QPixmap 将图片转换成圆形，出现锯齿，使用dpi优化

网上一般的将QPixmap的图片转换成圆形图片使用的大概是这样的例子

static QPixmap PixmapToRound(const QPixmap& src, int diameter /*直径*/) {
    if (src.isNull()) {
        return QPixmap();
    }
    QSize size(diameter, diameter);
    QBitmap mask(size);
    QPainter painter(&mask);
    painter.setRenderHint(QPainter::Antialiasing);
    painter.setRenderHint(QPainter::SmoothPixmapTransform);
    painter.fillRect(0, 0, size.width(), size.height(), Qt::white);
    painter.setBrush(QColor(0, 0, 0));
    painter.drawRoundedRect(0, 0, size.width(), size.height(), 99, 99);
    QPixmap image = src.scaled(size);
    image.setMask(mask);
    return image;
}

        但是当屏幕的缩放比不为1时，这种方式会有明显的锯齿。如下所示：

因此需要考虑dpi,如下

static QPixmap PixmapToRound1(const QPixmap& pixmap, int diameter /*直径*/,qreal dpi) {

    QSize realSize = QSize(pixmap.width() * dpi, pixmap.height() * dpi);
    QPixmap resultPixmap(realSize);

    resultPixmap.fill(Qt::transparent);
    QPainter painter(&resultPixmap);
    painter.setRenderHints(QPainter::Antialiasing | QPainter::SmoothPixmapTransform);

    QPainterPath path;
    path.addEllipse(0, 0, realSize.width(), realSize.height());
    painter.setClipPath(path);



    QPoint drawLeftTop(0, 0);
    QSize drawSize = realSize;

    drawSize.setWidth(realSize.width());
    drawSize.setHeight(realSize.width() * pixmap.height() / pixmap.width());
    drawLeftTop.setX(0);
    drawLeftTop.setY((realSize.height() - drawSize.height()) / 2);


    // scaled draw
    painter.drawPixmap(drawLeftTop.x(), drawLeftTop.y(), drawSize.width(), drawSize.height(), pixmap.scaled(drawSize, Qt::IgnoreAspectRatio, Qt::SmoothTransformation));

    resultPixmap.setDevicePixelRatio(dpi);
    return resultPixmap;
}

 

实际效果如上，会发现锯齿少了很多。

示例：

 

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include 
#include 
#include 

static QPixmap PixmapToRound(const QPixmap& src, int diameter /*直径*/) {
    if (src.isNull()) {
        return QPixmap();
    }
    QSize size(diameter, diameter);
    QBitmap mask(size);
    QPainter painter(&mask);
    painter.setRenderHint(QPainter::Antialiasing);
    painter.setRenderHint(QPainter::SmoothPixmapTransform);
    painter.fillRect(0, 0, size.width(), size.height(), Qt::white);
    painter.setBrush(QColor(0, 0, 0));
    painter.drawRoundedRect(0, 0, size.width(), size.height(), 99, 99);
    QPixmap image = src.scaled(size);
    image.setMask(mask);
    return image;
}


static QPixmap PixmapToRound1(const QPixmap& pixmap, int diameter /*直径*/,qreal dpi) {

    QSize realSize = QSize(pixmap.width() * dpi, pixmap.height() * dpi);
    QPixmap resultPixmap(realSize);

    resultPixmap.fill(Qt::transparent);
    QPainter painter(&resultPixmap);
    painter.setRenderHints(QPainter::Antialiasing | QPainter::SmoothPixmapTransform);

    QPainterPath path;
    path.addEllipse(0, 0, realSize.width(), realSize.height());
    painter.setClipPath(path);



    QPoint drawLeftTop(0, 0);
    QSize drawSize = realSize;

    drawSize.setWidth(realSize.width());
    drawSize.setHeight(realSize.width() * pixmap.height() / pixmap.width());
    drawLeftTop.setX(0);
    drawLeftTop.setY((realSize.height() - drawSize.height()) / 2);


    // scaled draw
    painter.drawPixmap(drawLeftTop.x(), drawLeftTop.y(), drawSize.width(), drawSize.height(), pixmap.scaled(drawSize, Qt::IgnoreAspectRatio, Qt::SmoothTransformation));

    resultPixmap.setDevicePixelRatio(dpi);
    return resultPixmap;
}



MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QPixmap pixmap(":/new/prefix1/message.png");

    ui->label_1->setPixmap(pixmap);
    ui->label_1->setScaledContents(true);

    QPixmap pixmap1 = PixmapToRound(pixmap, pixmap.width());
    ui->label_2->setPixmap(pixmap1);
    ui->label_2->setScaledContents(true);

    QPixmap pixmap2 = PixmapToRound1(pixmap, pixmap.width(),devicePixelRatioF());
    ui->label_3->setPixmap(pixmap2);
    ui->label_3->setScaledContents(true);
}

MainWindow::~MainWindow()
{
    delete ui;
}

代码资源路径：pixmapTocircle.7z-图像处理文档类资源-CSDN下载