mtl: Add support for readbacks and offscreen frames

examples/rhi/offscreen_metal/main.cpp

+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the examples of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:BSD$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** BSD License Usage
+** Alternatively, you may use this file under the terms of the BSD license
+** as follows:
+**
+** "Redistribution and use in source and binary forms, with or without
+** modification, are permitted provided that the following conditions are
+** met:
+**   * Redistributions of source code must retain the above copyright
+**     notice, this list of conditions and the following disclaimer.
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+**     notice, this list of conditions and the following disclaimer in
+**     the documentation and/or other materials provided with the
+**     distribution.
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+**     contributors may be used to endorse or promote products derived
+**     from this software without specific prior written permission.
+**
+**
+** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+** A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+** OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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+** LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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+** (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+** OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE."
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include <QGuiApplication>
+#include <QImage>
+#include <QFileInfo>
+#include <QFile>
+#include <QBakedShader>
+#include <QRhiMetalInitParams>
+
+static float vertexData[] = { // Y up (note m_proj), CCW
+     0.0f,   0.5f,   1.0f, 0.0f, 0.0f,
+    -0.5f,  -0.5f,   0.0f, 1.0f, 0.0f,
+     0.5f,  -0.5f,   0.0f, 0.0f, 1.0f,
+};
+
+static QBakedShader getShader(const QString &name)
+{
+    QFile f(name);
+    if (f.open(QIODevice::ReadOnly))
+        return QBakedShader::fromSerialized(f.readAll());
+
+    return QBakedShader();
+}
+
+int main(int argc, char **argv)
+{
+    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
+    QGuiApplication app(argc, argv);
+
+    QRhiMetalInitParams params;
+    QRhi *r = QRhi::create(QRhi::Metal, &params);
+    if (!r) {
+        qWarning("Failed to initialize RHI");
+        return 1;
+    }
+
+    QRhiTexture *tex = r->newTexture(QRhiTexture::RGBA8, QSize(1280, 720), 1, QRhiTexture::RenderTarget | QRhiTexture::UsedAsTransferSource);
+    tex->build();
+    QRhiTextureRenderTarget *rt = r->newTextureRenderTarget({ tex });
+    QRhiRenderPassDescriptor *rp = rt->newCompatibleRenderPassDescriptor();
+    rt->setRenderPassDescriptor(rp);
+    rt->build();
+
+    QMatrix4x4 proj = r->clipSpaceCorrMatrix();
+    proj.perspective(45.0f, 1280 / 720.f, 0.01f, 1000.0f);
+    proj.translate(0, 0, -4);
+
+    QRhiBuffer *vbuf = r->newBuffer(QRhiBuffer::Immutable, QRhiBuffer::VertexBuffer, sizeof(vertexData));
+    vbuf->build();
+
+    QRhiBuffer *ubuf = r->newBuffer(QRhiBuffer::Dynamic, QRhiBuffer::UniformBuffer, 68);
+    ubuf->build();
+
+    QRhiShaderResourceBindings *srb = r->newShaderResourceBindings();
+    srb->setBindings({
+        QRhiShaderResourceBinding::uniformBuffer(0, QRhiShaderResourceBinding::VertexStage | QRhiShaderResourceBinding::FragmentStage, ubuf)
+    });
+    srb->build();
+
+    QRhiGraphicsPipeline *ps = r->newGraphicsPipeline();
