c++ - 如何平滑自定义 QML 元素的绘制？

Question

我现在尝试创建自定义 QML 元素，派生自 QQuickItem .所以我覆盖了 QQuickItem::updatePaintNode现在想画一条线。我的代码:

QSGNode *StrikeLine::updatePaintNode(QSGNode *oldNode, QQuickItem::UpdatePaintNodeData *)
{
    QSGGeometryNode *node = 0;

    QSGGeometry *geometry;
    QSGFlatColorMaterial *material;
    node = static_cast<QSGGeometryNode *>(oldNode);
    if(!node) {
        node = new QSGGeometryNode;
        geometry = new QSGGeometry(QSGGeometry::defaultAttributes_Point2D(), 2);
        geometry->setDrawingMode(GL_LINES);
        geometry->setLineWidth(3);
        material = new QSGFlatColorMaterial;
        material->setColor(QColor(255, 0, 0));
        node->setGeometry(geometry);
        node->setFlag(QSGNode::OwnsGeometry);
        node->setMaterial(material);
        node->setFlag(QSGNode::OwnsMaterial);
        getColor();
    } else {
        geometry = node->geometry();
        material = static_cast<QSGFlatColorMaterial *>(node->material());
    }
    geometry->vertexDataAsPoint2D()[0].set(p_startPoint.x(), p_startPoint.y());
    geometry->vertexDataAsPoint2D()[1].set(p_endPoint.x(), p_endPoint.y());
    material->setColor(getColor());
    node->markDirty(QSGNode::DirtyGeometry);

    return node;
}

但是我的线看起来很丑。边缘很粗糙，看起来完全像 DOS 图形。所以我的问题 - 我怎样才能应用平滑的绘画？我现在可能是一些着色器或其他东西，但我找不到任何文档。

Answer 1

场景图支持两种类型的抗锯齿。矩形和图像等基元将通过沿基元边缘添加更多顶点来消除锯齿，从而使边缘淡化为透明。这种方法称为顶点抗锯齿。如果您请求多采样 OpenGL 上下文，场景图将更喜欢基于多采样的抗锯齿 (MSAA)。

顶点抗锯齿可以在相邻图元的边之间产生接缝，即使这两条边在数学上相同。多重采样抗锯齿没有。

多重采样抗锯齿

多重采样抗锯齿是一项硬件功能，其中硬件计算基元中每个像素的覆盖值。一些硬件可以以非常低的成本进行多重采样，而其他硬件可能需要更多内存和更多 GPU 周期来渲染帧。

要启用多重采样抗锯齿，您应该设置 QSurfaceFormat样本大于 0 使用 QQuickWindow::setFormat()

QQuickView view;
QSurfaceFormat format = view.format();
format.setSamples(16);
view.setFormat(format);
view.show();

顶点抗锯齿

可以使用 Item::antialiasing 在每个项目的基础上启用和禁用顶点抗锯齿属性(property)。无论底层硬件支持什么，它都会工作并产生更高质量的抗锯齿，无论是对于正常渲染的图元还是捕获到帧缓冲区对象中的图元。

使用顶点抗锯齿的缺点是每个启用了抗锯齿的图元都必须混合。在batching方面，这意味着渲染器需要做更多的工作来确定基元是否可以批处理，并且由于与场景中的其他元素重叠，这也可能导致批处理减少，这可能会影响性能。

将顶点抗锯齿应用于自定义 QML 元素，源自 QQuickItem ，请按照以下步骤操作:

1) 创建自定义 Material 和OpenGL着色器程序。

smoothcolormaterial.h

#include <QSGMaterial>
#include <QSGMaterialShader>

//----------------------------------------------------------------------

class QSGSmoothColorMaterial : public QSGMaterial
{
public:
    QSGSmoothColorMaterial();
    int compare(const QSGMaterial *other) const;
protected:
    virtual QSGMaterialType *type() const;
    virtual QSGMaterialShader *createShader() const;
};

