When it opens, LightWork will present a vintage photograph and toolbar containing a few items. The following screenshot shows it all:

This bucolic image is an early color photograph, shot in 1902 by Adolf Miethe, a German photographer, professor, and inventor. The photo almost looks like a painting due to its coarse grain and pastel colors. However, it is a true example of a color photographic process. Miethe captured a scene on three photographic plates behind three different filters: red, green, and blue. Then, to recreate a multicolored image, he superimposed the three images using a projection process with different colors of light or a printing process with different dyes. Similar techniques are still in use today in our digital sensors, monitors, and printers.

LightWork will also be capable of displaying a live preview from a camera. However, for our purposes, a static (still) image is also a useful preview. A static image (unlike a camera preview...

Create an Xcode project named LightWork using the Single View Application template. Configure the project according to the instructions in the Configuring the project section in Chapter 1, Setting Up Software and Hardware. Next, we will take a few additional configuration steps because LightWork depends on more frameworks and requires a camera.

As discussed in the previous chapter, we may declare GUI elements in a view controller's source code and lay them out in a storyboard. Thus, to begin, let's open ViewController.m and define the private interface of our ViewController class. The interface depends on headers from the iOS SDK's Photos and Social frameworks as well as the OpenCV framework. Moreover, it depends on headers that define the public interfaces of our own classes, ViewController and VideoCamera. The latter class will handle many aspects of camera input and video display and we will write it later in the Controlling the camera section. Let's import these dependencies by adding the following code at the start of ViewController.m:

#import <Photos/Photos.h>
#import <Social/Social.h>

#import <opencv2/core.hpp>
#import <opencv2/imgcodecs.hpp>
#import <opencv2/imgcodecs/ios.h>
#import <opencv2/imgproc.hpp>

#import "ViewController.h"
#import ...

The iOS SDK and OpenCV provide several programming interfaces for camera control. Within the iOS SDK, AVFoundation is the general-purpose framework for all recording and playback of audiovisual (AV) content. AVFoundation provides complete access to the iOS camera's parameters, including the image format, focus, exposure, flash, frame rate, and digital zoom (crop factor). However, AVFoundation does not solve any GUI problems. The application developer may create a custom camera GUI, use a higher-level framework that provides a GUI, or automate the camera so that it operates without GUI input. AVFoundation is sufficiently flexible to support any of these designs, but this flexibility comes at a price as AVFoundation is complex.

The iOS SDK implements a standard camera GUI in the UIImagePickerController class, which...

As we have seen, OpenCV and the iOS SDK work with various formats for color and grayscale images and sometimes we need to convert between formats. Let's step back from the code for a few moments to discuss the differences between formats and the problems that can arise if we do not perform the correct conversions.

Remember that we want to show an activity indicator and disable all the toolbar items while LightWork is busy saving or sharing a photo. Conversely, when LightWork is no longer busy with the photo, we want to hide the activity indicator and re-enable the toolbar items. As these actions affect the GUI, we must ensure that they run on the app's main thread.

To run code on a specific thread, we can make a post to the thread's event queue. The iOS SDK provides a C function, dispatch_async, which takes a target queue and code block as arguments. Another C function, dispatch_get_main_queue(), enables us to get the main thread's event queue. Let's use these functions in the following helper method, which starts the busy mode:

- (void)startBusyMode {
  dispatch_async(dispatch_get_main_queue(), ^{
    [self.activityIndicatorView startAnimating...