Fix device pixel ratio with QRhi for native text

Calculating values based on the dpr is not possible in the sync phase,
because the strictly correct dpr (that takes redirections into textures
into account) is not known until QQuickWindowPrivate::renderSceneGraph().

The text material implementation attempts to dig out something directly
from the context's associated surface, but this does not match the way
QQuickWindow calculates the value (although it would work without causing
any trouble in many cases).

This is of course incompatible with the QRhi-based abstraction since
neither the context nor the associated window (if there is one even) is
known to materials.

To solve this, create a proper solution that makes the
QQuickWindow-calculated dpr available already in the sync phase (so in
updatePaintNode() implementations): have QQuickWindow calculate
calculate the dpr in syncSceneGraph(), and pass it down via the
rendercontext.

Only the rhi-based code path is touched in this patch. The direct OpenGL
path could be fixed in a similar manner (by migrating to
rc->devicePixelRatio() in the sync phase and state.devicePixelRatio() in
the render phase), but that is left as a future exercise.

Task-number: QTBUG-78610
Change-Id: Id9d9d4b1fd5b9730a64834fbbf61c74af4a8ed07
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

5 files changed, 29 insertions, 5 deletions

diff --git a/src/quick/scenegraph/qsgcontext.cpp b/src/quick/scenegraph/qsgcontext.cpp
index 97fd49e4c7..17eb1e312c 100644
--- a/src/quick/scenegraph/qsgcontext.cpp
+++ b/src/quick/scenegraph/qsgcontext.cpp
@@ -341,6 +341,11 @@ void QSGRenderContext::invalidate()
 {
 }
 
+void QSGRenderContext::prepareSync(qreal devicePixelRatio)
+{
+    Q_UNUSED(devicePixelRatio);
+}
+
 void QSGRenderContext::beginNextFrame(QSGRenderer *renderer,
                                       RenderPassCallback mainPassRecordingStart,
                                       RenderPassCallback mainPassRecordingEnd,
diff --git a/src/quick/scenegraph/qsgcontext_p.h b/src/quick/scenegraph/qsgcontext_p.h
index 4e712ab7c3..244bcfabd1 100644
--- a/src/quick/scenegraph/qsgcontext_p.h
+++ b/src/quick/scenegraph/qsgcontext_p.h
@@ -174,6 +174,7 @@ public:
 
     using RenderPassCallback = void (*)(void *);
 
+    virtual void prepareSync(qreal devicePixelRatio);
     virtual void beginNextFrame(QSGRenderer *renderer,
                                 RenderPassCallback mainPassRecordingStart,
                                 RenderPassCallback mainPassRecordingEnd,
diff --git a/src/quick/scenegraph/qsgdefaultglyphnode_p.cpp b/src/quick/scenegraph/qsgdefaultglyphnode_p.cpp
index 4eba40b7a1..be6ef25feb 100644
--- a/src/quick/scenegraph/qsgdefaultglyphnode_p.cpp
+++ b/src/quick/scenegraph/qsgdefaultglyphnode_p.cpp
@@ -751,15 +751,15 @@ void QSGTextMaskMaterial::updateCache(QFontEngine::GlyphFormat glyphFormat)
         void *cacheKey;
         if (m_rhi) {
             cacheKey = m_rhi;
-            // ### no idea what the QWindow is (esp. since we are not even
-            // rendering at this point), and anyway is the original logic correct
-            // even...
-            devicePixelRatio = qGuiApp->devicePixelRatio();
+            // Get the dpr the modern way. This value retrieved via the
+            // rendercontext matches what RenderState::devicePixelRatio()
+            // exposes to the material shaders later on.
+            devicePixelRatio = m_rc->currentDevicePixelRatio();
         } else {
             ctx = const_cast<QOpenGLContext *>(QOpenGLContext::currentContext());
             Q_ASSERT(ctx != nullptr);
             cacheKey = ctx;
-            devicePixelRatio = qsg_device_pixel_ratio(ctx);
+            devicePixelRatio = qsg_device_pixel_ratio(ctx); // this is technically incorrect, see other branch above
         }
 
         QTransform glyphCacheTransform = QTransform::fromScale(devicePixelRatio, devicePixelRatio);
diff --git a/src/quick/scenegraph/qsgdefaultrendercontext.cpp b/src/quick/scenegraph/qsgdefaultrendercontext.cpp
index 4ccaa91cce..2bf4a83a8e 100644
--- a/src/quick/scenegraph/qsgdefaultrendercontext.cpp
+++ b/src/quick/scenegraph/qsgdefaultrendercontext.cpp
@@ -200,6 +200,11 @@ void QSGDefaultRenderContext::invalidate()
     emit invalidated();
 }
 
+void QSGDefaultRenderContext::prepareSync(qreal devicePixelRatio)
+{
+    m_currentDevicePixelRatio = devicePixelRatio;
+}
+
 static QBasicMutex qsg_framerender_mutex;
 
 void QSGDefaultRenderContext::beginNextFrame(QSGRenderer *renderer,
diff --git a/src/quick/scenegraph/qsgdefaultrendercontext_p.h b/src/quick/scenegraph/qsgdefaultrendercontext_p.h
index 6a3462ae2b..2fdb3a48dd 100644
--- a/src/quick/scenegraph/qsgdefaultrendercontext_p.h
+++ b/src/quick/scenegraph/qsgdefaultrendercontext_p.h
@@ -104,6 +104,7 @@ public:
     void initialize(const QSGRenderContext::InitParams *params) override;
     void invalidate() override;
 
+    void prepareSync(qreal devicePixelRatio) override;
     void beginNextFrame(QSGRenderer *renderer,
                         RenderPassCallback mainPassRecordingStart,
                         RenderPassCallback mainPassRecordingEnd,
@@ -151,6 +152,17 @@ public:
         return m_currentFrameRenderPass;
     }
 
+    qreal currentDevicePixelRatio() const
+    {
+        // Valid starting from QQuickWindow::syncSceneGraph(). This takes the
+        // redirections, e.g. QQuickWindow::setRenderTarget(), into account.
+        // This calculation logic matches what the renderer does, so this is
+        // the same value that gets exposed in RenderState::devicePixelRatio()
+        // to material shaders. This getter is useful to perform dpr-related
+        // operations in the sync phase (in updatePaintNode()).
+        return m_currentDevicePixelRatio;
+    }
+
 protected:
     static QString fontKey(const QRawFont &font);
 
@@ -166,6 +178,7 @@ protected:
     QSGRhiAtlasTexture::Manager *m_rhiAtlasManager;
     QRhiCommandBuffer *m_currentFrameCommandBuffer;
     QRhiRenderPassDescriptor *m_currentFrameRenderPass;
+    qreal m_currentDevicePixelRatio;
 };
 
 QT_END_NAMESPACE