Add begin/endOffscreen in the api

only implemented for Vulkan for now

src/rhi/qrhi.cpp

@@ -415,6 +415,16 @@ QRhi::FrameOpResult QRhi::endFrame(QRhiSwapChain *swapChain)
     return d->endFrame(swapChain);
 }
 
+QRhi::FrameOpResult QRhi::beginOffscreenFrame(QRhiCommandBuffer **cb)
+{
+    return d->beginOffscreenFrame(cb);
+}
+
+QRhi::FrameOpResult QRhi::endAndWaitOffscreenFrame()
+{
+    return d->endAndWaitOffscreenFrame();
+}
+
 void QRhi::beginPass(QRhiRenderTarget *rt,
                      QRhiCommandBuffer *cb,
                      const QRhiColorClearValue &colorClearValue,

src/rhi/qrhi.h

@@ -967,6 +967,22 @@ public:
     FrameOpResult beginFrame(QRhiSwapChain *swapChain);
     FrameOpResult endFrame(QRhiSwapChain *swapChain);
 
+    /*
+      Rendering without a swapchain is possible as well. This is synchronous in
+      the sense that end will wait for completion of the submitted commands.
+      The typical use case is to use it in completely offscreen applications,
+      e.g. to generate image sequences by rendering and reading back without
+      ever showing a window.
+          QRhiCommandBuffer *cb; // not owned
+          beginOffscreenFrame(&cb);
+          // ... the usual, set up a QRhiTextureRenderTarget, beginPass-endPass, etc.
+          endAndWaitOffscreenFrame();
+          // unlike a normal begin-end, the commands, including any readbacks
+          // have completed at this point
+     */
+    FrameOpResult beginOffscreenFrame(QRhiCommandBuffer **cb);
+    FrameOpResult endAndWaitOffscreenFrame();
+
     // Returns an instance to which updates can be queued. Batch instances are
     // pooled and never owned by the application. An instance is returned to
     // the pool after a beginPass() processes it or when it is "canceled" by

src/rhi/qrhi_p.h

@@ -83,6 +83,8 @@ public:
     virtual QRhiSwapChain *createSwapChain() = 0;
     virtual QRhi::FrameOpResult beginFrame(QRhiSwapChain *swapChain) = 0;
     virtual QRhi::FrameOpResult endFrame(QRhiSwapChain *swapChain) = 0;
+    virtual QRhi::FrameOpResult beginOffscreenFrame(QRhiCommandBuffer **cb) = 0;
+    virtual QRhi::FrameOpResult endAndWaitOffscreenFrame() = 0;
 
     virtual void beginPass(QRhiRenderTarget *rt,
                            QRhiCommandBuffer *cb,

src/rhi/qrhid3d11.cpp

@@ -497,6 +497,17 @@ QRhi::FrameOpResult QRhiD3D11::endFrame(QRhiSwapChain *swapChain)
     return QRhi::FrameOpSuccess;
 }
 
+QRhi::FrameOpResult QRhiD3D11::beginOffscreenFrame(QRhiCommandBuffer **cb)
+{
+    Q_UNUSED(cb);
+    return QRhi::FrameOpError;
+}
+
+QRhi::FrameOpResult QRhiD3D11::endAndWaitOffscreenFrame()
+{
+    return QRhi::FrameOpError;
+}
+
 static inline bool isCompressedFormat(QRhiTexture::Format format)
 {
     return format >= QRhiTexture::BC1 && format <= QRhiTexture::BC7;

src/rhi/qrhid3d11_p.h

@@ -374,6 +374,8 @@ public:
     QRhiSwapChain *createSwapChain() override;
     QRhi::FrameOpResult beginFrame(QRhiSwapChain *swapChain) override;
     QRhi::FrameOpResult endFrame(QRhiSwapChain *swapChain) override;
+    QRhi::FrameOpResult beginOffscreenFrame(QRhiCommandBuffer **cb) override;
+    QRhi::FrameOpResult endAndWaitOffscreenFrame() override;
 
     void beginPass(QRhiRenderTarget *rt,
                    QRhiCommandBuffer *cb,

src/rhi/qrhigles2.cpp

@@ -399,6 +399,17 @@ QRhi::FrameOpResult QRhiGles2::endFrame(QRhiSwapChain *swapChain)
     return QRhi::FrameOpSuccess;
 }
 
