Question

I’m passing an array of images (IplImage**) to an object in C++ using OpenCV. I’m then trying to iterate over that array and resize them all to a fixed size (150×150)

I’m doing it this way:

for(int i = 0; i< this->numTrainingFaces; i++)
{
    IplImage* frame_copy = cvCreateImage( cvSize(150,150), this->faceImageArray[0]->depth, this->faceImageArray[0]->nChannels );
    cout << "Created image" << endl;
    cvResize(this->faceImageArray[i], frame_copy);
    cout << "Resized image" << endl;
    IplImage* grey_image = cvCreateImage( cvSize( frame_copy->width, frame_copy->height ), IPL_DEPTH_8U, 1 );
    cout << "Created grey image" << endl;
    cvCvtColor( frame_copy, grey_image, CV_RGB2GRAY );
    cout << "Converted image" << endl;
    this->faceImageArray[i] = grey_image;
    cvReleaseImage(&frame_copy);
    cvReleaseImage(&grey_image);
}

But I’m getting this output, and I’m not sure why:

Created image
Resized image
Created grey image
Converted image
Created image
OpenCV Error: Assertion failed (src.type() == dst.type()) in cvResize, file /build/buildd/opencv-2.1.0/src/cv/cvimgwarp.cpp, line 3102
terminate called after throwing an instance of 'cv::Exception'
  what():  /build/buildd/opencv-2.1.0/src/cv/cvimgwarp.cpp:3102: error: (-215) src.type() == dst.type() in function cvResize

Aborted

I’m basically just trying to replace the image in the array with the resized one in as few steps as possible.

Edit:

Revised my code as follows:

for(int i = 0; i< this->numTrainingFaces; i++)
{

    IplImage* frame_copy = cvCreateImage( cvSize(150,150), this->faceImageArray[i]->depth, this->faceImageArray[i]->nChannels );
    cvResize(this->faceImageArray[i], frame_copy);
    IplImage* grey_image = cvCreateImage( cvSize( frame_copy->width, frame_copy->height ), IPL_DEPTH_8U, 1 );
    cvCvtColor( frame_copy, grey_image, CV_RGB2GRAY );
    faceImageArray[i] = cvCreateImage( cvSize(grey_image->width, grey_image->height), grey_image->depth, grey_image->nChannels);
    cvCopy(grey_image,faceImageArray[i]);

    cvReleaseImage(&frame_copy);
    cvReleaseImage(&grey_image);
}

Then later on I’m performing some PCA, and get this output:

OpenCV Error: Null pointer (Null pointer to the written object) in cvWrite, file /build/buildd/opencv-2.1.0/src/cxcore/cxpersistence.cpp, line 4740

But I don’t think my code has got to the point where I’m explicitly calling cvWrite, so it must be part of the library. I can give a full implementation if necessary – is there anything in my code that’s going to create a null pointer?

My full source code is here: http://pastie.org/1447022 – a class is created, variables are set, then learn() is called.

Answer 1

• Why do you use the C interface when you’re using C++? The C++ interface is far friendlier and you wouldn’t have had this problem with it.
• Your error is simple – your image differ in type, so you can’t use the type from the first image for all images.

The line

IplImage* frame_copy = cvCreateImage( cvSize(150,150), this->faceImageArray[0]->depth, this->faceImageArray[0]->nChannels );

should be

IplImage* frame_copy = cvCreateImage( cvSize(150,150), this->faceImageArray[i]->depth, this->faceImageArray[i]->nChannels );

Also, this-> is usually a no-op. faceImageArray[0]->depth, faceImageArray[0]->nChannels should suffice.