rhi: d3d11: Switch the default swap effect and scaling mode

[ChangeLog][QtGui][QRhi] With Direct3D 11 the default swap effect is now
DXGI_SWAP_EFFECT_FLIP_SEQUENTIAL combined with DXGI_SCALING_NONE. This
should provide a better (less "jumpy") resizing experience for Qt Quick
content in particular, because the stretch effect for size-mismatched
present is not ideal for user interfaces. We'd rather want to keep the
size as-is and have the white border on the right/bottom (which we will
have anyway, inevitably). This is also closer to what one gets with
OpenGL (though that may depend on the driver as well). For Vulkan on
Windows, the behavior will remain very similar to what
DXGI_SCALING_STRETCH does, depending on the implementation probably,
because the Vulkan spec fails to address the handling of scaling modes
for size-mismatched presents. To get the old D3D behavior, i.e.
FLIP_DISCARD+SCALING_STRETCH, set the environment variable
QT_D3D_FLIP_DISCARD to a non-zero value.

Pick-to: 6.3
Fixes: QTBUG-99637
Change-Id: Ic8a219fbf3cdb6458b7ec9149bf27e771d9f7ddd
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

2 files changed, 48 insertions, 26 deletions

diff --git a/src/gui/rhi/qrhid3d11.cpp b/src/gui/rhi/qrhid3d11.cpp
index 90e0214df8..fd64080421 100644
--- a/src/gui/rhi/qrhid3d11.cpp
+++ b/src/gui/rhi/qrhid3d11.cpp
@@ -50,8 +50,8 @@
 QT_BEGIN_NAMESPACE
 
 /*
-  Direct3D 11 backend. Provides a double-buffered flip model (FLIP_DISCARD)
-  swapchain. Textures and "static" buffers are USAGE_DEFAULT, leaving it to
+  Direct3D 11 backend. Provides a double-buffered flip model swapchain.
+  Textures and "static" buffers are USAGE_DEFAULT, leaving it to
   UpdateSubResource to upload the data in any way it sees fit. "Dynamic"
   buffers are USAGE_DYNAMIC and updating is done by mapping with WRITE_DISCARD.
   (so here QRhiBuffer keeps a copy of the buffer contents and all of it is
@@ -202,18 +202,19 @@ bool QRhiD3D11::create(QRhi::Flags flags)
 
     dxgiFactory = createDXGIFactory2();
     if (dxgiFactory != nullptr) {
-        supportsFlipDiscardSwapchain = !qEnvironmentVariableIntValue("QT_D3D_NO_FLIP");
+        supportsFlipSwapchain = !qEnvironmentVariableIntValue("QT_D3D_NO_FLIP");
     } else {
         dxgiFactory = createDXGIFactory1();
-        supportsFlipDiscardSwapchain = false;
+        supportsFlipSwapchain = false;
     }
 
     if (dxgiFactory == nullptr)
         return false;
 
     supportsAllowTearing = false;
-    if (supportsFlipDiscardSwapchain) {
-        // For a FLIP_DISCARD swapchain Present(0, 0) is not necessarily
+    forceFlipDiscard = false;
+    if (supportsFlipSwapchain) {
+        // For a FLIP_* swapchain Present(0, 0) is not necessarily
         // sufficient to get non-blocking behavior, try using ALLOW_TEARING
         // when available.
         IDXGIFactory5 *factory5 = nullptr;
@@ -223,12 +224,18 @@ bool QRhiD3D11::create(QRhi::Flags flags)
                 supportsAllowTearing = allowTearing;
             factory5->Release();
         }
+        // if we default to FLIP_SEQUENTIAL, have a way to request FLIP_DISCARD
+        forceFlipDiscard = qEnvironmentVariableIntValue("QT_D3D_FLIP_DISCARD");
     }
 
-    qCDebug(QRHI_LOG_INFO, "FLIP_DISCARD swapchain supported = %s, ALLOW_TEARING supported = %s",
-            supportsFlipDiscardSwapchain ? "true" : "false",
+    qCDebug(QRHI_LOG_INFO, "FLIP_* swapchain supported = %s, ALLOW_TEARING supported = %s",
+            supportsFlipSwapchain ? "true" : "false",
             supportsAllowTearing ? "true" : "false");
 
