickNinePatchData::coordsForSize(qreal size) const
     return coords;
 }
 
+/*
+    Adds the 0 index coordinate if appropriate, and the one at "size".
+*/
 void QQuickNinePatchData::fill(const QList<qreal> &coords, qreal size)
 {
     data.clear();
@@ -174,6 +177,31 @@ public:
     QQuickNinePatchData yDivs;
 };
 
+/*
+    Examines each pixel in a horizontal or vertical (if offset is equal to the image's width)
+    line, storing the start and end index ("coordinate") of each 9-patch line.
+
+    For instance, in the 7x3 (9x5 actual size) 9-patch image below, which has no horizontal
+    stretchable area, it would return {}:
+
+     +-----+
+     |     |
+     +-----+
+
+    If indices 3 to 5 were marked, it would return {2, 5}:
+
+       xxx
+     +-----+
+     |     |
+     +-----+
+
+    If indices 3 and 5 were marked, it would store {0, 2, 3, 4, 5, 7}:
+
+       x x
+     +-----+
+     |     |
+     +-----+
+*/
 static QList<qreal> readCoords(const QRgb *data, int from, int count, int offset, QRgb color)
 {
     int p1 = -1;
@@ -182,12 +210,16 @@ static QList<qreal> readCoords(const QRgb *data, int from, int count, int offset
         int p2 = from + i * offset;
         if (data[p2] == color) {
             // colored pixel
-            if (p1 == -1)
+            if (p1 == -1) {
+                // This is the start of a 9-patch line.
                 p1 = i;
+            }
         } else {
             // empty pixel
             if (p1 != -1) {
+                // This is the end of a 9-patch line; add the start and end indices as coordinates...
                 coords << p1 << i;
+                // ... and reset p1 so that we can search for the next one.
                 p1 = -1;
             }
         }
@@ -195,6 +227,12 @@ static QList<qreal> readCoords(const QRgb *data, int from, int count, int offset
     return coords;
 }
 
+/*
+    Called whenever a 9-patch image is set as the image's source.
+
+    Reads the 9-patch lines from the source image and sets the
+    inset and padding properties accordingly.
+*/
 void QQuickNinePatchImagePrivate::updatePatches()
 {
     if (ninePatch.isNull())
@@ -299,6 +337,8 @@ void QQuickNinePatchImagePrivate::updateInsets(const QList<qreal> &horizontal, c
 QQuickNinePatchImage::QQuickNinePatchImage(QQuickItem *parent)
     : QQuickImage(*(new QQuickNinePatchImagePrivate), parent)
 {
+    Q_D(QQuickNinePatchImage);
+    d->smooth = qEnvironmentVariableIntValue("QT_QUICK_CONTROLS_IMAGINE_SMOOTH");
 }
 
 qreal QQuickNinePatchImage::topPadding() const
@@ -432,6 +472,8 @@ QSGNode *QQuickNinePatchImage::updatePaintNode(QSGNode *oldNode, UpdatePaintNode
 
     QSGTexture *texture = window()->createTextureFromImage(image);
     patchNode->initialize(texture, sz * d->devicePixelRatio, image.size(), d->xDivs, d->yDivs, d->devicePixelRatio);
+    auto patchNodeMaterial = static_cast<QSGTextureMaterial *>(patchNode->material());
+    patchNodeMaterial->setFiltering(d->smooth ? QSGTexture::Linear : QSGTexture::Nearest);
     return patchNode;
 }