Display multiple image in same window with OpenCV

Question

I have three firewire cameras which can automatically synchronize via hardware and I'm trying to capture frames, save them into my hard disk and display them in a window.

Everything works fine, but at the moment, I'm only able to display frames in three different windows at the same time.

I'd like to display the frames in a single window but I do not know how to do this. At this link there is cvShowManyImages() function but you have to pass three frames at the same time but I have a for() loop which can consider one frame at a time.

This is the code which I'm using:

  for ( int j = 0; j < k_numImages; j++ )
    {
        // Display the timestamps for all cameras to show that the image
        // capture is synchronized for each image
        for ( unsigned int i = 0; i < numCameras; i++ )
        {
            Image image;
            error = ppCameras[i]->RetrieveBuffer( &image );
            if (error != PGRERROR_OK)
            {
                PrintError( error );
                return -1;
            }
            IplImage* destImage = ConvertImageToOpenCV(&image);
            char titolo[50];
            sprintf(titolo, "titolo%d", i);
            cvShowImage(titolo, destImage);
            waitKey(1);
        }
   }

It works well but creates a different windows for each camera while I want to display all cameras frames in the same window.

Can you help me, please?

EDIT: this is the ConvertImageToOpenCV() function.

IplImage* ConvertImageToOpenCV(Image* pImage)
{
    IplImage* cvImage = NULL;
    bool bColor = true;
    CvSize mySize;
    mySize.height = pImage->GetRows();
    mySize.width = pImage->GetCols();

    switch ( pImage->GetPixelFormat() )
    {
        case PIXEL_FORMAT_MONO8:     cvImage = cvCreateImageHeader(mySize, 8, 1 );
                                     cvImage->depth = IPL_DEPTH_8U;
                                     cvImage->nChannels = 1;
                                     bColor = false;
                                     break;
        case PIXEL_FORMAT_411YUV8:   cvImage = cvCreateImageHeader(mySize, 8, 3 );
                                     cvImage->depth = IPL_DEPTH_8U;
                                     cvImage->nChannels = 3;
                                     break;
        case PIXEL_FORMAT_422YUV8:   cvImage = cvCreateImageHeader(mySize, 8, 3 );
                                     cvImage->depth = IPL_DEPTH_8U;
                                     cvImage->nChannels = 3;
                                     break;
        case PIXEL_FORMAT_444YUV8:   cvImage = cvCreateImageHeader(mySize, 8, 3 );
                                     cvImage->depth = IPL_DEPTH_8U;
                                     cvImage->nChannels = 3;
                                     break;
        case PIXEL_FORMAT_RGB8:      cvImage = cvCreateImageHeader(mySize, 8, 3 );
                                     cvImage->depth = IPL_DEPTH_8U;
                                     cvImage->nChannels = 3;
                                     break;
        case PIXEL_FORMAT_MONO16:    cvImage = cvCreateImageHeader(mySize, 16, 1 );
                                     cvImage->depth = IPL_DEPTH_16U;
                                     cvImage->nChannels = 1;
                                     bColor = false;
                                     break;
        case PIXEL_FORMAT_RGB16:     cvImage = cvCreateImageHeader(mySize, 16, 3 );
                                     cvImage->depth = IPL_DEPTH_16U;
                                     cvImage->nChannels = 3;
                                     break;
        case PIXEL_FORMAT_S_MONO16:  cvImage = cvCreateImageHeader(mySize, 16, 1 );
                                     cvImage->depth = IPL_DEPTH_16U;
                                     cvImage->nChannels = 1;
                                     bColor = false;
                                     break;
        case PIXEL_FORMAT_S_RGB16:   cvImage = cvCreateImageHeader(mySize, 16, 3 );
                                     cvImage->depth = IPL_DEPTH_16U;
                                     cvImage->nChannels = 3;
                                     break;
        case PIXEL_FORMAT_RAW8:      cvImage = cvCreateImageHeader(mySize, 8, 3 );
                                     cvImage->depth = IPL_DEPTH_8U;
                                     cvImage->nChannels = 3;
                                     break;
        case PIXEL_FORMAT_RAW16:     cvImage = cvCreateImageHeader(mySize, 8, 3 );
                                     cvImage->depth = IPL_DEPTH_8U;
                                     cvImage->nChannels = 3;
                                     break;
        case PIXEL_FORMAT_MONO12:    printf("Not supported by OpenCV");
                                     bColor = false;
                                     break;
        case PIXEL_FORMAT_RAW12:     printf("Not supported by OpenCV");
                                     break;
        case PIXEL_FORMAT_BGR:       cvImage = cvCreateImageHeader(mySize, 8, 3 );
                                     cvImage->depth = IPL_DEPTH_8U;
                                     cvImage->nChannels = 3;
                                     break;
        case PIXEL_FORMAT_BGRU:      cvImage = cvCreateImageHeader(mySize, 8, 4 );
                                     cvImage->depth = IPL_DEPTH_8U;
                                     cvImage->nChannels = 4;
                                     break;
        case PIXEL_FORMAT_RGBU:      cvImage = cvCreateImageHeader(mySize, 8, 4 );
                                     cvImage->depth = IPL_DEPTH_8U;
                                     cvImage->nChannels = 4;
                                     break;
        default: printf("Some error occured...\n");
                 return NULL;
    }

