Issue with stitching images using openCV for iOS

I'm trying to adopt the code from here:

http://ift.tt/1cmchkL

into my program. However, I've run up against a wall. This code stitches images together that are already existing. The program I'm trying to make will stitch images together that the user took. The error I'm getting is that when I pass the images to the stitch function, it is saying they are of invalid size (0 x 0).

Here is the stitching function:

- (IBAction)stitchImages:(UIButton *)sender {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{


        NSArray* imageArray = [NSArray arrayWithObjects:
                               chosenImage, chosenImage2, nil];
        UIImage* stitchedImage = [CVWrapper processWithArray:imageArray]; // error occurring within processWithArray function
        dispatch_async(dispatch_get_main_queue(), ^{

            NSLog (@"stitchedImage %@",stitchedImage);
            UIImageView *imageView = [[UIImageView alloc] initWithImage:stitchedImage];
            self.imageView = imageView;
            [self.scrollView addSubview:imageView];
            self.scrollView.backgroundColor = [UIColor blackColor];
            self.scrollView.contentSize = self.imageView.bounds.size;
            self.scrollView.maximumZoomScale = 4.0;
            self.scrollView.minimumZoomScale = 0.5;
            self.scrollView.contentOffset = CGPointMake(-(self.scrollView.bounds.size.width-self.imageView.bounds.size.width)/2, -(self.scrollView.bounds.size.height-self.imageView.bounds.size.height)/2);
            [self.spinner stopAnimating];

        });


    });

}

chosenImage and chosenImage2 are images the user has taken using these two functions:

- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info {
    savedImage = info[UIImagePickerControllerOriginalImage];

    // display photo in the correct UIImageView
    switch(image_location){
        case 1:
            chosenImage = info[UIImagePickerControllerOriginalImage];
            self.imageView2.image = chosenImage;
            image_location++;
            break;
        case 2:
            chosenImage2 = info[UIImagePickerControllerOriginalImage];
            self.imageView3.image = chosenImage2;
            image_location--;
            break;

    }
    // if user clicked "take photo", it should save photo
    // if user clicked "select photo", it should not save photo
    /*if (should_save){
     UIImageWriteToSavedPhotosAlbum(chosenImage, nil, nil, nil);
     }*/
    [picker dismissViewControllerAnimated:YES completion:NULL];

}

- (IBAction)takePhoto:(UIButton *)sender {
    UIImagePickerController *picker = [[UIImagePickerController alloc] init];
    picker.delegate = self;
    picker.allowsEditing = NO;
    picker.sourceType = UIImagePickerControllerSourceTypeCamera;
    //last_pressed = 1;
    should_save = 1;
    [self presentViewController:picker animated:YES completion:NULL];
}

The stitchesImages function passes an array of these two images to this function:

+ (UIImage*) processWithArray:(NSArray*)imageArray
{
    if ([imageArray count]==0){
        NSLog (@"imageArray is empty");
        return 0;
        }
    cv::vector<cv::Mat> matImages;

    for (id image in imageArray) {
        if ([image isKindOfClass: [UIImage class]]) {
            cv::Mat matImage = [image CVMat3];
            NSLog (@"matImage: %@",image);
            matImages.push_back(matImage);
        }
    }
    NSLog (@"stitching...");
    cv::Mat stitchedMat = stitch (matImages); // error occurring within stitch function
    UIImage* result =  [UIImage imageWithCVMat:stitchedMat];
    return result;
}

This is where the program is running into a problem. When it is passed images that are saved locally in the application file, it works fine. However, when it is passed images that are saved in variables (chosenImage and chosenImage2), it doesn't work.

Here is the stitch function that is being called in the processWithArray function and is causing the error:

cv::Mat stitch (vector<Mat>& images)
{
    imgs = images;
    Mat pano;
    Stitcher stitcher = Stitcher::createDefault(try_use_gpu);
    Stitcher::Status status = stitcher.stitch(imgs, pano);

    if (status != Stitcher::OK)
        {
        cout << "Can't stitch images, error code = " << int(status) << endl;
            //return 0;
        }
    return pano;
}

The error is "Can't stitch images, error code = 1".