场景图 - 自定义材质

展示如何在 Qt Quick 场景图中实现自定义材质。

自定义材质示例展示了如何实现使用自定义顶点和片段着色器渲染的项目。

着色器和材质

主要功能在片段着色器中

// Copyright (C) 2023 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause

#version 440

layout(location = 0) in vec2 vTexCoord;

layout(location = 0) out vec4 fragColor;

// uniform block: 84 bytes
layout(std140, binding = 0) uniform buf {
    mat4 qt_Matrix; // offset 0
    float qt_Opacity; // offset 64
    float zoom; // offset 68
    vec2 center; // offset 72
    int limit; // offset 80
} ubuf;

void main()
{
    vec4 color1 = vec4(1.0, 0.85, 0.55, 1);
    vec4 color2 = vec4(0.226, 0.0, 0.615, 1);

    float aspect_ratio = -ubuf.qt_Matrix[0][0]/ubuf.qt_Matrix[1][1];
    vec2 z, c;

    c.x = (vTexCoord.x - 0.5) / ubuf.zoom + ubuf.center.x;
    c.y = aspect_ratio * (vTexCoord.y - 0.5) / ubuf.zoom + ubuf.center.y;

    int i;
    z = c;
    for (i = 0; i < ubuf.limit; i++) {
        float x = (z.x * z.x - z.y * z.y) + c.x;
        float y = (z.y * z.x + z.x * z.y) + c.y;

        if ((x * x + y * y) > 4.0) break;
        z.x = x;
        z.y = y;
    }

    if (i == ubuf.limit) {
        fragColor = vec4(0.0, 0.0, 0.0, 1.0);
    } else {
        float f = (i * 1.0) / ubuf.limit;
        fragColor = mix(color1, color2, sqrt(f));
    }
}

片段和顶点着色器组合成一个 QSGMaterialShader 子类。

class CustomShader : public QSGMaterialShader
{
public:
    CustomShader()
    {
        setShaderFileName(VertexStage, QLatin1String(":/scenegraph/custommaterial/shaders/mandelbrot.vert.qsb"));
        setShaderFileName(FragmentStage, QLatin1String(":/scenegraph/custommaterial/shaders/mandelbrot.frag.qsb"));
    }
    bool updateUniformData(RenderState &state,
                           QSGMaterial *newMaterial, QSGMaterial *oldMaterial) override;
};

QSGMaterial 子类封装了着色器以及渲染状态。在本示例中，我们添加了与着色器统一变量相对应的状态信息。材质负责通过重新实现 QSGMaterial::createShader() 来创建着色器。

class CustomMaterial : public QSGMaterial
{
public:
    CustomMaterial();
    QSGMaterialType *type() const override;
    int compare(const QSGMaterial *other) const override;

    QSGMaterialShader *createShader(QSGRendererInterface::RenderMode) const override
    {
        return new CustomShader;
    }

    struct {
        float center[2];
        float zoom;
        int limit;
        bool dirty;
    } uniforms;
};

为了更新统一数据，我们重新实现了 QSGMaterialShader::updateUniformData()。

bool CustomShader::updateUniformData(RenderState &state, QSGMaterial *newMaterial, QSGMaterial *oldMaterial)
{
    bool changed = false;
    QByteArray *buf = state.uniformData();
    Q_ASSERT(buf->size() >= 84);

    if (state.isMatrixDirty()) {
        const QMatrix4x4 m = state.combinedMatrix();
        memcpy(buf->data(), m.constData(), 64);
        changed = true;
    }

    if (state.isOpacityDirty()) {
        const float opacity = state.opacity();
        memcpy(buf->data() + 64, &opacity, 4);
        changed = true;
    }

    auto *customMaterial = static_cast<CustomMaterial *>(newMaterial);
    if (oldMaterial != newMaterial || customMaterial->uniforms.dirty) {
        memcpy(buf->data() + 68, &customMaterial->uniforms.zoom, 4);
        memcpy(buf->data() + 72, &customMaterial->uniforms.center, 8);
        memcpy(buf->data() + 80, &customMaterial->uniforms.limit, 4);
        customMaterial->uniforms.dirty = false;
        changed = true;
    }
    return changed;
}

项和节点

我们创建一个自定义项来展示我们的新材质

#include <QQuickItem>

class CustomItem : public QQuickItem
{
    Q_OBJECT
    Q_PROPERTY(qreal zoom READ zoom WRITE setZoom NOTIFY zoomChanged)
    Q_PROPERTY(int iterationLimit READ iterationLimit WRITE setIterationLimit NOTIFY iterationLimitChanged)
    Q_PROPERTY(QPointF center READ center WRITE setCenter NOTIFY centerChanged)
    QML_ELEMENT

public:
    explicit CustomItem(QQuickItem *parent = nullptr);

    qreal zoom() const
    {
        return m_zoom;
    }

    int iterationLimit() const
    {
        return m_limit;
    }

    QPointF center() const
    {
        return m_center;
    }

public slots:
    void setZoom(qreal zoom);

    void setIterationLimit(int iterationLimit);

    void setCenter(QPointF center);

signals:
    void zoomChanged(qreal zoom);

    void iterationLimitChanged(int iterationLimit);

    void centerChanged(QPointF center);

protected:
    QSGNode *updatePaintNode(QSGNode *, UpdatePaintNodeData *) override;
    void geometryChange(const QRectF &newGeometry, const QRectF &oldGeometry) override;

private:
    bool m_geometryChanged = true;
    qreal m_zoom;
    bool m_zoomChanged = true;
    int m_limit;
    bool m_limitChanged = true;
    QPointF m_center;
    bool m_centerChanged = true;
};