+
+    QRhiGraphicsPipeline::TargetBlend premulAlphaBlend;
+    premulAlphaBlend.enable = true;
+    ps->setTargetBlends({ premulAlphaBlend });
+
+    const QBakedShader vs = getShader(QLatin1String(":/color.vert.qsb"));
+    if (!vs.isValid())
+        qFatal("Failed to load shader pack (vertex)");
+    const QBakedShader fs = getShader(QLatin1String(":/color.frag.qsb"));
+    if (!fs.isValid())
+        qFatal("Failed to load shader pack (fragment)");
+
+    ps->setShaderStages({
+        { QRhiGraphicsShaderStage::Vertex, vs },
+        { QRhiGraphicsShaderStage::Fragment, fs }
+    });
+
+    QRhiVertexInputLayout inputLayout;
+    inputLayout.bindings = {
+        { 5 * sizeof(float) }
+    };
+    inputLayout.attributes = {
+        { 0, 0, QRhiVertexInputLayout::Attribute::Float2, 0 },
+        { 0, 1, QRhiVertexInputLayout::Attribute::Float3, 2 * sizeof(float) }
+    };
+
+    ps->setVertexInputLayout(inputLayout);
+    ps->setShaderResourceBindings(srb);
+    ps->setRenderPassDescriptor(rp);
+    ps->build();
+
+    for (int frame = 0; frame < 20; ++frame) {
+        QRhiCommandBuffer *cb;
+        if (r->beginOffscreenFrame(&cb) != QRhi::FrameOpSuccess) {
+            qDebug("Failed to start frame");
+            break;
+        }
+
+        qDebug("Generating offscreen frame %d", frame);
+        QRhiResourceUpdateBatch *u = r->nextResourceUpdateBatch();
+        if (frame == 0)
+            u->uploadStaticBuffer(vbuf, vertexData);
+
+        static float rotation = 0.0f;
+        QMatrix4x4 mvp = proj;
+        mvp.rotate(rotation, 0, 1, 0);
+        u->updateDynamicBuffer(ubuf, 0, 64, mvp.constData());
+        rotation += 5.0f;
+        static float opacity = 1.0f;
+        static int opacityDir= 1;
+        u->updateDynamicBuffer(ubuf, 64, 4, &opacity);
+        opacity += opacityDir * 0.005f;
+        if (opacity < 0.0f || opacity > 1.0f) {
+            opacityDir *= -1;
+            opacity = qBound(0.0f, opacity, 1.0f);
+        }
+
+        cb->beginPass(rt, { 0, 1, 0, 1 }, { 1, 0 }, u);
+        cb->setGraphicsPipeline(ps);
+        cb->setViewport({ 0, 0, 1280, 720 });
+        cb->setVertexInput(0, { { vbuf, 0 } });
+        cb->draw(3);
+
+        u = r->nextResourceUpdateBatch();
+        QRhiReadbackDescription rb(tex);
+        QRhiReadbackResult rbResult;
+        rbResult.completed = [frame] { qDebug("  - readback %d completed", frame); };
+        u->readBackTexture(rb, &rbResult);
+
+        cb->endPass(u);
+
+        qDebug("Submit and wait");
+        r->endOffscreenFrame();
+
+        // No finish() or waiting for the completed callback is needed here
+        // since the endOffscreenFrame() implies a wait for completion.
+        if (!rbResult.data.isEmpty()) {
+            const uchar *p = reinterpret_cast<const uchar *>(rbResult.data.constData());
+            QImage image(p, rbResult.pixelSize.width(), rbResult.pixelSize.height(), QImage::Format_RGBA8888);
+            QString fn = QString::asprintf("frame%d.png", frame);
+            fn = QFileInfo(fn).absoluteFilePath();
+            qDebug("Saving into %s", qPrintable(fn));
+            image.save(fn);
+        } else {
+            qWarning("Readback failed!");
+        }
+    }
+
+    ps->releaseAndDestroy();
+    srb->releaseAndDestroy();
+    ubuf->releaseAndDestroy();
+    vbuf->releaseAndDestroy();
+
+    rt->releaseAndDestroy();
+    rp->releaseAndDestroy();
+    tex->releaseAndDestroy();
+
+    delete r;
+
+    return 0;
+}

