在 Qt 中通过 Windows GDI 绘图

Question

我正在尝试在 QGraphicsView paintEvent 中使用 Windows GDI，但注意到一些绘图问题，例如当我调整 window 大小时或最小化和最大化时绘图消失或闪烁。当我使用 Qt 而不是 GDI 时，它工作得很好。这是代码：

[更新代码]

#include "CustomView.h"

#include <QPainter>
#include <QPaintEngine>
#include <windows.h>
#include <objidl.h>
#include <gdiplus.h>
using namespace Gdiplus;
#pragma comment (lib,"Gdiplus.lib")

CustomView::CustomView(QWidget *parent)
    : QGraphicsView(parent)
{
    setAutoFillBackground(true);
    setViewportUpdateMode(QGraphicsView::FullViewportUpdate);
    setAttribute(Qt::WA_NativeWindow, true);
}

CustomView::~CustomView()
{

}

void CustomView::paintEvent(QPaintEvent * event)
{
    QPainter painter(viewport());
    painter.beginNativePainting();
    // Drawing by Windows GDI
    HWND hwnd = (HWND)viewport()->winId();
    HDC hdc = GetDC(hwnd);

    QString text("Test GDI Paint");
    RECT rect;
    GetClientRect(hwnd, &rect);

    HBRUSH hbrRed = CreateSolidBrush(RGB(0, 255, 0));
    FillRect(hdc, &rect, hbrRed);

    Ellipse(hdc, 50, 50, rect.right - 100, rect.bottom - 100);
    SetTextAlign(hdc, TA_CENTER | TA_BASELINE);
    TextOutW(hdc, width() / 2, height() / 2, (LPCWSTR)text.utf16(), text.size());
    ReleaseDC(hwnd, hdc);
    painter.endNativePainting();

    QGraphicsView::paintEvent(event);

    // Drawing the same by Qt
    //  QPainter painter(viewport());
    //  painter.save() ;
    //  QBrush GreenBrush(Qt::green);
    //  QBrush WhiteBrush(Qt::white);
    //  QRect ViewportRect = viewport()->rect();
    //  painter.fillRect(ViewportRect, GreenBrush);
    //  painter.drawEllipse(50, 50, ViewportRect.right() - 100, ViewportRect.bottom() - 100);
    //  QPainterPath EllipsePath;
    //  EllipsePath.addEllipse(50, 50, ViewportRect.right() - 100, ViewportRect.bottom() - 100);
    //  painter.fillPath(EllipsePath, WhiteBrush);
    //  painter.drawText(ViewportRect.width() / 2, ViewportRect.height() / 2, "Test Qt Paint");
    //  painter.restore();
}

这是整个项目（Visual Studio 2010 + Qt 5.4.1）[更新存档] https://dl.dropboxusercontent.com/u/105132532/Whosebug/Qt5TestApplication.7z

有什么想法吗？

解决方案（根据Kuba Ober的回答编辑了代码）

#include "CustomView.h"

#include <QPainter>
#include <QPaintEngine>
#include <windows.h>
#include <objidl.h>
#include <gdiplus.h>

using namespace Gdiplus;
#pragma comment (lib,"Gdiplus.lib")


CustomView::CustomView(QWidget *parent)
    : QGraphicsView(parent)
{
    // This brings the original paint engine alive.
    QGraphicsView::paintEngine();
    setAttribute(Qt::WA_NativeWindow);
    setAttribute(Qt::WA_PaintOnScreen);
    setRenderHint(QPainter::Antialiasing);
}

CustomView::~CustomView()
{

}

QPaintEngine* CustomView::paintEngine() const
{
    return NULL;
}

bool CustomView::event(QEvent * event) {
    if (event->type() == QEvent::Paint)
    {
        bool result = QGraphicsView::event(event);
        drawGDI();
        return result;
    }
    else if (event->type() == QEvent::UpdateRequest)
    {
        bool result = QGraphicsView::event(event);
        drawGDI();
        return result;
    }
    return QGraphicsView::event(event);
}

void CustomView::drawGDI()
{
    // Drawing by Windows GDI
    HWND hwnd = (HWND)viewport()->winId();
    HDC hdc = GetDC(hwnd);

    QString text("Test GDI Paint");
    RECT rect;
    GetClientRect(hwnd, &rect);

    HBRUSH hbrRed = CreateSolidBrush(RGB(0, 255, 0));
    FillRect(hdc, &rect, hbrRed);

    Ellipse(hdc, 50, 50, rect.right - 100, rect.bottom - 100);
    SetTextAlign(hdc, TA_CENTER | TA_BASELINE);
    TextOutW(hdc, width() / 2, height() / 2, (LPCWSTR)text.utf16(), text.size());
    ReleaseDC(hwnd, hdc);
}

