[问题记录]Qt使用QPainter在QImage、QBitmap、QPixmap上面绘图时出现杂色_在qimage上绘图

目录

1. 问题现象

2. 问题原因

3. 解决方案

1. 问题现象

使用QPainter，在QImage上绘图，能明显看到顶部有杂色，在QBitmap上则更明显，唯一在QPicture上绘图没出现该问题。

代码

void Widget::paintImage()
{
    QPainter painter;
    QImage qImage(100, 100, QImage::Format_ARGB32);
    painter.begin(&qImage);
    painter.setBrush(Qt::yellow);
    painter.setPen(QPen(Qt::green, 1));
    painter.drawRect(10, 10, 60, 60);
    painter.drawText(10, 10, 60, 60, Qt::AlignCenter, tr("QImage"));
    painter.setBrush(QColor(0, 0, 0, 100));
    painter.drawRect(50, 50, 40, 40);
    painter.end();
 
    QPixmap qPixmap(100, 100);
    painter.begin(&qPixmap);
    painter.setPen(QPen(Qt::green, 1));
    painter.setBrush(Qt::yellow);
    painter.drawRect(10, 10, 60, 60);
    painter.drawText(10, 10, 60, 60, Qt::AlignCenter, tr("QQPixmapImage"));
    painter.setBrush(QColor(0, 0, 0, 100));
    painter.drawRect(50, 50, 40, 40);
    painter.end();
 
    QBitmap qBitmap(100, 100);
    painter.begin(&qBitmap);
    painter.setPen(QPen(Qt::green, 1));
    painter.setBrush(Qt::yellow);
    painter.drawRect(10, 10, 60, 60);
    painter.drawText(10, 10, 60, 60, Qt::AlignCenter, tr("QBitmap"));
    painter.setBrush(QColor(0, 0, 0, 100));
    painter.drawRect(50, 50, 40, 40);
    painter.end();
 
    QPicture qPicture;
    painter.begin(&qPicture);
    painter.setPen(QPen(Qt::green, 1));
    painter.setBrush(Qt::yellow);
    painter.drawRect(10, 10, 60, 60);
    painter.drawText(10, 10, 60, 60, Qt::AlignCenter, tr("QPicture"));
    painter.setBrush(QColor(0, 0, 0, 100));
    painter.drawRect(50, 50, 40, 40);
    painter.end();
 
    painter.begin(this);
    painter.drawImage(50, 20, qImage);
    painter.drawPixmap(200, 20, qPixmap);
    painter.drawPixmap(50, 170, qBitmap);
    painter.drawPicture(200, 170, qPicture);
}

2. 问题原因

只能是推测，未证实

QImage、QPixmap、QBitmap主要作用还是文件，文件初始化时，如果没有填充背景，那初始的像素就是不确定的，即显示的杂色。

也可能是和电脑显示驱动关联，使用到显示驱动初始化的图片，驱动不兼容等导致的杂色。

3. 解决方案

在创建相关对象后，先使用fill填充背景，再绘图

void Widget::paintImage()
{
    QPainter painter;
    QImage qImage(100, 100, QImage::Format_ARGB32);
    qImage.fill(QColor(0, 0, 0, 0));
...
 
    QPixmap qPixmap(100, 100);
    qPixmap.fill();
...
 
    QBitmap qBitmap(100, 100);
    qBitmap.fill();
...
}

效果如下