examples/rhi/offscreen_metal/offscreen_metal.pro

+TEMPLATE = app
+CONFIG += console
+
+QT += shadertools rhi
+
+SOURCES = \
+    main.cpp
+
+RESOURCES = offscreen_metal.qrc
+
+target.path = $$[QT_INSTALL_EXAMPLES]/rhi/offscreen_metal
+INSTALLS += target

examples/rhi/offscreen_metal/offscreen_metal.qrc

+<!DOCTYPE RCC><RCC version="1.0">
+<qresource>
+  <file alias="color.vert.qsb">../shared/color.vert.qsb</file>
+  <file alias="color.frag.qsb">../shared/color.frag.qsb</file>
+</qresource>
+</RCC>

examples/rhi/rhi.pro

@@ -25,5 +25,6 @@ win32 {
 
 mac {
     SUBDIRS += \
-        plainqwindow_metal
+        plainqwindow_metal \
+        offscreen_metal
 }

examples/rhi/shared/examplewindow.cpp

@@ -114,9 +114,14 @@ void ExampleWindow::init()
     m_sc->setWindow(this);
     m_sc->setDepthStencil(m_ds);
     m_sc->setSampleCount(m_sampleCount);
+    QRhiSwapChain::Flags scFlags = 0;
+#ifdef READBACK_SWAPCHAIN
+    scFlags |= QRhiSwapChain::UsedAsTransferSource;
+#endif
 #ifdef USE_SRGB_SWAPCHAIN
-    m_sc->setFlags(QRhiSwapChain::sRGB);
+    scFlags |= QRhiSwapChain::sRGB;
 #endif
+    m_sc->setFlags(scFlags);
 
     m_scrp = m_sc->newCompatibleRenderPassDescriptor();
     m_sc->setRenderPassDescriptor(m_scrp);

src/rhi/qrhi.h

@@ -835,18 +835,19 @@ Q_DECLARE_TYPEINFO(QRhiGraphicsPipeline::TargetBlend, Q_MOVABLE_TYPE);
 class Q_RHI_EXPORT QRhiSwapChain : public QRhiResource
 {
 public:
-    enum SurfaceImportFlag {
+    enum Flag {
         SurfaceHasPreMulAlpha = 1 << 0,
         SurfaceHasNonPreMulAlpha = 1 << 1,
-        sRGB = 1 << 2
+        sRGB = 1 << 2,
+        UsedAsTransferSource = 1 << 3 // will be read back
     };
-    Q_DECLARE_FLAGS(SurfaceImportFlags, SurfaceImportFlag)
+    Q_DECLARE_FLAGS(Flags, Flag)
 
     QWindow *window() const { return m_window; }
     void setWindow(QWindow *window) { m_window = window; }
 
-    SurfaceImportFlags flags() const { return m_flags; }
-    void setFlags(SurfaceImportFlags f) { m_flags = f; }
+    Flags flags() const { return m_flags; }
+    void setFlags(Flags f) { m_flags = f; }
 
     QRhiRenderBuffer *depthStencil() const { return m_depthStencil; }
     void setDepthStencil(QRhiRenderBuffer *ds) { m_depthStencil = ds; }
@@ -896,7 +897,7 @@ public:
 protected:
     QRhiSwapChain(QRhiImplementation *rhi);
     QWindow *m_window = nullptr;
-    SurfaceImportFlags m_flags;
+    Flags m_flags;
     QRhiRenderBuffer *m_depthStencil = nullptr;
     int m_sampleCount = 1;
     QRhiRenderPassDescriptor *m_renderPassDesc = nullptr;
@@ -905,7 +906,7 @@ protected:
     void *m_reserved;
 };
 
