Using OpenCV with Qt

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Compatibility

Platform(s): Maemo

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Article

Keywords: Qt, Graphics, OpenCV, Maemo

Created: gnuton (18 Jun 2010)

Last edited: hamishwillee (11 Oct 2012)

Introduction

In this article we will see how to use OpenCV in Qt.

OpenCV (Open Source Computer Vision Library) is a library of programming functions mainly aimed at real time computer vision, developed by Intel and now supported by Willow Garage. It is free for use under the open source BSD license. The library is cross-platform. It focuses mainly on real-time image processing. If the library finds Intel's Integrated Performance Primitives on the system, it will use these proprietary optimized routines to accelerate itself.

The IplImage and Qt

In order to take advantage of the 500 algorithms offered by OpenCV, Qt applications have to manage iplImages. Therefore, converting QImage to/from iplImage is very important.

Converting QImage to iplImage

This snippet will convert QImages into iplImage with depth 8 and 3 channels. QImage can store data in several formats. This code accepts just 24-bit QImage::Format_RGB888 and the QImage::Format_RGB32. Apha values in the 32-bit format will be removed during the conversion.

  static IplImage* qImage2IplImage(const QImage& qImage)
  {
    int width = qImage.width();
    int height = qImage.height();
 
    // Creates a iplImage with 3 channels
    IplImage *img = cvCreateImage(cvSize(width, height), IPL_DEPTH_8U, 3);
    char * imgBuffer = img->imageData;
 
    //Remove alpha channel
    int jump = (qImage.hasAlphaChannel()) ? 4 : 3;
 
    for (int y=0;y<img->height;y++){
      QByteArray a((const char*)qImage.scanLine(y), qImage.bytesPerLine());
      for (int i=0; i<a.size(); i+=jump){
          //Swap from RGB to BGR
          imgBuffer[2] = a[i];
          imgBuffer[1] = a[i+1];
          imgBuffer[0] = a[i+2];
          imgBuffer+=3;
      }
  }
 
  return img;
  }

Converting iplImage to QImage

This snippet will convert a iplImage with depth 8 and 1 or 3 channels into a 8/24-bit QImage.

Note: This code won't work for images with different depth and number of channels.

static QImage IplImage2QImage(const IplImage *iplImage)
{
    int height = iplImage->height;
    int width = iplImage->width;
 
    if  (iplImage->depth == IPL_DEPTH_8U && iplImage->nChannels == 3)
    {
      const uchar *qImageBuffer = (const uchar*)iplImage->imageData;
      QImage img(qImageBuffer, width, height, QImage::Format_RGB888);
      return img.rgbSwapped();
    } else if  (iplImage->depth == IPL_DEPTH_8U && iplImage->nChannels == 1){
	const uchar *qImageBuffer = (const uchar*)iplImage->imageData;
	QImage img(qImageBuffer, width, height, QImage::Format_Indexed8);
 
	QVector<QRgb> colorTable;
	for (int i = 0; i < 256; i++){
	    colorTable.push_back(qRgb(i, i, i));
	}
	img.setColorTable(colorTable);
	return img;
    }else{
      qWarning() << "Image cannot be converted.";
      return QImage();
    }
}

Useful Links

 http://opencv.willowgarage.com/wiki/ 
 http://www.cs.iit.edu/~agam/cs512/lect-notes/opencv-intro/opencv-intro.html 
 http://doc.qt.nokia.com/4.6/qimage.html