Add readback api skeleton

not yet implemented

examples/rhi/offscreen_vulkan/main.cpp

@@ -94,9 +94,12 @@ int main(int argc, char **argv)
     rt->setRenderPassDescriptor(rp);
     rt->build();
 
-    QRhiCommandBuffer *cb;
-    if (r->beginOffscreenFrame(&cb) == QRhi::FrameOpSuccess) {
-        qDebug("Generating offscreen frame");
+    for (int frame = 0; frame < 20; ++frame) {
+        QRhiCommandBuffer *cb;
+        if (r->beginOffscreenFrame(&cb) != QRhi::FrameOpSuccess)
+            break;
+
+        qDebug("Generating offscreen frame %d", frame);
         r->beginPass(rt, cb, { 0, 1, 0, 1 }, { 1, 0 }, nullptr);
         r->endPass(cb);
 

src/rhi/qrhi.cpp

@@ -487,6 +487,11 @@ void QRhi::drawIndexed(QRhiCommandBuffer *cb, quint32 indexCount,
     d->drawIndexed(cb, indexCount, instanceCount, firstIndex, vertexOffset, firstInstance);
 }
 
+void QRhi::readback(QRhiCommandBuffer *cb, QRhiReadback *rb)
+{
+    d->readback(cb, rb);
+}
+
 QVector<int> QRhi::supportedSampleCounts() const
 {
     return d->supportedSampleCounts();

src/rhi/qrhi.h

@@ -280,6 +280,13 @@ Q_DECLARE_TYPEINFO(QRhiTextureUploadDescription::Layer::MipLevel, Q_MOVABLE_TYPE
 Q_DECLARE_TYPEINFO(QRhiTextureUploadDescription::Layer, Q_MOVABLE_TYPE);
 Q_DECLARE_TYPEINFO(QRhiTextureUploadDescription, Q_MOVABLE_TYPE);
 
+struct Q_RHI_EXPORT QRhiReadback
+{
+    // ###
+};
+
+Q_DECLARE_TYPEINFO(QRhiReadback, Q_MOVABLE_TYPE);
+
 class Q_RHI_EXPORT QRhiResource
 {
 public:
@@ -976,9 +983,9 @@ public:
           QRhiCommandBuffer *cb; // not owned
           beginOffscreenFrame(&cb);
           // ... the usual, set up a QRhiTextureRenderTarget, beginPass-endPass, etc.
+          readback(cb, someTexture, rb);
           endAndWaitOffscreenFrame();
-          // unlike a normal begin-end, the commands, including any readbacks
-          // have completed at this point
+          // the results are available in rb
      */
     FrameOpResult beginOffscreenFrame(QRhiCommandBuffer **cb);
     FrameOpResult endAndWaitOffscreenFrame();
@@ -1030,6 +1037,9 @@ public:
                      quint32 instanceCount = 1, quint32 firstIndex = 0,
                      qint32 vertexOffset = 0, quint32 firstInstance = 0);
 
+    // Must only be called outside a begin-endPass section.
+    void readback(QRhiCommandBuffer *cb, QRhiReadback *rb);
+
     QVector<int> supportedSampleCounts() const;
 
     int ubufAlignment() const;

src/rhi/qrhi_p.h

@@ -113,6 +113,8 @@ public:
                              quint32 instanceCount, quint32 firstIndex,
                              qint32 vertexOffset, quint32 firstInstance) = 0;
 
+    virtual void readback(QRhiCommandBuffer *cb, QRhiReadback *rb) = 0;
+
     virtual QVector<int> supportedSampleCounts() const = 0;
     virtual int ubufAlignment() const = 0;
     virtual bool isYUpInFramebuffer() const = 0;

src/rhi/qrhid3d11.cpp

@@ -453,6 +453,12 @@ void QRhiD3D11::drawIndexed(QRhiCommandBuffer *cb, quint32 indexCount,
     cbD->commands.append(cmd);
 }
 