-Q_DECLARE_OPERATORS_FOR_FLAGS(QRhiSwapChain::SurfaceImportFlags)
+Q_DECLARE_OPERATORS_FOR_FLAGS(QRhiSwapChain::Flags)
 
 class Q_RHI_EXPORT QRhiCommandBuffer : public QRhiResource
 {
@@ -1159,8 +1160,10 @@ public:
       is to use it in completely offscreen applications, e.g. to generate image
       sequences by rendering and reading back without ever showing a window.
       Usage in on-screen applications (so beginFrame, endFrame,
-      beginOffscreenFrame, endOffscreenFrame, beginFrame, ...) is possible too but
-      it does break parallelism so should be done only infrequently.
+      beginOffscreenFrame, endOffscreenFrame, beginFrame, ...) is possible too
+      but it does reduce parallelism (offscreen frames do not let the CPU -
+      potentially - generate another frame while the GPU is still processing
+      the previous one) so should be done only infrequently.
           QRhiReadbackResult rbResult;
           QRhiCommandBuffer *cb; // not owned
           beginOffscreenFrame(&cb);

src/rhi/qrhimetal.mm

@@ -65,6 +65,8 @@ QT_BEGIN_NAMESPACE
 
 struct QRhiMetalData
 {
+    QRhiMetalData(QRhiImplementation *rhi) : ofr(rhi) { }
+
     id<MTLDevice> dev;
     id<MTLCommandQueue> cmdQueue;
 
@@ -104,8 +106,28 @@ struct QRhiMetalData
         };
     };
     QVector<DeferredReleaseEntry> releaseQueue;
+
+    struct OffscreenFrame {
+        OffscreenFrame(QRhiImplementation *rhi) : cbWrapper(rhi) { }
+        bool active = false;
+        QMetalCommandBuffer cbWrapper;
+    } ofr;
+
+    struct ActiveReadback {
+        int activeFrameSlot = -1;
+        QRhiReadbackDescription desc;
+        QRhiReadbackResult *result;
+        id<MTLBuffer> buf;
+        quint32 bufSize;
+        QSize pixelSize;
+        QRhiTexture::Format format;
+    };
+    QVector<ActiveReadback> activeReadbacks;
 };
 
+Q_DECLARE_TYPEINFO(QRhiMetalData::DeferredReleaseEntry, Q_MOVABLE_TYPE);
+Q_DECLARE_TYPEINFO(QRhiMetalData::ActiveReadback, Q_MOVABLE_TYPE);
+
 struct QMetalBufferData
 {
     bool managed;
@@ -143,15 +165,17 @@ struct QMetalRenderTargetData
     QSize pixelSize;
     int colorAttCount = 0;
     int dsAttCount = 0;
+
+    struct ColorAtt {
+        id<MTLTexture> tex = nil;
+        int layer = 0;
+        int level = 0;
+        id<MTLTexture> resolveTex = nil;
+        int resolveLayer = 0;
+        int resolveLevel = 0;
+    };
+
     struct {
-        struct ColorAtt {
-            id<MTLTexture> tex = nil;
-            int layer = 0;
-            int level = 0;
-            id<MTLTexture> resolveTex = nil;
-            int resolveLayer = 0;
-            int resolveLevel = 0;
-        };
         ColorAtt colorAtt[QMetalRenderPassDescriptor::MAX_COLOR_ATTACHMENTS];
         id<MTLTexture> dsTex = nil;
         bool hasStencil = false;
@@ -179,12 +203,13 @@ struct QMetalSwapChainData
     id<MTLCommandBuffer> cb[QMTL_FRAMES_IN_FLIGHT];
     MTLRenderPassDescriptor *rp = nullptr;
     id<MTLTexture> msaaTex[QMTL_FRAMES_IN_FLIGHT];
+    QRhiTexture::Format rhiColorFormat;
     MTLPixelFormat colorFormat;
 };
 
 QRhiMetal::QRhiMetal(QRhiInitParams *params)
 {
-    d = new QRhiMetalData;
+    d = new QRhiMetalData(this);
 
     QRhiMetalInitParams *metalparams = static_cast<QRhiMetalInitParams *>(params);
     importedDevice = metalparams->importExistingDevice;
@@ -220,6 +245,7 @@ void QRhiMetal::create()
 void QRhiMetal::destroy()
 {
     executeDeferredReleases(true);
+    finishActiveReadbacks(true);
 
     if (d->cmdQueue) {
         [d->cmdQueue release];
@@ -558,6 +584,7 @@ QRhi::FrameOpResult QRhiMetal::beginFrame(QRhiSwapChain *swapChain)
         return QRhi::FrameOpSwapChainOutOfDate;
     }
 
