QRhiWidget Class

class ExampleRhiWidget : public QRhiWidget
{
public:
    ExampleRhiWidget(QWidget *parent = nullptr) : QRhiWidget(parent) { }
    void initialize(QRhiCommandBuffer *cb) override;
    void render(QRhiCommandBuffer *cb) override;
private:
    QRhi *m_rhi = nullptr;
    std::unique_ptr<QRhiBuffer> m_vbuf;
    std::unique_ptr<QRhiBuffer> m_ubuf;
    std::unique_ptr<QRhiShaderResourceBindings> m_srb;
    std::unique_ptr<QRhiGraphicsPipeline> m_pipeline;
    QMatrix4x4 m_viewProjection;
    float m_rotation = 0.0f;
};

float vertexData[] = {
     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,
};

QShader getShader(const QString &name)
{
    QFile f(name);
    return f.open(QIODevice::ReadOnly) ? QShader::fromSerialized(f.readAll()) : QShader();
}

void ExampleRhiWidget::initialize(QRhiCommandBuffer *cb)
{
    if (m_rhi != rhi()) {
        m_pipeline.reset();
        m_rhi = rhi();
    }

    if (!m_pipeline) {
        m_vbuf.reset(m_rhi->newBuffer(QRhiBuffer::Immutable, QRhiBuffer::VertexBuffer, sizeof(vertexData)));
        m_vbuf->create();

        m_ubuf.reset(m_rhi->newBuffer(QRhiBuffer::Dynamic, QRhiBuffer::UniformBuffer, 64));
        m_ubuf->create();

        m_srb.reset(m_rhi->newShaderResourceBindings());
        m_srb->setBindings({
            QRhiShaderResourceBinding::uniformBuffer(0, QRhiShaderResourceBinding::VertexStage, m_ubuf.get()),
        });
        m_srb->create();

        m_pipeline.reset(m_rhi->newGraphicsPipeline());
        m_pipeline->setShaderStages({
            { QRhiShaderStage::Vertex, getShader(QLatin1String(":/shader_assets/color.vert.qsb")) },
            { QRhiShaderStage::Fragment, getShader(QLatin1String(":/shader_assets/color.frag.qsb")) }
        });
        QRhiVertexInputLayout inputLayout;
        inputLayout.setBindings({
            { 5 * sizeof(float) }
        });
        inputLayout.setAttributes({
            { 0, 0, QRhiVertexInputAttribute::Float2, 0 },
            { 0, 1, QRhiVertexInputAttribute::Float3, 2 * sizeof(float) }
        });
        m_pipeline->setVertexInputLayout(inputLayout);
        m_pipeline->setShaderResourceBindings(m_srb.get());
        m_pipeline->setRenderPassDescriptor(renderTarget()->renderPassDescriptor());
        m_pipeline->create();

        QRhiResourceUpdateBatch *resourceUpdates = m_rhi->nextResourceUpdateBatch();
        resourceUpdates->uploadStaticBuffer(m_vbuf.get(), vertexData);
        cb->resourceUpdate(resourceUpdates);
    }

    const QSize outputSize = colorTexture()->pixelSize();
    m_viewProjection = m_rhi->clipSpaceCorrMatrix();
    m_viewProjection.perspective(45.0f, outputSize.width() / (float) outputSize.height(), 0.01f, 1000.0f);
    m_viewProjection.translate(0, 0, -4);
}

void ExampleRhiWidget::render(QRhiCommandBuffer *cb)
{
    QRhiResourceUpdateBatch *resourceUpdates = m_rhi->nextResourceUpdateBatch();
    m_rotation += 1.0f;
    QMatrix4x4 modelViewProjection = m_viewProjection;
    modelViewProjection.rotate(m_rotation, 0, 1, 0);
    resourceUpdates->updateDynamicBuffer(m_ubuf.get(), 0, 64, modelViewProjection.constData());

    const QColor clearColor = QColor::fromRgbF(0.4f, 0.7f, 0.0f, 1.0f);
    cb->beginPass(renderTarget(), clearColor, { 1.0f, 0 }, resourceUpdates);

    cb->setGraphicsPipeline(m_pipeline.get());
    const QSize outputSize = colorTexture()->pixelSize();
    cb->setViewport(QRhiViewport(0, 0, outputSize.width(), outputSize.height()));
    cb->setShaderResources();
    const QRhiCommandBuffer::VertexInput vbufBinding(m_vbuf.get(), 0);
    cb->setVertexInput(0, 1, &vbufBinding);
    cb->draw(3);