+    qCDebug(QRHI_LOG_INFO, "Default swap effect: %s",
+            supportsFlipSwapchain ? (forceFlipDiscard ? "FLIP_DISCARD" : "FLIP_SEQUENTIAL")
+                                  : "DISCARD");
+
     if (!importedDeviceAndContext) {
         IDXGIAdapter1 *adapter;
         int requestedAdapterIndex = -1;
@@ -4584,15 +4591,15 @@ bool QD3D11SwapChain::createOrResize()
         return false;
 
     QRHI_RES_RHI(QRhiD3D11);
-    bool useFlipDiscard = rhiD->supportsFlipDiscardSwapchain;
+    bool useFlipModel = rhiD->supportsFlipSwapchain;
 
     // Take a shortcut for alpha: whatever the platform plugin does to enable
     // transparency for our QWindow will be sufficient on the legacy (DISCARD)
-    // path. For FLIP_DISCARD we'd need to use DirectComposition (create a
+    // path. For FLIP_* we'd need to use DirectComposition (create a
     // IDCompositionDevice/Target/Visual), avoid that for now. (this though
     // means HDR and semi-transparent windows cannot be combined)
     if (m_flags.testFlag(SurfaceHasPreMulAlpha) || m_flags.testFlag(SurfaceHasNonPreMulAlpha)) {
-        useFlipDiscard = false;
+        useFlipModel = false;
         if (window->requestedFormat().alphaBufferSize() <= 0)
             qWarning("Swapchain says surface has alpha but the window has no alphaBufferSize set. "
                      "This may lead to problems.");
@@ -4603,9 +4610,9 @@ bool QD3D11SwapChain::createOrResize()
 
     // A non-flip swapchain can do Present(0) as expected without
     // ALLOW_TEARING, and ALLOW_TEARING is not compatible with it at all so the
-    // flag must not be set then. Whereas for flip-discard we should use it, if
+    // flag must not be set then. Whereas for flip we should use it, if
     // supported, to get better results for 'unthrottled' presentation.
-    if (swapInterval == 0 && useFlipDiscard && rhiD->supportsAllowTearing)
+    if (swapInterval == 0 && useFlipModel && rhiD->supportsAllowTearing)
         swapChainFlags |= DXGI_SWAP_CHAIN_FLAG_ALLOW_TEARING;
 
     if (!swapChain) {
@@ -4615,7 +4622,7 @@ bool QD3D11SwapChain::createOrResize()
         srgbAdjustedColorFormat = m_flags.testFlag(sRGB) ? DEFAULT_SRGB_FORMAT : DEFAULT_FORMAT;
 
         HRESULT hr;
-        if (useFlipDiscard) {
+        if (useFlipModel) {
             DXGI_COLOR_SPACE_TYPE hdrColorSpace = DXGI_COLOR_SPACE_RGB_FULL_G22_NONE_P709; // SDR
             DXGI_OUTPUT_DESC1 hdrOutputDesc;
             if (outputDesc1ForWindow(m_window, rhiD->activeAdapter, &hdrOutputDesc) && m_format != SDR) {
@@ -4644,11 +4651,11 @@ bool QD3D11SwapChain::createOrResize()
                 }
             }
 
-            // We use FLIP_DISCARD which implies a buffer count of 2 (as opposed to the
-            // old DISCARD with back buffer count == 1). This makes no difference for
-            // the rest of the stuff except that automatic MSAA is unsupported and
-            // needs to be implemented via a custom multisample render target and an
-            // explicit resolve.
+            // We use a FLIP model swapchain which implies a buffer count of 2
+            // (as opposed to the old DISCARD with back buffer count == 1).
+            // This makes no difference for the rest of the stuff except that
+            // automatic MSAA is unsupported and needs to be implemented via a
+            // custom multisample render target and an explicit resolve.
 