+    currentSwapChain = swapChainD;
     currentFrameSlot = swapChainD->currentFrame;
     if (swapChainD->ds)
         swapChainD->ds->lastActiveFrameSlot = currentFrameSlot;
@@ -568,7 +595,6 @@ QRhi::FrameOpResult QRhiMetal::beginFrame(QRhiSwapChain *swapChain)
     swapChainD->d->cb[currentFrameSlot] = [d->cmdQueue commandBufferWithUnretainedReferences];
 
     swapChainD->cbWrapper.d->cb = swapChainD->d->cb[currentFrameSlot];
-    swapChainD->cbWrapper.resetState();
 
     id<MTLTexture> scTex = swapChainD->d->curDrawable.texture;
     id<MTLTexture> resolveTex = nil;
@@ -582,6 +608,8 @@ QRhi::FrameOpResult QRhiMetal::beginFrame(QRhiSwapChain *swapChain)
     swapChainD->rtWrapper.d->fb.hasStencil = swapChainD->ds ? true : false;
 
     executeDeferredReleases();
+    swapChainD->cbWrapper.resetState();
+    finishActiveReadbacks();
 
     return QRhi::FrameOpSuccess;
 }
@@ -603,6 +631,7 @@ QRhi::FrameOpResult QRhiMetal::endFrame(QRhiSwapChain *swapChain)
     [swapChainD->d->cb[currentFrameSlot] commit];
 
     swapChainD->currentFrame = (swapChainD->currentFrame + 1) % QMTL_FRAMES_IN_FLIGHT;
+    currentSwapChain = nullptr;
 
     ++finishedFrameCount;
     return QRhi::FrameOpSuccess;
@@ -610,13 +639,45 @@ QRhi::FrameOpResult QRhiMetal::endFrame(QRhiSwapChain *swapChain)
 
 QRhi::FrameOpResult QRhiMetal::beginOffscreenFrame(QRhiCommandBuffer **cb)
 {
-    Q_UNUSED(cb);
-    return QRhi::FrameOpError;
+    Q_ASSERT(!inFrame);
+    inFrame = true;
+
+    // Switch to the next slot manually. Swapchains do not know about this
+    // which is good. So for example a - unusual but possible - onscreen,
+    // onscreen, offscreen, onscreen, onscreen, onscreen sequence of
+    // begin/endFrame leads to 0, 1, 0, 0, 1, 0. This works because the
+    // offscreen frame is synchronous in the sense that we wait for execution
+    // to complete in endFrame, and so no resources used in that frame are busy
+    // anymore in the next frame.
+    currentFrameSlot = (currentFrameSlot + 1) % QMTL_FRAMES_IN_FLIGHT;
+
+    d->ofr.active = true;
+    *cb = &d->ofr.cbWrapper;
+    d->ofr.cbWrapper.d->cb = [d->cmdQueue commandBufferWithUnretainedReferences];
+
+    executeDeferredReleases();
+    d->ofr.cbWrapper.resetState();
+    finishActiveReadbacks();
+
+    return QRhi::FrameOpSuccess;
 }
 
 QRhi::FrameOpResult QRhiMetal::endOffscreenFrame()
 {
-    return QRhi::FrameOpError;
+    Q_ASSERT(d->ofr.active);
+    d->ofr.active = false;
+    Q_ASSERT(inFrame);
+    inFrame = false;
+
+    [d->ofr.cbWrapper.d->cb commit];
+
+    // offscreen frames wait for completion, unlike swapchain ones
+    [d->ofr.cbWrapper.d->cb waitUntilCompleted];
+
+    finishActiveReadbacks(true);
+
+    ++finishedFrameCount;
+    return QRhi::FrameOpSuccess;
 }
 
