qimage 与cv::mat 转化--> QPixmap

最近qt项目里有段图像处理与显示的程序设计：

1. opencv对于摄像头读取与处理比较方便，所以采用opencv前段采集图像与进行图像处理。

2. qt界面只负责显示图片：这一块主要依赖的是label标签来显示，需要的文件是格式是QPixmap。

void MainWindow::imgShow()
{
    //qimg.load("/home/yhzhao/Desktop/12.png");
    ui->labelImage->setPixmap(QPixmap::fromImage(qimg));
    qDebug()<<"Tigger";

}

3. cv::Mat --> QImage --> QPixmap 图片的转换。

4. 定时事件读取摄像头：固定频率读取摄像头图像

5.主要注意点是：cv::Mat --> QImage ：这两者内存管理机制不一样

QImage Widget::Mat2QImage(cv::Mat const& src)
{
     cv::Mat temp; // make the same cv::Mat
     cvtColor(src, temp,CV_BGR2RGB); // cvtColor Makes a copt, that what i need
     QImage dest((const uchar *) temp.data, temp.cols, temp.rows, temp.step, QImage::Format_RGB888);
     dest.bits(); // enforce deep copy, see documentation
     // of QImage::QImage ( const uchar * data, int width, int height, Format format )
     return dest;
}

cv::Mat Widget::QImage2Mat(QImage const& src)
{
     cv::Mat tmp(src.height(),src.width(),CV_8UC3,(uchar*)src.bits(),src.bytesPerLine());
     cv::Mat result; // deep copy just in case (my lack of knowledge with open cv)
     cvtColor(tmp, result,CV_BGR2RGB);
     return result;
}

namedWindow("edges",0);
        qimg = Mat2QImage(pCam_qr_estimation->frame);
        img_test = QImage2Mat(qimg);
        imshow("edges", img_test );

这两者内存管理机制不一样。

Memory management.

cv::Mat

doesn't work like

QImage

in this mater. Remember that

QImage

is using copy on write mechanism and shares memory for each copy.

cv::Mat

also shares memory but it doesn't do copy
on write (I'm also new with open cv (2 weeks) so I can't explain yet exactly how it works but I've stumbled on some crushes because of that)!

Another thing is that when you are creating

QImage

from memory image is using this memory and doesn't take ownership of it.

Final outcome is that on Linux and Qt5 your code is crashes because of problems with memory management. On your screen shot you can see at the top of second window that something strange is going on and you see some memory trash.

QImage的构造函数进行转换，主要是使用cv::Mat的data来构造一个QImage类型，这样做确实可以达到转换目的，但是，因此这样构造出来的QImage本身并不保存data，因此，在QImage的生存周期内，必须保证cv::Mat中的数据不会被释放。上面的这个问题也是比较容易解决的，主要是通过调用QImage::bits函数来强制QImage进行深层次复制，使得QImage自己保存一份data的副本，这样就可以保证在cv::Mat中的数据被释放的时候，QImage还能正常使用。

参考：

http://stackoverflow.com/questions/17127762/cvmat-to-qimage-and-back
http://blog.csdn.net/loveaborn/article/details/7680834
OpenCV学习(2)--cv::Mat转化为QImage