    if(bColor) {
        if(!bInitialized)
        {
            colorImage.SetData(new unsigned char[pImage->GetCols() * pImage->GetRows()*3], pImage->GetCols() * pImage->GetRows()*3);
            bInitialized = true;
        }

        pImage->Convert(PIXEL_FORMAT_BGR, &colorImage); //needs to be as BGR to be saved

        cvImage->width = colorImage.GetCols();
        cvImage->height = colorImage.GetRows();
        cvImage->widthStep = colorImage.GetStride();

        cvImage->origin = 0; //interleaved color channels

        cvImage->imageDataOrigin = (char*)colorImage.GetData(); //DataOrigin and Data same pointer, no ROI
        cvImage->imageData         = (char*)(colorImage.GetData());
        cvImage->widthStep      = colorImage.GetStride();
        cvImage->nSize = sizeof (IplImage);
        cvImage->imageSize = cvImage->height * cvImage->widthStep;
    }
    else
    {
        cvImage->imageDataOrigin = (char*)(pImage->GetData());
        cvImage->imageData         = (char*)(pImage->GetData());
        cvImage->widthStep         = pImage->GetStride();
        cvImage->nSize             = sizeof (IplImage);
        cvImage->imageSize         = cvImage->height * cvImage->widthStep;

        //at this point cvImage contains a valid IplImage
     }
    return cvImage;
}

Answer 1

I could not test the code below for obvious reasons, but it illustrates one approach:

for ( int j = 0; j < k_numImages; j++ )
{
    // Display the timestamps for all cameras to show that the image
    // capture is synchronized for each image

    IplImage* destImage[3]; // A-ha moment

    for ( unsigned int i = 0; i < numCameras; i++ )
    {
        Image image;
        error = ppCameras[i]->RetrieveBuffer( &image );
        if (error != PGRERROR_OK)
        {
            PrintError( error );
            return -1;
        }

        /* Since ConvertImageToOpenCV() doesn't copy the image data,
         * we need to do that ourselves, because when this loop is done Image 
         * is destroyed and the data is lost.
         */
        IplImage* tmp = ConvertImageToOpenCV(&image);
        destImage[i] = cvCreateImage(cvGetSize(tmp), tmp->depth, tmp->nChannels);
        cvCopy(tmp, destImage[i], NULL); 

        char titolo[50];
        sprintf(titolo, "titolo%d", i);
    }

    cvShowManyImages("all", 3, destImage[0], destImage[1], destImage[2]);
    waitKey(0);

    // when you finish using them, release the allocated resources to prevent memory leaks
    cvReleaseImage(&destImage[0]);
    cvReleaseImage(&destImage[1]);
    cvReleaseImage(&destImage[2]);                 
}

The idea is to create an array of IplImage* to store the images retrieved by the cameras, so after the loop you have access to all those 3 images and be able to display them on a single window.

EDIT:

Just to summarize out private chat, the problem is that cvShowManyImages() takes colored (3-channel) images, and his camera was returning grayscale (single-channel) images. The solution was simply to change the implementation of cvShowManyImages(), from:

DispImage = cvCreateImage( cvSize(100 + size*w, 60 + size*h), 8, 3 );

To:

DispImage = cvCreateImage( cvSize(100 + size*w, 60 + size*h), 8, 1 );