 QRhi::FrameOpResult QRhiMetal::finish()
@@ -827,6 +888,60 @@ void QRhiMetal::enqueueResourceUpdates(QRhiCommandBuffer *cb, QRhiResourceUpdate
                                   destinationOrigin: MTLOriginMake(dx, dy, 0)];
     }
 
+    for (const QRhiResourceUpdateBatchPrivate::TextureRead &u : ud->textureReadbacks) {
+        QRhiMetalData::ActiveReadback aRb;
+        aRb.activeFrameSlot = currentFrameSlot;
+        aRb.desc = u.rb;
+        aRb.result = u.result;
+
+        QMetalTexture *texD = QRHI_RES(QMetalTexture, aRb.desc.texture);
+        QMetalSwapChain *swapChainD = nullptr;
+        id<MTLTexture> src;
+        QSize srcSize;
+        if (texD) {
+            if (texD->samples > 1) {
+                qWarning("Multisample texture cannot be read back");
+                continue;
+            }
+            aRb.pixelSize = texD->m_pixelSize;
+            if (u.rb.level > 0) {
+                aRb.pixelSize.setWidth(qFloor(float(qMax(1, aRb.pixelSize.width() >> u.rb.level))));
+                aRb.pixelSize.setHeight(qFloor(float(qMax(1, aRb.pixelSize.height() >> u.rb.level))));
+            }
+            aRb.format = texD->m_format;
+            src = texD->d->tex;
+            srcSize = texD->m_pixelSize;
+        } else {
+            Q_ASSERT(currentSwapChain);
+            swapChainD = QRHI_RES(QMetalSwapChain, currentSwapChain);
+            aRb.pixelSize = swapChainD->pixelSize;
+            aRb.format = swapChainD->d->rhiColorFormat;
+            // Multisample swapchains need nothing special since resolving
+            // happens when ending a renderpass.
+            const QMetalRenderTargetData::ColorAtt &colorAtt(swapChainD->rtWrapper.d->fb.colorAtt[0]);
+            src = colorAtt.resolveTex ? colorAtt.resolveTex : colorAtt.tex;
+            srcSize = swapChainD->rtWrapper.d->pixelSize;
+        }
+
+        quint32 bpl = 0;
+        textureFormatInfo(aRb.format, aRb.pixelSize, &bpl, &aRb.bufSize);
+        aRb.buf = [d->dev newBufferWithLength: aRb.bufSize options: MTLResourceStorageModeShared];
+
+        ensureBlit();
+        [blitEnc copyFromTexture: src
+                                  sourceSlice: aRb.desc.layer
+                                  sourceLevel: aRb.desc.level
+                                  sourceOrigin: MTLOriginMake(0, 0, 0)
+                                  sourceSize: MTLSizeMake(srcSize.width(), srcSize.height(), 1)
+                                  toBuffer: aRb.buf
+                                  destinationOffset: 0
+                                  destinationBytesPerRow: bpl
+                                  destinationBytesPerImage: 0
+                                  options: MTLBlitOptionNone];
+
+        d->activeReadbacks.append(aRb);
+    }
+
     for (const QRhiResourceUpdateBatchPrivate::TextureMipGen &u : ud->textureMipGens) {
         ensureBlit();
         [blitEnc generateMipmapsForTexture: QRHI_RES(QMetalTexture, u.tex)->d->tex];
@@ -974,6 +1089,31 @@ void QRhiMetal::executeDeferredReleases(bool forced)
     }
 }
 
