UIKit Image Classes and Functions

The UIKit framework has three classes and one protocol that are related to images in some way:

UIImage

Objects of this class represent images in the UIKit framework. You can create them from several different sources, including files and Quartz image objects. Methods of the class enable you to draw images to the current graphics context using different blend modes and opacity values.

The UIImage class automatically handles any required transformations for you, such as applying the proper scale factor (taking into consideration high-resolution displays) and, when given Quartz images, modifying the coordinate system of the image so that it matches the default coordinate system of UIKit (where the y origin is at the upper left).

UIImageView

Objects of this class are views that display either a single image or animate a series of images. If an image is to be the sole content of a view, use the UIImageView class instead of drawing the image.

UIImagePickerController and UIImagePickerControllerDelegate

This class and protocol give your app a way to obtain images (photos) and movies supplied by the user. The class presents and manages user interfaces for choosing and taking photos and movies. When users pick a photo, it delivers the selected UIImage object to the delegate, which must implement the protocol methods.

In addition to these classes, UIKit declares functions that you can call to perform a variety of tasks with images:

• Drawing into an image-backed graphics context. The UIGraphicsBeginImageContext function creates an offscreen bitmap graphics context. You can draw in this graphics context and then extract a UIImage object from it. (See Drawing Images for additional information.)

• Getting or caching image data. Each UIImage object has a backing Core Graphics image object (CGImageRef) that you can access directly. You can then pass the Core Graphics object to the Image I/O framework to save the data. You can also convert the image data in a UIImage object to either a PNG or JPEG format by calling the UIImagePNGRepresentation or UIImageJPEGRepresentation functions. You can then access the bytes in the data object and you can write the image data to a file.

• Writing an image to the Photo Album on a device. Call the UIImageWriteToSavedPhotosAlbum function, passing in a UIImage object, to put that image in the Photo Album on a device.

Drawing Images identifies scenarios when you would use these UIKit classes and functions.

Other Image-Related Frameworks

You can use several system frameworks other than UIKit to create, access, modify, and write images. If you find that you cannot accomplish a certain image-related task using a UIKit method or function, a function of one of these lower-level frameworks might be able do what you want. Some of these functions might require a Core Graphics image object (CGImageRef). You can access the CGImageRef object backing a UIImage object through the CGImage property.

The Core Graphics framework of Quartz is the most important of the lower-level system frameworks. Several of its functions correspond to UIKit functions and methods; for example, some Core Graphics functions allow you to create and draw to bitmap graphics contexts, while others let you create images from various sources. However, Core Graphics offers more options for handling images. With Core Graphics you can create and apply image masks, create images from portions of existing images, apply color spaces, and access a number of additional image attributes, including bytes per row, bits per pixel, and rendering intent.

The Image I/O framework is closely related to Core Graphics. It allows an app to read and write most image file formats, including the standard web formats, high dynamic range images, and raw camera data. It features fast image encoding and decoding, image metadata, and image caching.

Assets Library is a framework that allows an app to access assets managed by the Photos app. You can get an asset either by representation (for example, PNG or JPEG) or URL. From the representation or URL you can obtain a Core Graphics image object or the raw image data. The framework also lets you write images to the Saved Photos Album.

Supported Image Formats

Table C-2 lists the image formats supported directly by iOS. Of these formats, the PNG format is the one most recommended for use in your apps. Generally, the image formats that UIKit supports are the same formats supported by the Image I/O framework.