Answer 1

打算通过 GDI 仅绘画的 QWidget 必须：

重新实现 paintEngine 到 return nullptr.
设置 WA_PaintOnScreen 属性。
可选择设置 WA_NativeWindow 属性。这只会加快小部件的第一次重绘。
重新实现 QObject::event 并捕获 Paint 和 UpdateRequest 事件。这些事件应该导致调用执行 GDI 绘制的方法。事件不得转发到基地class。

此外，通过 GDI 在通过 paintEvent/QPainter 绘制的 内容之上绘制的 QWidget 必须另外:

在构造函数中调用基class的paintEngine()方法一次。这将为小部件实例化本机绘制引擎。
在 QObject::event 实现中，必须在执行 GDI 之前 调用基础 class 的 event绘画。这将使用光栅绘制引擎绘制内容，并且 return 控制你在它上面透支一些其他内容。

下面的示例显示了如何在 Qt 的绘制系统完成的绘图之上进行叠加绘制。当然，由于绘制是在Qt已经绘制好的top内容上进行的，所以会出现闪烁。

在 Qt 的后备存储之上进行 GDI 绘制，避免闪烁，也是可能的，但需要一种稍微不同的方法。

#include <QApplication> #include <QGraphicsObject> #include <QPropertyAnimation> #include <QGraphicsView> #include <QPainter> #include <QPaintEvent> #include <windows.h> class View : public QGraphicsView { public: View(QWidget *parent = 0) : QGraphicsView(parent) { // This brings the original paint engine alive. QGraphicsView::paintEngine(); //setViewportUpdateMode(QGraphicsView::FullViewportUpdate); setAttribute(Qt::WA_NativeWindow); setAttribute(Qt::WA_PaintOnScreen); setRenderHint(QPainter::Antialiasing); } QPaintEngine * paintEngine() const Q_DECL_OVERRIDE { return 0; } bool event(QEvent * event) Q_DECL_OVERRIDE { if (event->type() == QEvent::Paint) { bool result = QGraphicsView::event(event); paintOverlay(); return result; } if (event->type() == QEvent::UpdateRequest) { bool result = QGraphicsView::event(event); paintOverlay(); return result; } return QGraphicsView::event(event); } void resizeEvent(QResizeEvent *) { fitInView(-2, -2, 4, 4, Qt::KeepAspectRatio); } virtual void paintOverlay(); }; void View::paintOverlay() { // We're called after the native painter has done its thing HWND hwnd = (HWND)viewport()->winId(); HDC hdc = GetDC(hwnd); HBRUSH hbrGreen = CreateHatchBrush(HS_BDIAGONAL, RGB(0, 255, 0)); RECT rect; GetClientRect(hwnd, &rect); SetBkMode(hdc, TRANSPARENT); SelectObject(hdc, hbrGreen); Rectangle(hdc, 0, 0, rect.right, rect.bottom); SelectObject(hdc, GetStockObject(NULL_BRUSH)); Ellipse(hdc, 50, 50, rect.right - 100, rect.bottom - 100); QString text("Test GDI Paint"); SetTextAlign(hdc, TA_CENTER | TA_BASELINE); TextOutW(hdc, width() / 2, height() / 2, (LPCWSTR)text.utf16(), text.size()); DeleteObject(hbrGreen); ReleaseDC(hwnd, hdc); } class EmptyGraphicsObject : public QGraphicsObject { public: EmptyGraphicsObject() {} QRectF boundingRect() const { return QRectF(0, 0, 0, 0); } void paint(QPainter *, const QStyleOptionGraphicsItem *, QWidget *) {} }; void setupScene(QGraphicsScene &s) { QGraphicsObject * obj = new EmptyGraphicsObject; QGraphicsRectItem * rect = new QGraphicsRectItem(-1, 0.3, 2, 0.3, obj); QPropertyAnimation * anim = new QPropertyAnimation(obj, "rotation", &s); s.addItem(obj); rect->setPen(QPen(Qt::darkBlue, 0.1)); anim->setDuration(2000); anim->setStartValue(0); anim->setEndValue(360); anim->setEasingCurve(QEasingCurve::InBounce); anim->setLoopCount(-1); anim->start(); } int main(int argc, char *argv[]) { QApplication a(argc, argv); QGraphicsScene s; setupScene(s); View view; view.setScene(&s); view.show(); return a.exec(); }

在 Qt 中通过 Windows GDI 绘图

Drawing by Windows GDI inside of Qt

qt

drawing

gdi

qgraphicsview