All of the new source code will go in ViewController.m. Open the file. After the import statements, let's define the following enumeration to identify the available blending modes:

enum BlendMode {
  None,
  Average,
  Multiply,
  Screen,
  HUD
};

Now, we need to add several new variables and methods to the private interface of our ViewController class. To provide callbacks to a standard image picker, our view controller must implement two protocols, UIImagePickerControllerDelegate and UINavigationControllerDelegate. We also need two more cv::Mat variables to store the selected foreground image in its original format and a converted format that is appropriate for the current background and blending mode. The blending mode is a variable, too. Note the highlighted changes in this block of code:

@interface ViewController () <CvVideoCameraDelegate,
    UIImagePickerControllerDelegate,
    UINavigationControllerDelegate> {
  cv::Mat originalStillMat...