+QRhi::FrameOpResult QRhiGles2::beginOffscreenFrame(QRhiCommandBuffer **cb)
+{
+    Q_UNUSED(cb);
+    return QRhi::FrameOpError;
+}
+
+QRhi::FrameOpResult QRhiGles2::endAndWaitOffscreenFrame()
+{
+    return QRhi::FrameOpError;
+}
+
 void QRhiGles2::commitResourceUpdates(QRhiResourceUpdateBatch *resourceUpdates)
 {
     QRhiResourceUpdateBatchPrivate *ud = QRhiResourceUpdateBatchPrivate::get(resourceUpdates);

src/rhi/qrhigles2_p.h

@@ -332,6 +332,8 @@ public:
     QRhiSwapChain *createSwapChain() override;
     QRhi::FrameOpResult beginFrame(QRhiSwapChain *swapChain) override;
     QRhi::FrameOpResult endFrame(QRhiSwapChain *swapChain) override;
+    QRhi::FrameOpResult beginOffscreenFrame(QRhiCommandBuffer **cb) override;
+    QRhi::FrameOpResult endAndWaitOffscreenFrame() override;
 
     void beginPass(QRhiRenderTarget *rt,
                    QRhiCommandBuffer *cb,

src/rhi/qrhimetal.mm

@@ -557,6 +557,17 @@ QRhi::FrameOpResult QRhiMetal::endFrame(QRhiSwapChain *swapChain)
     return QRhi::FrameOpSuccess;
 }
 
+QRhi::FrameOpResult QRhiMetal::beginOffscreenFrame(QRhiCommandBuffer **cb)
+{
+    Q_UNUSED(cb);
+    return QRhi::FrameOpError;
+}
+
+QRhi::FrameOpResult QRhiMetal::endAndWaitOffscreenFrame()
+{
+    return QRhi::FrameOpError;
+}
+
 MTLRenderPassDescriptor *QRhiMetalData::createDefaultRenderPass(bool hasDepthStencil,
                                                                 const QRhiColorClearValue &colorClearValue,
                                                                 const QRhiDepthStencilClearValue &depthStencilClearValue)

src/rhi/qrhimetal_p.h

@@ -263,6 +263,8 @@ public:
     QRhiSwapChain *createSwapChain() override;
     QRhi::FrameOpResult beginFrame(QRhiSwapChain *swapChain) override;
     QRhi::FrameOpResult endFrame(QRhiSwapChain *swapChain) override;
+    QRhi::FrameOpResult beginOffscreenFrame(QRhiCommandBuffer **cb) override;
+    QRhi::FrameOpResult endAndWaitOffscreenFrame() override;
 
     void beginPass(QRhiRenderTarget *rt,
                    QRhiCommandBuffer *cb,

src/rhi/qrhivulkan_p.h

@@ -325,6 +325,8 @@ public:
     QRhiSwapChain *createSwapChain() override;
     QRhi::FrameOpResult beginFrame(QRhiSwapChain *swapChain) override;
     QRhi::FrameOpResult endFrame(QRhiSwapChain *swapChain) override;
+    QRhi::FrameOpResult beginOffscreenFrame(QRhiCommandBuffer **cb) override;
+    QRhi::FrameOpResult endAndWaitOffscreenFrame() override;
 
     void beginPass(QRhiRenderTarget *rt,
                    QRhiCommandBuffer *cb,
@@ -392,8 +394,6 @@ public:
     QRhi::FrameOpResult endWrapperFrame(QRhiSwapChain *swapChain);
     QRhi::FrameOpResult beginNonWrapperFrame(QRhiSwapChain *swapChain);
     QRhi::FrameOpResult endNonWrapperFrame(QRhiSwapChain *swapChain);
-    QRhi::FrameOpResult beginOffscreenFrame(QRhiCommandBuffer **cb);
-    QRhi::FrameOpResult endAndWaitOffscreenFrame();
     void prepareNewFrame(QRhiCommandBuffer *cb);
     void finishFrame();
     void commitResourceUpdates(QRhiCommandBuffer *cb, QRhiResourceUpdateBatch *resourceUpdates);