    cb->endPass();

    update();
}

#version 440
layout(location = 0) in vec4 position;
layout(location = 1) in vec3 color;
layout(location = 0) out vec3 v_color;
layout(std140, binding = 0) uniform buf {
    mat4 mvp;
};

void main()
{
    v_color = color;
    gl_Position = mvp * position;
}

#version 440
layout(location = 0) in vec3 v_color;
layout(location = 0) out vec4 fragColor;

void main()
{
    fragColor = vec4(v_color, 1.0);
}

Member Function Documentation

[explicit] QRhiWidget::QRhiWidget(QWidget *parent = nullptr, Qt::WindowFlags f = {})

Constructs a widget which is a child of parent, with widget flags set to f.

[override virtual noexcept] QRhiWidget::~QRhiWidget()

Destructor.

QRhiWidget::Api QRhiWidget::api() const

Returns the currently set graphics API (QRhi backend).

See also setApi().

[protected] QRhiTexture *QRhiWidget::colorTexture() const

Returns the texture serving as the color buffer for the widget.

Must only be called from initialize() and render().

Unlike the depth-stencil buffer and the QRhiRenderTarget, this texture is always available and is managed by the QRhiWidget, independent of the value of autoRenderTarget.

See also msaaColorBuffer(), depthStencilBuffer(), renderTarget(), and resolveTexture().

[protected] QRhiRenderBuffer *QRhiWidget::depthStencilBuffer() const

Returns the depth-stencil buffer used by the widget's rendering.

Must only be called from initialize() and render().

Available only when autoRenderTarget is true. Otherwise the returned value is nullptr and it is up the reimplementation of initialize() to create and manage a depth-stencil buffer and a QRhiTextureRenderTarget.

See also colorTexture() and renderTarget().

[override virtual protected] bool QRhiWidget::event(QEvent *e)

Reimplements: QWidget::event(QEvent *event).

[signal] void QRhiWidget::frameSubmitted()

This signal is emitted after the widget's top-level window has finished composition and has submitted a frame.

QImage QRhiWidget::grabFramebuffer() const

Renders a new frame, reads the contents of the texture back, and returns it as a QImage.

When an error occurs, a null QImage is returned.

The returned QImage will have a format of QImage::Format_RGBA8888, QImage::Format_RGBA16FPx4, QImage::Format_RGBA32FPx4, or QImage::Format_BGR30, depending on colorBufferFormat().

QRhiWidget does not know the renderer's approach to blending and composition, and therefore cannot know if the output has alpha premultiplied in the RGB color values. Thus _Premultiplied QImage formats are never used for the returned QImage, even when it would be appropriate. It is up to the caller to reinterpret the resulting data as it sees fit.

The function is named grabFramebuffer() for consistency with QOpenGLWidget and QQuickWidget. It is not the only way to get CPU-side image data out of the QRhiWidget's content: calling QWidget::grab() on a QRhiWidget, or an ancestor of it, is functional as well (returning a QPixmap). Besides working directly with QImage, another advantage of grabFramebuffer() is that it may be slightly more performant, simply because it does not have to go through the rest of QWidget infrastructure but can right away trigger rendering a new frame and then do the readback.

See also setColorBufferFormat().

[virtual protected] void QRhiWidget::initialize(QRhiCommandBuffer *cb)

Called when the widget is initialized for the first time, when the associated texture's size, format, or sample count changes, or when the QRhi and texture change for any reason. The function is expected to maintain (create if not yet created, adjust and rebuild if the size has changed) the graphics resources used by the rendering code in render().

To query the QRhi, QRhiTexture, and other related objects, call rhi(), colorTexture(), depthStencilBuffer(), and renderTarget().

When the widget size changes, the QRhi object, the color buffer texture, and the depth stencil buffer objects are all the same instances (so the getters return the same pointers) as before, but the color and depth/stencil buffers will likely have been rebuilt, meaning the size and the underlying native texture resource may be different than in the last invocation.

Reimplementations should also be prepared that the QRhi object and the color buffer texture may change between invocations of this function. One special case where the objects will be different is when performing a grabFramebuffer() with a widget that is not yet shown, and then making the widget visible on-screen within a top-level widget. There the grab will happen with a dedicated QRhi that is then replaced with the top-level window's associated QRhi in subsequent initialize() and render() invocations. Another, more common case is when the widget is reparented so that it belongs to a new top-level window. In this case the QRhi and all related resources managed by the QRhiWidget will be different instances than before in the subsequent call to this function. Is is then important that all existing QRhi resources previously created by the subclass are destroyed because they belong to the previous QRhi that should not be used by the widget anymore.

