关键代码如下：

std::vector<uint8_t> buffer;
cv::imencode(".png", mat, buffer);
QByteArray byteArray = QByteArray::fromRawData((const char*)buffer.data(), buffer.size());
QString base64Image(byteArray.toBase64());
qDebug() << base64Image;

打印如下：

"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"

这里我们找个在线解析的来试试：

这里要注意HTML中img标签使用的规则：

Base64 在HTML中的使用

<img width="40" height="30" src="data:image/jpg;base64,/9j/4QMZRXhpZgAASUkqAAgAAAAL...." />

此时图片正常显示了。