QQuickImage: fix devicePixelRatio being lost for grabToImage() results

QQuickItemGrabResult::render() correctly records the window's
devicePixelRatio on the grabbed QImage. But when that image is exposed
under an "itemgrabber:" URL via QQuickPixmap::QQuickPixmap(const QUrl &,
const QImage &), the ratio was discarded: QQuickPixmapData's
devicePixelRatio was left at its default of 1, and
QQuickImageBase::loadPixmap() unconditionally resets its own
devicePixelRatio to 1.0, only restoring a real value for scalable image
formats (SVG/PDF) or "@2x" local files. An itemgrabber: URL, being a
raster in-memory image, matches neither.

As a result, an Image loaded from a grabToImage() result via "Image {
source: result.url }" (with no explicit width/height/sourceSize) ended
up devicePixelRatio times too big in device-independent pixels, even
though the underlying pixel data and DPR metadata were both already
correct.

Fix this by propagating image.devicePixelRatio() into QQuickPixmapData
when QQuickPixmap wraps a raw QImage, exposing it via a new
QQuickPixmap::devicePixelRatio() accessor, and having
QQuickImageBase::requestFinished() adopt that ratio for "itemgrabber:"
URLs specifically, once the pixmap has actually loaded (the ratio isn't
knowable ahead of load the way it is for "@2x"/scalable formats).

Add tst_qquickimage::grabToImageDevicePixelRatio() to cover this.

Fixes: QTBUG-150289
Change-Id: Iaf72a76c779ee208fac45e539543f9c6cb9738f7
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

5 files changed, 52 insertions, 1 deletions

diff --git a/src/quick/items/qquickimagebase.cpp b/src/quick/items/qquickimagebase.cpp
index 592d9dec1a..17850a29fd 100644
--- a/src/quick/items/qquickimagebase.cpp
+++ b/src/quick/items/qquickimagebase.cpp
@@ -377,6 +377,11 @@ void QQuickImageBase::requestFinished()
         d->status = Error;
         d->setProgress(0);
     } else {
+        // itemgrabber: images carry their own devicePixelRatio (set by whatever grabbed
+        // them), which isn't known ahead of load the way "@2x" and scalable-image-format
+        // ratios are, so pick it up here once the pixmap has actually loaded.
+        if (d->url.scheme() == QQuickPixmap::itemGrabberScheme)
+            d->devicePixelRatio = d->currentPix->devicePixelRatio();
         d->status = Ready; // do not emit statusChanged until after setImplicitSize
         d->setProgress(1);
     }
diff --git a/src/quick/util/qquickpixmap_p.h b/src/quick/util/qquickpixmap_p.h
index 972235f172..ad66e014b0 100644
--- a/src/quick/util/qquickpixmap_p.h
+++ b/src/quick/util/qquickpixmap_p.h
@@ -144,6 +144,7 @@ public:
     QRect rect() const;
     int width() const;
     int height() const;
+    qreal devicePixelRatio() const;
 
     void load(QQmlEngine *, const QUrl &);
     void load(QQmlEngine *, const QUrl &, QQuickPixmap::Options options);
diff --git a/src/quick/util/qquickpixmapcache.cpp b/src/quick/util/qquickpixmapcache.cpp
index 288d7afbce..7e693a6587 100644
--- a/src/quick/util/qquickpixmapcache.cpp
+++ b/src/quick/util/qquickpixmapcache.cpp
@@ -1709,7 +1709,8 @@ QQuickPixmap::QQuickPixmap(QQmlEngine *engine, const QUrl &url, const QRect &reg
 QQuickPixmap::QQuickPixmap(const QUrl &url, const QImage &image)
 {
     d = new QQuickPixmapData(url, new QQuickDefaultTextureFactory(image), image.size(), QRect(), QSize(),
-                             QQuickImageProviderOptions(), QQuickImageProviderOptions::UsePluginDefaultTransform);
+                             QQuickImageProviderOptions(), QQuickImageProviderOptions::UsePluginDefaultTransform,
+                             0, 1, image.devicePixelRatio());
     d->addToCache();
 }
 
@@ -1860,6 +1861,11 @@ int QQuickPixmap::height() const
         return 0;
 }
 
+qreal QQuickPixmap::devicePixelRatio() const
+{
+    return d ? d->devicePixelRatio : 1.0;
+}
+
 QRect QQuickPixmap::rect() const
 {
     if (d && d->textureFactory)
diff --git a/tests/auto/quick/qquickimage/data/grabToImageDevicePixelRatio.qml b/tests/auto/quick/qquickimage/data/grabToImageDevicePixelRatio.qml
new file mode 100644
index 0000000000..4a7f9ad79c
--- /dev/null
+++ b/tests/auto/quick/qquickimage/data/grabToImageDevicePixelRatio.qml
@@ -0,0 +1,12 @@
+import QtQuick
+
+Image {
+    id: sourceImage
+    objectName: "sourceImage"
+    source: "rect.png"
+
+    Image {
+        id: grabbedImage
+        objectName: "grabbedImage"
+    }
+}
diff --git a/tests/auto/quick/qquickimage/tst_qquickimage.cpp b/tests/auto/quick/qquickimage/tst_qquickimage.cpp
index cbd6282e2b..2bdad33777 100644
--- a/tests/auto/quick/qquickimage/tst_qquickimage.cpp
+++ b/tests/auto/quick/qquickimage/tst_qquickimage.cpp
@@ -21,6 +21,7 @@
 #include <QQuickView>
 #include <QQuickImageProvider>
 #include <QQmlAbstractUrlInterceptor>
+#include <QtQuick/QQuickItemGrabResult>
 
 #include <QtQuickTestUtils/private/qmlutils_p.h>
 #include <QtQuickTestUtils/private/testhttpserver_p.h>
@@ -83,6 +84,7 @@ private slots:
     void multiFrame();
     void colorSpace();
     void devicePixelRatio();
+    void grabToImageDevicePixelRatio();
 
 private:
     QQmlEngine engine;
@@ -1344,6 +1346,31 @@ void tst_qquickimage::devicePixelRatio()
     QCOMPARE(obj2->sourceSize(), (devicePixelRatio * origSize).toSize());
 }
 
+void tst_qquickimage::grabToImageDevicePixelRatio()
+{
+    QQuickView window;
+    QVERIFY(QQuickTest::showView(window, testFileUrl("grabToImageDevicePixelRatio.qml")));
+
+    if (window.effectiveDevicePixelRatio() <= 1)
+        QSKIP("This test only demonstrates the bug on a HiDPI (devicePixelRatio > 1) screen");
+
+    QQuickImage *sourceImage = qmlobject_cast<QQuickImage *>(window.rootObject());
+    QVERIFY(sourceImage);
+    QQuickImage *grabbedImage = sourceImage->findChild<QQuickImage *>("grabbedImage");
+    QVERIFY(grabbedImage);
+
+    auto grabResult = sourceImage->grabToImage();
+    QVERIFY(grabResult);
+    QSignalSpy readySpy(grabResult.data(), &QQuickItemGrabResult::ready);
+    QVERIFY(readySpy.wait());
+    grabbedImage->setSource(grabResult->url());
+    QTRY_COMPARE(grabbedImage->status(), QQuickImageBase::Ready);
+
+    // grabToImage() results loaded via Image.source ignore devicePixelRatio: on a HiDPI screen,
+    // grabbedImage ends up devicePixelRatio times too big instead of matching sourceImage.
+    QCOMPARE(grabbedImage->size(), sourceImage->size());
+}
+
 QTEST_MAIN(tst_qquickimage)
 
 #include "tst_qquickimage.moc"