Request correct alpha composition on the rhi code path

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

1 files changed, 9 insertions, 1 deletions

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();