When autoRenderTarget is true, which is the default, a depth-stencil QRhiRenderBuffer and a QRhiTextureRenderTarget associated with colorTexture() (or msaaColorBuffer()) and the depth-stencil buffer are created and managed automatically. Reimplementations of initialize() and render() can query those objects via depthStencilBuffer() and renderTarget(). When autoRenderTarget is set to false, these objects are no longer created and managed automatically. Rather, it will be up the the initialize() implementation to create buffers and set up the render target as it sees fit. When manually managing additional color or depth-stencil attachments for the render target, their size and sample count must always follow the size and sample count of colorTexture() / msaaColorBuffer(), otherwise rendering or 3D API validation errors may occur.

The subclass-created graphics resources are expected to be released in the destructor implementation of the subclass.

cb is the QRhiCommandBuffer for the current frame of the widget. The function is called with a frame being recorded, but without an active render pass. The command buffer is provided primarily to allow enqueuing resource updates without deferring to render().

See also render().

bool QRhiWidget::isDebugLayerEnabled() const

Returns true if a debug or validation layer will be requested if applicable to the graphics API in use.

See also setDebugLayerEnabled().

[protected] QRhiRenderBuffer *QRhiWidget::msaaColorBuffer() const

Returns the renderbuffer serving as the multisample color buffer for the widget.

Must only be called from initialize() and render().

When sampleCount is larger than 1, and so multisample antialising is enabled, the returned QRhiRenderBuffer has a matching sample count and serves as the color buffer. Graphics pipelines used to render into this buffer must be created with the same sample count, and the depth-stencil buffer's sample count must match as well. The multisample content is expected to be resolved into the texture returned from resolveTexture(). When autoRenderTarget is true, renderTarget() is set up automatically to do this, by setting up msaaColorBuffer() as the renderbuffer of color attachment 0 and resolveTexture() as its resolveTexture.

When MSAA is not in use, the return value is nullptr. Use colorTexture() instead then.

Depending on the underlying 3D graphics API, there may be no practical difference between multisample textures and color renderbuffers with a sample count larger than 1 (QRhi may just map both to the same native resource type). Some older APIs however may differentiate between textures and renderbuffers. In order to support OpenGL ES 3.0, where multisample renderbuffers are available, but multisample textures are not, QRhiWidget always performs MSAA by using a multisample QRhiRenderBuffer as the color attachment (and never a multisample QRhiTexture).

See also colorTexture(), depthStencilBuffer(), renderTarget(), and resolveTexture().

[override virtual protected] void QRhiWidget::paintEvent(QPaintEvent *e)

Reimplements: QWidget::paintEvent(QPaintEvent *event).

Handles paint events.

Calling QWidget::update() will lead to sending a paint event e, and thus invoking this function. The sending of the event is asynchronous and will happen at some point after returning from update(). This function will then, after some preparation, call the virtual render() to update the contents of the QRhiWidget's associated texture. The widget's top-level window will then composite the texture with the rest of the window.

[virtual protected] void QRhiWidget::releaseResources()

Called when the need to early-release the graphics resources arises.

This normally does not happen for a QRhiWidget that is added to a top-level widget's child hierarchy and it then stays there for the rest of its and the top-level's lifetime. Thus in many cases there is no need to reimplement this function, e.g. because the application only ever has a single top-level widget (native window). However, when reparenting of the widget (or an ancestor of it) is involved, reimplementing this function will become necessary in robust, well-written QRhiWidget subclasses.

When this function is called, the implementation is expected to destroy all QRhi resources (QRhiBuffer, QRhiTexture, etc. objects), similarly to how it is expected to do this in the destructor. Nulling out, using a smart pointer, or setting a resources-invalid flag is going to be required as well, because initialize() will eventually get called afterwards. Note however that deferring the releasing of resources to the subsequent initialize() is wrong. If this function is called, the resource must be dropped before returning. Also note that implementing this function does not replace the class destructor (or smart pointers): the graphics resources must still be released in both.