+void QRhiMetal::finishActiveReadbacks(bool forced)
+{
+    QVarLengthArray<std::function<void()>, 4> completedCallbacks;
+
+    for (int i = d->activeReadbacks.count() - 1; i >= 0; --i) {
+        const QRhiMetalData::ActiveReadback &aRb(d->activeReadbacks[i]);
+        if (forced || currentFrameSlot == aRb.activeFrameSlot || aRb.activeFrameSlot < 0) {
+            aRb.result->format = aRb.format;
+            aRb.result->pixelSize = aRb.pixelSize;
+            aRb.result->data.resize(aRb.bufSize);
+            void *p = [aRb.buf contents];
+            memcpy(aRb.result->data.data(), p, aRb.bufSize);
+            [aRb.buf release];
+
+            if (aRb.result->completed)
+                completedCallbacks.append(aRb.result->completed);
+
+            d->activeReadbacks.removeAt(i);
+        }
+    }
+
+    for (auto f : completedCallbacks)
+        f();
+}
+
 QMetalBuffer::QMetalBuffer(QRhiImplementation *rhi, Type type, UsageFlags usage, int size)
     : QRhiBuffer(rhi, type, usage, size),
       d(new QMetalBufferData)
@@ -2116,6 +2256,7 @@ void QMetalSwapChain::chooseFormats()
     samples = rhiD->effectiveSampleCount(m_sampleCount);
     // pick a format that is allowed for CAMetalLayer.pixelFormat
     d->colorFormat = m_flags.testFlag(sRGB) ? MTLPixelFormatBGRA8Unorm_sRGB : MTLPixelFormatBGRA8Unorm;
+    d->rhiColorFormat = QRhiTexture::BGRA8;
 }
 
 bool QMetalSwapChain::buildOrResize()
@@ -2136,6 +2277,9 @@ bool QMetalSwapChain::buildOrResize()
     if (d->colorFormat != d->layer.pixelFormat)
         d->layer.pixelFormat = d->colorFormat;
 
+    if (m_flags.testFlag(UsedAsTransferSource))
+        d->layer.framebufferOnly = NO;
+
     m_currentPixelSize = surfacePixelSize();
     pixelSize = m_currentPixelSize;
 

src/rhi/qrhimetal_p.h

@@ -315,6 +315,7 @@ public:
     void create();
     void destroy();
     void executeDeferredReleases(bool forced = false);
+    void finishActiveReadbacks(bool forced = false);
     void enqueueResourceUpdates(QRhiCommandBuffer *cb, QRhiResourceUpdateBatch *resourceUpdates);
     void executeBufferHostWritesForCurrentFrame(QMetalBuffer *bufD);
     int effectiveSampleCount(int sampleCount) const;
@@ -324,6 +325,7 @@ public:
     int currentFrameSlot = 0;
     int finishedFrameCount = 0;
     bool inPass = false;
+    QMetalSwapChain *currentSwapChain = nullptr;
 
     QRhiMetalData *d = nullptr;
 };

src/rhi/qrhivulkan.cpp

@@ -940,7 +940,7 @@ bool QRhiVulkan::recreateSwapChain(QRhiSwapChain *swapChain)
 
     VkImageUsageFlags usage = VK_IMAGE_USAGE_COLOR_ATTACHMENT_BIT;
     swapChainD->supportsReadback = (surfaceCaps.supportedUsageFlags & VK_IMAGE_USAGE_TRANSFER_SRC_BIT);
-    if (swapChainD->supportsReadback)
+    if (swapChainD->supportsReadback && swapChainD->m_flags.testFlag(QRhiSwapChain::UsedAsTransferSource))
         usage |= VK_IMAGE_USAGE_TRANSFER_SRC_BIT;
 
     qDebug("Creating new swapchain of %d buffers, size %dx%d",

todo.txt

-mtl: rhi without a window, offscreen frame
-mtl: readback (tex, backbuffer)
-
+mtl: finish()
 gl: tex formats (texture, readback)
 gl: srgb
 
@@ -55,6 +53,8 @@ dxc for d3d as an alternative to fxc?
 hlsl -> dxc -> spirv -> spirv-cross hmmm...
 
 +++ done
+mtl: readback (tex, backbuffer)
+mtl: rhi without a window, offscreen frame
 mtl: compressed textures
 mtl: srgb (tex, swapchain buf)
 change how resolve is done