//----------------------------------------------------------------------

class QSGSmoothColorMaterialShader : public QSGMaterialShader
{
public:
    QSGSmoothColorMaterialShader();
    virtual void updateState(const RenderState &state, QSGMaterial *newEffect, QSGMaterial *oldEffect);
    virtual char const *const *attributeNames() const;
private:
    void initialize();
    int m_matrixLoc;
    int m_opacityLoc;
    int m_pixelSizeLoc;
};

smoothcolormaterial.cpp

QSGSmoothColorMaterial::QSGSmoothColorMaterial()
{
    setFlag(RequiresFullMatrixExceptTranslate, true);
    setFlag(Blending, true);
}

int QSGSmoothColorMaterial::compare(const QSGMaterial *other) const
{
    Q_UNUSED(other)
    return 0;
}

QSGMaterialType *QSGSmoothColorMaterial::type() const
{
    static QSGMaterialType type;
    return &type;
}

QSGMaterialShader *QSGSmoothColorMaterial::createShader() const
{
    return new QSGSmoothColorMaterialShader();
}

//----------------------------------------------------------------------

QSGSmoothColorMaterialShader::QSGSmoothColorMaterialShader()
    : QSGMaterialShader()
{
    setShaderSourceFile(QOpenGLShader::Vertex, QStringLiteral(":/shaders/smoothcolor.vert"));
    setShaderSourceFile(QOpenGLShader::Fragment, QStringLiteral(":/shaders/smoothcolor.frag"));
}

void QSGSmoothColorMaterialShader::updateState(const QSGMaterialShader::RenderState &state, QSGMaterial *newEffect, QSGMaterial *oldEffect)
{
    Q_UNUSED(newEffect)

    if (state.isOpacityDirty())
        program()->setUniformValue(m_opacityLoc, state.opacity());

    if (state.isMatrixDirty())
        program()->setUniformValue(m_matrixLoc, state.combinedMatrix());

    if (oldEffect == 0) {
        // The viewport is constant, so set the pixel size uniform only once.
        QRect r = state.viewportRect();
        program()->setUniformValue(m_pixelSizeLoc, 2.0f / r.width(), 2.0f / r.height());
    }
}

const char * const *QSGSmoothColorMaterialShader::attributeNames() const
{
    static char const *const attributes[] = {
        "vertex",
        "vertexColor",
        "vertexOffset",
        0
    };
    return attributes;
}

void QSGSmoothColorMaterialShader::initialize()
{
    m_matrixLoc = program()->uniformLocation("matrix");
    m_opacityLoc = program()->uniformLocation("opacity");
    m_pixelSizeLoc = program()->uniformLocation("pixelSize");
}

片段着色器

varying lowp vec4 color;

void main()
{
    gl_FragColor = color;
}

顶点着色器

uniform highp vec2 pixelSize;
uniform highp mat4 matrix;
uniform lowp float opacity;

attribute highp vec4 vertex;
attribute lowp vec4 vertexColor;
attribute highp vec2 vertexOffset;

varying lowp vec4 color;

void main()
{
    highp vec4 pos = matrix * vertex;
    gl_Position = pos;

    if (vertexOffset.x != 0.) {
        highp vec4 delta = matrix[0] * vertexOffset.x;
        highp vec2 dir = delta.xy * pos.w - pos.xy * delta.w;
        highp vec2 ndir = .5 * pixelSize * normalize(dir / pixelSize);
        dir -= ndir * delta.w * pos.w;
        highp float numerator = dot(dir, ndir * pos.w * pos.w);
        highp float scale = 0.0;
        if (numerator < 0.0)
            scale = 1.0;
        else
            scale = min(1.0, numerator / dot(dir, dir));
        gl_Position += scale * delta;
    }

    if (vertexOffset.y != 0.) {
        highp vec4 delta = matrix[1] * vertexOffset.y;
        highp vec2 dir = delta.xy * pos.w - pos.xy * delta.w;
        highp vec2 ndir = .5 * pixelSize * normalize(dir / pixelSize);
        dir -= ndir * delta.w * pos.w;
        highp float numerator = dot(dir, ndir * pos.w * pos.w);
        highp float scale = 0.0;
        if (numerator < 0.0)
            scale = 1.0;
        else
            scale = min(1.0, numerator / dot(dir, dir));
        gl_Position += scale * delta;
    }

    color = vertexColor * opacity;
}

2) 创建自定义 AttributeSet为 QSGGeometry .