See the Cube RHI Widget Example for an example of this in action. There the button that toggles the QRhiWidget between being a child widget (due to having a parent widget) and being a top-level widget (due to having no parent widget), will trigger invoking this function since the associated top-level widget, native window, and QRhi all change during the lifetime of the QRhiWidget, with the previously used QRhi getting destroyed which implies an early-release of the associated resources managed by the still-alive QRhiWidget.

Another case when this function is called is when grabFramebuffer() is used with a QRhiWidget that is not added to a visible window, i.e. the rendering is performed offscreen. If later on this QRhiWidget is made visible, or added to a visible widget hierarchy, the associated QRhi will change from the temporary one used for offscreen rendering to the window's dedicated one, thus triggering this function as well.

See also initialize().

[virtual protected] void QRhiWidget::render(QRhiCommandBuffer *cb)

Called when the widget contents (i.e. the contents of the texture) need updating.

There is always at least one call to initialize() before this function is called.

To request updates, call QWidget::update(). Calling update() from within render() will lead to updating continuously, throttled by vsync.

cb is the QRhiCommandBuffer for the current frame of the widget. The function is called with a frame being recorded, but without an active render pass.

See also initialize().

[signal] void QRhiWidget::renderFailed()

This signal is emitted whenever the widget is supposed to render to its backing texture (either due to a widget update or due to a call to grabFramebuffer()), but there is no QRhi for the widget to use, likely due to issues related to graphics configuration.

This signal may be emitted multiple times when a problem arises. Do not assume it is emitted only once. Connect with Qt::SingleShotConnection if the error handling code is to be notified only once.

[protected] QRhiRenderTarget *QRhiWidget::renderTarget() const

Returns the render target object that must be used with QRhiCommandBuffer::beginPass() in reimplementations of render().

Must only be called from initialize() and render().

Available only when autoRenderTarget is true. Otherwise the returned value is nullptr and it is up the reimplementation of initialize() to create and manage a depth-stencil buffer and a QRhiTextureRenderTarget.

When creating graphics pipelines, a QRhiRenderPassDescriptor is needed. This can be queried from the returned QRhiTextureRenderTarget by calling renderPassDescriptor().

See also colorTexture() and depthStencilBuffer().

[override virtual protected] void QRhiWidget::resizeEvent(QResizeEvent *e)

Reimplements: QWidget::resizeEvent(QResizeEvent *event).

Handles resize events that are passed in the e event parameter. Calls the virtual function initialize().

[protected] QRhiTexture *QRhiWidget::resolveTexture() const

Returns the non-multisample texture to which the multisample content is resolved.

The result is nullptr when multisample antialiasing is not enabled.

Must only be called from initialize() and render().

With MSAA enabled, this is the texture that gets composited with the rest of the QWidget content on-screen. However, the QRhiWidget's rendering must target the (multisample) QRhiRenderBuffer returned from msaaColorBuffer(). When autoRenderTarget is true, this is taken care of by the QRhiRenderTarget returned from renderTarget(). Otherwise, it is up to the subclass code to correctly configure a render target object with both the color buffer and resolve textures.

See also colorTexture().

[protected] QRhi *QRhiWidget::rhi() const

Returns the current QRhi object.

Must only be called from initialize() and render().

void QRhiWidget::setApi(QRhiWidget::Api api)

Sets the graphics API and QRhi backend to use to api.

The default value depends on the platform: Metal on macOS and iOS, Direct 3D 11 on Windows, OpenGL otherwise.

The api can only be set once for the widget and its top-level window, once it is done and takes effect, the window can only use that API and QRhi backend to render. Attempting to set another value, or to add another QRhiWidget with a different api will not function as expected.

See also setColorBufferFormat(), setDebugLayerEnabled(), and api().

[protected] void QRhiWidget::setAutoRenderTarget(bool enabled)

Controls if a depth-stencil QRhiRenderBuffer and a QRhiTextureRenderTarget is created and maintained automatically by the widget. The default value is true.

In automatic mode, the size and sample count of the depth-stencil buffer follows the color buffer texture's settings. In non-automatic mode, renderTarget() and depthStencilBuffer() always return nullptr and it is then up to the application's implementation of initialize() to take care of setting up and managing these objects.

Call this function with enabled set to false early on, for example in the derived class' constructor, to disable the automatic mode.

void QRhiWidget::setDebugLayerEnabled(bool enable)

Requests the debug or validation layer of the underlying graphics API when enable is true.

Applicable for Vulkan and Direct 3D.

By default this is disabled.

See also setApi() and isDebugLayerEnabled().