Allow readbacks for msaa swapchain

No user control over resolving (unlike normal textures) so readbacks must
do an implicit resolve, when needed.

examples/rhi/plainqwindow_gles2/main.cpp

@@ -104,6 +104,7 @@ int main(int argc, char **argv)
     QSurfaceFormat fmt;
     fmt.setDepthBufferSize(24);
     fmt.setStencilBufferSize(8);
+    //fmt.setSamples(4); // enable 4x MSAA (except for the render-to-texture pass)
     QSurfaceFormat::setDefaultFormat(fmt);
 
     GlWindow w;

src/rhi/qrhi.h

@@ -343,6 +343,10 @@ struct Q_RHI_EXPORT QRhiReadbackDescription
 {
     QRhiReadbackDescription() { } // source is the back buffer of the swapchain of the current frame (if the swapchain supports readback)
     QRhiReadbackDescription(QRhiTexture *texture_) : texture(texture_) { } // source is the specified texture
+    // Note that reading back an msaa image is only supported for swapchains
+    // (by inserting an implicit resolve). For multisample textures, do an
+    // explicit resolve first.
+
     QRhiTexture *texture = nullptr;
     int layer = 0;
     int level = 0;

src/rhi/qrhid3d11.cpp

@@ -897,6 +897,18 @@ void QRhiD3D11::enqueueResourceUpdates(QRhiCommandBuffer *cb, QRhiResourceUpdate
         } else {
             Q_ASSERT(contextState.currentSwapChain);
             swapChainD = QRHI_RES(QD3D11SwapChain, contextState.currentSwapChain);
+            if (swapChainD->sampleDesc.Count > 1) {
+                // Unlike with textures, reading back a multisample swapchain image
+                // has to be supported. Insert a resolve.
+                QD3D11CommandBuffer::Command rcmd;
+                rcmd.cmd = QD3D11CommandBuffer::Command::ResolveSubRes;
+                rcmd.args.resolveSubRes.dst = swapChainD->tex[swapChainD->currentFrame];
+                rcmd.args.resolveSubRes.dstSubRes = 0;
+                rcmd.args.resolveSubRes.src = swapChainD->msaaTex[swapChainD->currentFrame];
+                rcmd.args.resolveSubRes.srcSubRes = 0;
+                rcmd.args.resolveSubRes.format = swapChainD->colorFormat;
+                cbD->commands.append(rcmd);
+            }
             src = swapChainD->tex[swapChainD->currentFrame];
             dxgiFormat = swapChainD->colorFormat;
             pixelSize = swapChainD->pixelSize;

src/rhi/qrhigles2.cpp

@@ -1138,6 +1138,7 @@ void QRhiGles2::executeCommandBuffer(QRhiCommandBuffer *cb)
             } else {
                 result->pixelSize = currentSwapChain->pixelSize;
                 result->format = QRhiTexture::RGBA8;
+                // readPixels handles multisample resolving implicitly
             }
             result->data.resize(result->pixelSize.width() * result->pixelSize.height() * 4);
             f->glReadPixels(0, 0, result->pixelSize.width(), result->pixelSize.height(),

src/rhi/qrhivulkan.cpp

@@ -2201,6 +2201,9 @@ void QRhiVulkan::enqueueResourceUpdates(QRhiCommandBuffer *cb, QRhiResourceUpdat
             aRb.format = colorTextureFormatFromVkFormat(swapChainD->colorFormat, nullptr);
             if (aRb.format == QRhiTexture::UnknownFormat)
                 continue;
+
+            // Multisample swapchains need nothing special since resolving
+            // happens when ending a renderpass.
         }
         textureFormatInfo(aRb.format, aRb.pixelSize, nullptr, &aRb.bufSize);
 

todo.txt

@@ -32,7 +32,6 @@ vk: rendering hangs sometimes when minimize and back on some systems?
 mtl: cbuffers, textures, samplers set should be batched too
 cbuffer alignment rules - some things fail to translate (to hlsl e.g. with structs), which is fine but how to mitigate
 what does image copy do for compressed formats?
-does/should reading back an msaa swapchain buffer work? (breaks with vk, d3d at least)
 figure sg out for minimizing viewport/scissor cmds
 vk: test FrameOpDeviceLost somehow
 d3d device loss?
@@ -68,6 +67,7 @@ dxc for d3d as an alternative to fxc?
 hlsl -> dxc -> spirv -> spirv-cross hmmm...
 
 +++ done
+msaa swapchain readback
 d3d: resolveimage (color)
 vk: resolveimage (color)
 gl: resolveimage (color)