myquickitem.cpp

namespace
{
    struct Color4ub
    {
        unsigned char r, g, b, a;
    };

    inline Color4ub colorToColor4ub(const QColor &c)
    {
        Color4ub color = { uchar(c.redF() * c.alphaF() * 255),
                           uchar(c.greenF() * c.alphaF() * 255),
                           uchar(c.blueF() * c.alphaF() * 255),
                           uchar(c.alphaF() * 255)
                         };
        return color;
    }

    struct SmoothVertex
    {
        float x, y;
        Color4ub color;
        float dx, dy;
        void set(float nx, float ny, Color4ub ncolor, float ndx, float ndy)
        {
            x = nx; y = ny; color = ncolor;
            dx = ndx; dy = ndy;
        }
    };

    const QSGGeometry::AttributeSet &smoothAttributeSet()
    {
        static QSGGeometry::Attribute data[] = {
            QSGGeometry::Attribute::create(0, 2, GL_FLOAT, true),
            QSGGeometry::Attribute::create(1, 4, GL_UNSIGNED_BYTE, false),
            QSGGeometry::Attribute::create(2, 2, GL_FLOAT, false)
        };
        static QSGGeometry::AttributeSet attrs = { 3, sizeof(SmoothVertex), data };
        return attrs;
    }
}

3) 将自定义 Material 和自定义几何体应用于 QSGGeometryNode .

myquickitem.cpp

QSGNode *MyQuickItem::updatePaintNode(QSGNode *oldNode, QQuickItem::UpdatePaintNodeData *data)
{
     QSGGeometryNode *node = 0;

     QSGGeometry *geometry;
     QSGSmoothColorMaterial *material;
     node = static_cast<QSGGeometryNode *>(oldNode);
     if(!node) {
         node = new QSGGeometryNode;
         geometry = new QSGGeometry(smoothAttributeSet(), 0);
         geometry->setDrawingMode(GL_TRIANGLE_STRIP);
         material = new QSGSmoothColorMaterial();
         node->setGeometry(geometry);
         node->setFlag(QSGNode::OwnsGeometry);
         node->setMaterial(material);
         node->setFlag(QSGNode::OwnsMaterial);
     } else {
         geometry = node->geometry();
         material = static_cast<QSGSmoothColorMaterial *>(node->material());
     }

4) 获取指向顶点数据的指针。

 int vertexStride = geometry->sizeOfVertex();
 int vertexCount = 8;

 geometry->allocate(vertexCount, 0);
 SmoothVertex *smoothVertices = reinterpret_cast<SmoothVertex *>(geometry->vertexData());
 memset(smoothVertices, 0, vertexCount * vertexStride);

5) 设置顶点数据。

你需要4分。

 float lineWidth = 4;
 float tlX = 0;   float tlY = 0;               //top-left
 float blX = 0;   float blY = 0 + lineWidth;   //bottom-left
 float trX = 500; float trY = 100;             //top-right
 float brX = 500; float brY = 100 + lineWidth; //bottom-right
 float delta = lineWidth * 0.5f;

 Color4ub fillColor = colorToColor4ub(QColor(255,0,0,255));
 Color4ub transparent = { 0, 0, 0, 0 };

要绘制抗锯齿线，您应该设置 8 个顶点以绘制 6 个三角形(2 个用于线条，4 个用于抗锯齿)。顶点 0 和 2、1 和 3、4 和 6、5 和 7 具有相同的坐标，但是不同的颜色和相反的顶点偏移。

 smoothVertices[0].set(trX, trY, transparent, delta, -delta);
 smoothVertices[1].set(tlX, tlY, transparent, -delta, -delta);

 smoothVertices[2].set(trX, trY, fillColor, -delta, delta);
 smoothVertices[3].set(tlX, tlY, fillColor, delta, delta);
 smoothVertices[4].set(brX, brY, fillColor, -delta, -delta);
 smoothVertices[5].set(blX, blY, fillColor, delta, -delta);

 smoothVertices[6].set(brX, brY, transparent, delta, delta);
 smoothVertices[7].set(blX, blY, transparent, -delta, delta);


 node->markDirty(QSGNode::DirtyGeometry);

 return node;
 }