CustomItem 声明添加了三个属性，这三个属性对应于我们想要暴露给 QML 的统一变量。

    Q_PROPERTY(qreal zoom READ zoom WRITE setZoom NOTIFY zoomChanged)
    Q_PROPERTY(int iterationLimit READ iterationLimit WRITE setIterationLimit NOTIFY iterationLimitChanged)
    Q_PROPERTY(QPointF center READ center WRITE setCenter NOTIFY centerChanged)

与每个自定义 Qt Quick 项一样，实现分为两部分：除了 CustomItem 之外，它存在于 GUI 线程，我们还创建了存在于渲染线程的 QSGNode 子类。

class CustomNode : public QSGGeometryNode
{
public:
    CustomNode()
    {
        auto *m = new CustomMaterial;
        setMaterial(m);
        setFlag(OwnsMaterial, true);

        QSGGeometry *g = new QSGGeometry(QSGGeometry::defaultAttributes_TexturedPoint2D(), 4);
        QSGGeometry::updateTexturedRectGeometry(g, QRect(), QRect());
        setGeometry(g);
        setFlag(OwnsGeometry, true);
    }

    void setRect(const QRectF &bounds)
    {
        QSGGeometry::updateTexturedRectGeometry(geometry(), bounds, QRectF(0, 0, 1, 1));
        markDirty(QSGNode::DirtyGeometry);
    }

    void setZoom(qreal zoom)
    {
        auto *m = static_cast<CustomMaterial *>(material());
        m->uniforms.zoom = zoom;
        m->uniforms.dirty = true;
        markDirty(DirtyMaterial);
    }

    void setLimit(int limit)
    {
        auto *m = static_cast<CustomMaterial *>(material());
        m->uniforms.limit = limit;
        m->uniforms.dirty = true;
        markDirty(DirtyMaterial);
    }

    void setCenter(const QPointF &center)
    {
        auto *m = static_cast<CustomMaterial *>(material());
        m->uniforms.center[0] = center.x();
        m->uniforms.center[1] = center.y();
        m->uniforms.dirty = true;
        markDirty(DirtyMaterial);
    }
};

节点拥有材质的一个实例，并具备更新材质状态的逻辑。项维护相应的 QML 属性。由于项和材质位于不同的线程上，因此项需要复制来自材质的信息。

void CustomItem::setZoom(qreal zoom)
{
    if (qFuzzyCompare(m_zoom, zoom))
        return;

    m_zoom = zoom;
    m_zoomChanged = true;
    emit zoomChanged(m_zoom);
    update();
}

void CustomItem::setIterationLimit(int limit)
{
    if (m_limit == limit)
        return;

    m_limit = limit;
    m_limitChanged = true;
    emit iterationLimitChanged(m_limit);
    update();
}

void CustomItem::setCenter(QPointF center)
{
    if (m_center == center)
        return;

    m_center = center;
    m_centerChanged = true;
    emit centerChanged(m_center);
    update();
}

信息通过重新实现 QQuickItem::updatePaintNode() 从项复制到场景图中。当函数被调用时，两个线程处于同步点，因此可以安全地访问这两个类。

QSGNode *CustomItem::updatePaintNode(QSGNode *old, UpdatePaintNodeData *)
{
    auto *node = static_cast<CustomNode *>(old);

    if (!node)
        node = new CustomNode;

    if (m_geometryChanged)
        node->setRect(boundingRect());
    m_geometryChanged = false;

    if (m_zoomChanged)
        node->setZoom(m_zoom);
    m_zoomChanged = false;

    if (m_limitChanged)
        node->setLimit(m_limit);
    m_limitChanged = false;

    if (m_centerChanged)
        node->setCenter(m_center);
    m_centerChanged = false;

    return node;
}

示例的其余部分

该应用程序是一个简单的 QML 应用程序，包含一个 QGuiApplication 和一个 QQuickView，我们传递一个 .qml 文件。

在 QML 文件中，我们创建了要锚定的自定义项，使其填充根。

    CustomItem {
        property real t: 1
        anchors.fill: parent
        center: Qt.point(-0.748, 0.1);
        iterationLimit: 3 * (zoom + 30)
        zoom: t * t / 10
        NumberAnimation on t {
            from: 1
            to: 60
            duration: 30*1000;
            running: true
            loops: Animation.Infinite
        }
    }

为了使示例更有趣，我们添加了一个动画来更改缩放级别和迭代限制。中心保持不变。

示例项目 @ code.qt.io