使用原始 OpenGL 调用绘制 Qml 项目的问题

Question

我想使用原始 OpenGL 调用在 QtQuick 场景中绘制单个项目。我决定采用this question 中建议的方法。

我创建了一个源自QQuickFramebufferObject 的 Qt Quick 项目，并将其作为 Renderer 暴露给 QML：（代码基于 Qt 示例：Scene Graph - Rendering FBOs）

class FboInSGRenderer : public QQuickFramebufferObject {
    Q_OBJECT
public:
    Renderer *createRenderer() const;
};

源文件：

class LogoInFboRenderer : public QQuickFramebufferObject::Renderer {
    public:
        LogoInFboRenderer() { }

        void render() {
            int width = 1, height = 1;
            glEnable(GL_BLEND);
            glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
            glColor4f(0.0, 1.0, 0.0, 0.8);
            glBegin(GL_QUADS);
            glVertex2f(0, 0);
            glVertex2f(width, 0);
            glVertex2f(width, height);
            glVertex2f(0, height);
            glEnd();

            glLineWidth(2.5);
            glColor4f(0.0, 0.0, 0.0, 1.0);
            glBegin(GL_LINES);
            glVertex2f(0, 0);
            glVertex2f(width, height);
            glVertex2f(width, 0);
            glVertex2f(0, height);
            glEnd();

            update();
        }

        QOpenGLFramebufferObject *createFramebufferObject(const QSize &size) {
            QOpenGLFramebufferObjectFormat format;
            format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
            format.setSamples(4);
            return new QOpenGLFramebufferObject(size, format);
        }
};

QQuickFramebufferObject::Renderer *FboInSGRenderer::createRenderer() const {
    return new LogoInFboRenderer();
}

在 Qml 中我按如下方式使用它：

import QtQuick 2.4
import SceneGraphRendering 1.0

Rectangle {
    width: 400
    height: 400
    color: "purple"
    Renderer {
        id: renderer
        anchors.fill: parent
    }
}

我期待看到渲染的“X”会填满整个场景，但结果却如下所示：

其他实验似乎证实了绘制的形状总是其大小（宽度/高度）除以 2。

我还检查了createFramebufferObject 中的size 参数值是否正确。

查看文档让我找到了QQuickFramebufferObject 类中的属性textureFollowsItemSize，但默认设置为true。

我做错了什么还是应该将此行为视为 Qt 错误？

Answer 1

绘制的矩形是您预期大小的一半，因为默认坐标范围是 [-1, 1]，而不是您的代码假设的 [0, 1]。如果要使用 [0, 1] 比例，则应适当设置投影矩阵：

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);

Answer 2

正如 Qt 文档所说："Warning: It is crucial that OpenGL operations and interaction with the scene graph happens exclusively on the rendering thread, primarily during the updatePaintNode() call. The best rule of thumb is to only use classes with the "QSG" prefix inside the QQuickItem::updatePaintNode() function." 我是这样做的：

*.h

class MyQuickItem : public QQuickItem
{
    Q_OBJECT
public:
    MyQuickItem();
    ~MyQuickItem();

protected:
    QSGNode *updatePaintNode(QSGNode * oldNode, UpdatePaintNodeData * updatePaintNodeData);
    QSGNode *addNode(QSGGeometry *geometry, const QColor &color);
};

*.cpp

MyQuickItem::MyQuickItem()
{
    setFlag(QQuickItem::ItemHasContents,true);
}

MyQuickItem::~MyQuickItem()
{

}

QSGNode *MyQuickItem::updatePaintNode(QSGNode *oldNode, QQuickItem::UpdatePaintNodeData *updatePaintNodeData)
{
    Q_UNUSED(updatePaintNodeData)

    QSGTransformNode *root = static_cast<QSGTransformNode *>(oldNode);
    if(!root) root = new QSGTransformNode;
    QSGNode *node;
    QSGGeometry *geometry;

    QSGSimpleRectNode *rect = new QSGSimpleRectNode();
    rect->setColor(Qt::green);
    rect->setRect(boundingRect());
    root->appendChildNode(rect);

    geometry = new QSGGeometry(QSGGeometry::defaultAttributes_Point2D(), 2);
    geometry->setDrawingMode(GL_LINES);
    geometry->setLineWidth(5.0);
    geometry->vertexDataAsPoint2D()[0].set(x(), y());
    geometry->vertexDataAsPoint2D()[1].set(width(), height());
    node = addNode(geometry,Qt::blue);
    root->appendChildNode(node);

    geometry = new QSGGeometry(QSGGeometry::defaultAttributes_Point2D(), 2);
    geometry->setDrawingMode(GL_LINES);
    geometry->setLineWidth(5.0);
    geometry->vertexDataAsPoint2D()[0].set(width(), y());
    geometry->vertexDataAsPoint2D()[1].set(x(), height());
    node = addNode(geometry,Qt::blue);
    root->appendChildNode(node);

    return root;
}

QSGNode *MyQuickItem::addNode(QSGGeometry *geometry, const QColor &color)
{
    QSGFlatColorMaterial *material = new QSGFlatColorMaterial;
    material->setColor(color);
    QSGGeometryNode *node = new QSGGeometryNode;
    node->setGeometry(geometry);
    node->setFlag(QSGNode::OwnsGeometry);
    node->setMaterial(material);
    node->setFlag(QSGNode::OwnsMaterial);
    return node;
}

在 ma​​in.cpp

qmlRegisterType<MyQuickItem>("MyObjects", 1, 0, "MyObject");

及用法：

import QtQuick 2.3
import QtQuick.Window 2.2
import MyObjects 1.0    

Window {
    visible: true
    width: 360
    height: 360

    MyObject {
        anchors.fill: parent
    }
}