Request correct alpha composition on the rhi code path

Task-number: QTBUG-78089
Change-Id: I22f8bb5ec0af33397df14e064a0306bd4c5a5ef5
Reviewed-by: Christian Strømme <christian.stromme@qt.io>

3 files changed, 26 insertions, 2 deletions

diff --git a/src/quick/scenegraph/qsgrenderloop.cpp b/src/quick/scenegraph/qsgrenderloop.cpp
index f15105168e..25d10d37d4 100644
--- a/src/quick/scenegraph/qsgrenderloop.cpp
+++ b/src/quick/scenegraph/qsgrenderloop.cpp
@@ -561,6 +561,14 @@ void QSGGuiThreadRenderLoop::renderWindow(QQuickWindow *window)
 
         QRhiSwapChain::Flags flags = QRhiSwapChain::UsedAsTransferSource; // may be used in a grab
 
+        // QQ is always premul alpha. Decide based on alphaBufferSize in
+        // requestedFormat(). (the platform plugin can override format() but
+        // what matters here is what the application wanted, hence using the
+        // requested one)
+        const bool alpha = window->requestedFormat().alphaBufferSize() > 0;
+        if (alpha)
+            flags |= QRhiSwapChain::SurfaceHasPreMulAlpha;
+
         cd->swapchain = rhi->newSwapChain();
         cd->depthStencilForSwapchain = rhi->newRenderBuffer(QRhiRenderBuffer::DepthStencil,
                                                             QSize(),
@@ -568,7 +576,8 @@ void QSGGuiThreadRenderLoop::renderWindow(QQuickWindow *window)
                                                             QRhiRenderBuffer::UsedWithSwapChainOnly);
         cd->swapchain->setWindow(window);
         cd->swapchain->setDepthStencil(cd->depthStencilForSwapchain);
-        qCDebug(QSG_LOG_INFO, "MSAA sample count for the swapchain is %d", rhiSampleCount);
+        qCDebug(QSG_LOG_INFO, "MSAA sample count for the swapchain is %d. Alpha channel requested = %s",
+                rhiSampleCount, alpha ? "yes" : "no");
         cd->swapchain->setSampleCount(rhiSampleCount);
         cd->swapchain->setFlags(flags);
         cd->rpDescForSwapchain = cd->swapchain->newCompatibleRenderPassDescriptor();
diff --git a/src/quick/scenegraph/qsgthreadedrenderloop.cpp b/src/quick/scenegraph/qsgthreadedrenderloop.cpp
index 9bcb96a65a..eb6eec342c 100644
--- a/src/quick/scenegraph/qsgthreadedrenderloop.cpp
+++ b/src/quick/scenegraph/qsgthreadedrenderloop.cpp
@@ -924,6 +924,13 @@ void QSGRenderThread::run()
                 if (rhi && !cd->swapchain) {
                     cd->rhi = rhi;
                     QRhiSwapChain::Flags flags = QRhiSwapChain::UsedAsTransferSource; // may be used in a grab
+                    // QQ is always premul alpha. Decide based on alphaBufferSize in
+                    // requestedFormat(). (the platform plugin can override format() but
+                    // what matters here is what the application wanted, hence using the
+                    // requested one)
+                    const bool alpha = window->requestedFormat().alphaBufferSize() > 0;
+                    if (alpha)
+                        flags |= QRhiSwapChain::SurfaceHasPreMulAlpha;
                     cd->swapchain = rhi->newSwapChain();
                     cd->depthStencilForSwapchain = rhi->newRenderBuffer(QRhiRenderBuffer::DepthStencil,
                                                                         QSize(),
@@ -931,7 +938,8 @@ void QSGRenderThread::run()
                                                                         QRhiRenderBuffer::UsedWithSwapChainOnly);
                     cd->swapchain->setWindow(window);
                     cd->swapchain->setDepthStencil(cd->depthStencilForSwapchain);
-                    qCDebug(QSG_LOG_INFO, "MSAA sample count for the swapchain is %d", rhiSampleCount);
+                    qCDebug(QSG_LOG_INFO, "MSAA sample count for the swapchain is %d. Alpha channel requested = %s.",
+                            rhiSampleCount, alpha ? "yes" : "no");
                     cd->swapchain->setSampleCount(rhiSampleCount);
                     cd->swapchain->setFlags(flags);
                     cd->rpDescForSwapchain = cd->swapchain->newCompatibleRenderPassDescriptor();
diff --git a/tests/manual/nodetypes_ng/nodetypes_ng.cpp b/tests/manual/nodetypes_ng/nodetypes_ng.cpp
index 829002b538..8fe0e0dc98 100644
--- a/tests/manual/nodetypes_ng/nodetypes_ng.cpp
+++ b/tests/manual/nodetypes_ng/nodetypes_ng.cpp
@@ -301,6 +301,13 @@ int main(int argc, char **argv)
         fmt.setProfile(QSurfaceFormat::CoreProfile);
         view.setFormat(fmt);
     }
+    if (app.arguments().contains(QLatin1String("--transparent"))) {
+        qDebug("Requesting alpha channel for the window and using Qt::transparent as background");
+        QSurfaceFormat fmt = view.format();
+        fmt.setAlphaBufferSize(8);
+        view.setFormat(fmt);
+        view.setColor(Qt::transparent);
+    }
 
     view.engine()->rootContext()->setContextProperty(QLatin1String("helper"), &helper);