功能

1. 初始加载图片，对任意大小图片，显示固定尺寸的缩略图
2. 按住ctrl +滚轮：缩放
3. 按住鼠标拖动图片
4. 双击还原图片大小
5. 实时显示鼠标所指像素的rgb

头文件

#pragma once
#include <qlabel.h>
#include<QKeyEvent>
class PicLabel :
	public QLabel
{
	Q_OBJECT
public:
	PicLabel(QWidget *parent = Q_NULLPTR);
	~PicLabel();

	void SetPicPath(QString strPath);//设置图片路径
	void SetPic(QImage Image); //设置图片
	void setPenWidth(int w) { w = m_intPenWidth; };
	void setZoom(bool bZoom) { m_bZoom = bZoom; }//设置是否允许滚动缩放
protected:
	void paintEvent(QPaintEvent *event);                        //QPaint画图
	void wheelEvent(QWheelEvent *event) override;               //鼠标滚轮滚动
	void mouseDoubleClickEvent(QMouseEvent *event) override;    //鼠标双击还原图片大小事件
	void mousePressEvent(QMouseEvent *event) override;          //鼠标摁下
	void mouseMoveEvent(QMouseEvent *event) override;           //鼠标移动
	void mouseReleaseEvent(QMouseEvent *event) override;        //鼠标放开
	//键盘事件（按住ctrl 并滚动滚轮才可以放大或者缩小图片）
	void keyPressEvent(QKeyEvent *event);
	void  keyReleaseEvent(QKeyEvent *event);

signals:
	void SigColor(int, int, int);

private slots:
	void OnZoomInImage();       //图片放大
	void OnZoomOutImage();      //图片缩小
	void OnPresetImage();       //图片还原

private:
	QImage m_Image;           //原始图片
	QPixmap m_bTempPixmap; //当前画图展示的缩放移动后的图片

	QString m_strImagePath="";  //图片全路径名

	qreal ZoomValue = 1.0;  //鼠标缩放值
	int XPtInterval = 0;    //平移X轴的值
	int YPtInterval = 0;    //平移Y轴的值
	bool Pressed = false;   //鼠标是否被摁压
	bool m_bZoom = true; //是否按住滚动缩放
	QPoint OldPos;          //旧的鼠标位置
	QPoint m_nMovePos;
	int m_intPenWidth = 1;
 
};

cpp

 #include "PicLabel.h"
#include<QFile>
#include<QPainter>


PicLabel::PicLabel(QWidget *parent  ) :QLabel(parent)
{
	//开启鼠标追踪：即使没有按下鼠标键，移动鼠标时也可以捕获鼠标的移动事件。
	setMouseTracking(true);

}
PicLabel::~PicLabel()
{

}

void PicLabel::SetPicPath(QString strPath)//设置图片路径
{
	m_Image = QImage(); //清空
	m_strImagePath = strPath;
	if (! m_Image.load(m_strImagePath))
	{
		//如果load失败可能是格式错误
		QFile file(strPath);
		if (file.open(QIODevice::ReadOnly))
		{
			m_Image.loadFromData(file.readAll());
		}
	}
	update();
}

void PicLabel::SetPic(QImage Image)
{
	m_Image = Image;
}


void PicLabel::paintEvent(QPaintEvent *event)
{
	// 设置一个画家painter,  在空白的tmpPixMap上进行绘制的
	QPainter painter(this);
	QPixmap tmpPixMap(this->width(), this->height());
	tmpPixMap.fill(Qt::transparent);
	painter.begin(&tmpPixMap);

	// 根据窗口计算应该显示的图片的大小
	int width = qMin(m_Image.width(), this->width());
	int height = width * 1.0 / (m_Image.width() * 1.0 / m_Image.height());
	height = qMin(height, this->height());
	width = height * 1.0 * (m_Image.width() * 1.0 / m_Image.height());

	// 平移
	painter.translate(this->width() / 2 + XPtInterval, this->height() / 2 + YPtInterval);
	// 缩放
	painter.scale(ZoomValue, ZoomValue);

	// 绘制图像
	QRect picRect(-width / 2, -height / 2, width, height);
	painter.drawImage(picRect, m_Image);
	painter.end();

	m_bTempPixmap = tmpPixMap;
}

//键盘事件（按住ctrl 并滚动滚轮才可以放大或者缩小图片）
void PicLabel::keyPressEvent(QKeyEvent *event)
{
	if (event->key() == Qt::Key_Control)
	{
		setZoom(true);
	}
}
void  PicLabel::keyReleaseEvent(QKeyEvent *event)
{
	if (event->key() == Qt::Key_Control)
	{
		setZoom(false);
	}
}

//图片放大
void PicLabel::OnZoomInImage()
{
	ZoomValue += 0.05;
	update();
}

//图片缩小
void PicLabel::OnZoomOutImage()
{
	ZoomValue -= 0.05;
	if (ZoomValue <= 0)
	{
		ZoomValue = 0.05;
		return;
	}

	update();
}

//鼠标滚轮滚动
void PicLabel::wheelEvent(QWheelEvent *event)
{
	if (!m_bZoom)
		return;
	int value = event->delta();
	if (value > 0)  //放大
		OnZoomInImage();
	else            //缩小
		OnZoomOutImage();

	update();
}

//鼠标双击还原图片大小事件
void PicLabel::mouseDoubleClickEvent(QMouseEvent *event)
{
	OnPresetImage();
}

//图片还原
void PicLabel::OnPresetImage()
{
	ZoomValue = 1.0;
	XPtInterval = 0;
	YPtInterval = 0;
	update();
}

//鼠标摁下
void PicLabel::mousePressEvent(QMouseEvent *event)
{
	OldPos = event->pos();
	Pressed = true;
}

//鼠标松开
void PicLabel::mouseMoveEvent(QMouseEvent *event)
{
	if (!Pressed)//如果不是按下拖动，则显示实时坐标的rgb
	{
		m_nMovePos = event->pos();
		QRgb qRgb = m_bTempPixmap.toImage().pixel(m_nMovePos);
		emit SigColor(qRed(qRgb), qGreen(qRgb), qBlue(qRgb));
		return QWidget::mouseMoveEvent(event);
	}
	this->setCursor(Qt::SizeAllCursor);
	QPoint pos = event->pos();
	int xPtInterval = pos.x() - OldPos.x();
	int yPtInterval = pos.y() - OldPos.y();

	XPtInterval += xPtInterval;
	YPtInterval += yPtInterval;

	OldPos = pos;
	update();
}

 
void PicLabel::mouseReleaseEvent(QMouseEvent *event)
{
	Pressed = false;
	setCursor(Qt::ArrowCursor);
}