             DXGI_SWAP_CHAIN_DESC1 desc;
             memset(&desc, 0, sizeof(desc));
@@ -4658,8 +4665,22 @@ bool QD3D11SwapChain::createOrResize()
             desc.SampleDesc.Count = 1;
             desc.BufferUsage = DXGI_USAGE_RENDER_TARGET_OUTPUT;
             desc.BufferCount = BUFFER_COUNT;
-            desc.Scaling = DXGI_SCALING_STRETCH;
-            desc.SwapEffect = DXGI_SWAP_EFFECT(4); // DXGI_SWAP_EFFECT_FLIP_DISCARD
+
+            // Normally we'd want FLIP_DISCARD, but that comes with the default
+            // SCALING_STRETCH, as SCALING_NONE is documented to be only
+            // available for FLIP_SEQUENTIAl. The problem with stretch is that
+            // Qt Quick and similar apps typically running in resizable windows
+            // will not like how that looks in practice: the content will
+            // appear to be "jumping" around during a window resize. So choose
+            // sequential/none by default.
+            if (rhiD->forceFlipDiscard) {
+                desc.Scaling = DXGI_SCALING_STRETCH;
+                desc.SwapEffect = DXGI_SWAP_EFFECT_FLIP_DISCARD;
+            } else {
+                desc.Scaling = DXGI_SCALING_NONE;
+                desc.SwapEffect = DXGI_SWAP_EFFECT_FLIP_SEQUENTIAL;
+            }
+
             // Do not bother with AlphaMode, if won't work unless we go through
             // DirectComposition. Instead, we just take the other (DISCARD)
             // path for now when alpha is requested.
@@ -4694,8 +4715,8 @@ bool QD3D11SwapChain::createOrResize()
             }
         } else {
             // Fallback: use DISCARD mode. Regardless, keep on using our manual
-            // resolve for symmetry with the FLIP_DISCARD code path when MSAA
-            // is requested. This has no HDR support.
+            // resolve for symmetry with the FLIP_* code path when MSAA is
+            // requested. This has no HDR support.
 
             DXGI_SWAP_CHAIN_DESC desc;
             memset(&desc, 0, sizeof(desc));
@@ -4721,7 +4742,7 @@ bool QD3D11SwapChain::createOrResize()
         rhiD->dxgiFactory->MakeWindowAssociation(hwnd, DXGI_MWA_NO_WINDOW_CHANGES);
     } else {
         releaseBuffers();
-        const UINT count = useFlipDiscard ? BUFFER_COUNT : 1;
+        const UINT count = useFlipModel ? BUFFER_COUNT : 1;
         HRESULT hr = swapChain->ResizeBuffers(count, UINT(pixelSize.width()), UINT(pixelSize.height()),
                                               colorFormat, swapChainFlags);
         if (hr == DXGI_ERROR_DEVICE_REMOVED || hr == DXGI_ERROR_DEVICE_RESET) {
@@ -4734,7 +4755,7 @@ bool QD3D11SwapChain::createOrResize()
         }
     }
 
-    // This looks odd (for FLIP_DISCARD, esp. compared with backends for Vulkan
+    // This looks odd (for FLIP_*, esp. compared with backends for Vulkan
     // & co.) but the backbuffer is always at index 0, with magic underneath.
     // Some explanation from
     // https://docs.microsoft.com/en-us/windows/win32/direct3ddxgi/dxgi-1-4-improvements
diff --git a/src/gui/rhi/qrhid3d11_p_p.h b/src/gui/rhi/qrhid3d11_p_p.h
index 7ae4c28a02..9e12defbfd 100644
--- a/src/gui/rhi/qrhid3d11_p_p.h
+++ b/src/gui/rhi/qrhid3d11_p_p.h
@@ -728,8 +728,9 @@ public:
     ID3DUserDefinedAnnotation *annotations = nullptr;
     IDXGIAdapter1 *activeAdapter = nullptr;
     IDXGIFactory1 *dxgiFactory = nullptr;
-    bool supportsFlipDiscardSwapchain = false;
+    bool supportsFlipSwapchain = false;
     bool supportsAllowTearing = false;
+    bool forceFlipDiscard = false;
     bool deviceLost = false;
     QRhiD3D11NativeHandles nativeHandlesStruct;
     QRhiDriverInfo driverInfoStruct;