+void QRhiD3D11::readback(QRhiCommandBuffer *cb, QRhiReadback *rb)
+{
+    Q_UNUSED(cb);
+    Q_UNUSED(rb);
+}
+
 QRhi::FrameOpResult QRhiD3D11::beginFrame(QRhiSwapChain *swapChain)
 {
     Q_ASSERT(!inFrame);

src/rhi/qrhid3d11_p.h

@@ -405,6 +405,8 @@ public:
                      quint32 instanceCount, quint32 firstIndex,
                      qint32 vertexOffset, quint32 firstInstance) override;
 
+    void readback(QRhiCommandBuffer *cb, QRhiReadback *rb) override;
+
     QVector<int> supportedSampleCounts() const override;
     int ubufAlignment() const override;
     bool isYUpInFramebuffer() const override;

src/rhi/qrhigles2.cpp

@@ -366,6 +366,12 @@ void QRhiGles2::drawIndexed(QRhiCommandBuffer *cb, quint32 indexCount,
     cbD->commands.append(cmd);
 }
 
+void QRhiGles2::readback(QRhiCommandBuffer *cb, QRhiReadback *rb)
+{
+    Q_UNUSED(cb);
+    Q_UNUSED(rb);
+}
+
 QRhi::FrameOpResult QRhiGles2::beginFrame(QRhiSwapChain *swapChain)
 {
     Q_ASSERT(!inFrame);

src/rhi/qrhigles2_p.h

@@ -363,6 +363,8 @@ public:
                      quint32 instanceCount, quint32 firstIndex,
                      qint32 vertexOffset, quint32 firstInstance) override;
 
+    void readback(QRhiCommandBuffer *cb, QRhiReadback *rb) override;
+
     QVector<int> supportedSampleCounts() const override;
     int ubufAlignment() const override;
     bool isYUpInFramebuffer() const override;

src/rhi/qrhimetal.mm

@@ -496,6 +496,12 @@ void QRhiMetal::drawIndexed(QRhiCommandBuffer *cb, quint32 indexCount,
       baseInstance: firstInstance];
 }
 
+void QRhiMetal::readback(QRhiCommandBuffer *cb, QRhiReadback *rb)
+{
+    Q_UNUSED(cb);
+    Q_UNUSED(rb);
+}
+
 QRhi::FrameOpResult QRhiMetal::beginFrame(QRhiSwapChain *swapChain)
 {
     Q_ASSERT(!inFrame);

src/rhi/qrhimetal_p.h

@@ -294,6 +294,8 @@ public:
                      quint32 instanceCount, quint32 firstIndex,
                      qint32 vertexOffset, quint32 firstInstance) override;
 
+    void readback(QRhiCommandBuffer *cb, QRhiReadback *rb) override;
+
     QVector<int> supportedSampleCounts() const override;
     int ubufAlignment() const override;
     bool isYUpInFramebuffer() const override;

src/rhi/qrhivulkan.cpp

@@ -2315,6 +2315,12 @@ void QRhiVulkan::drawIndexed(QRhiCommandBuffer *cb, quint32 indexCount,
     df->vkCmdDrawIndexed(QRHI_RES(QVkCommandBuffer, cb)->cb, indexCount, instanceCount, firstIndex, vertexOffset, firstInstance);
 }
 
+void QRhiVulkan::readback(QRhiCommandBuffer *cb, QRhiReadback *rb)
+{
+    Q_UNUSED(cb);
+    Q_UNUSED(rb);
+}
+
 static inline VkBufferUsageFlagBits toVkBufferUsage(QRhiBuffer::UsageFlags usage)
 {
     int u = 0;

src/rhi/qrhivulkan_p.h

@@ -356,6 +356,8 @@ public:
                      quint32 instanceCount, quint32 firstIndex,
                      qint32 vertexOffset, quint32 firstInstance) override;
 
+    void readback(QRhiCommandBuffer *cb, QRhiReadback *rb) override;
+
     QVector<int> supportedSampleCounts() const override;
     int ubufAlignment() const override;
     bool isYUpInFramebuffer() const override;