NSImage和NSImageRep

The NSImage class provides the high-level interface for manipulating images in many different formats. Because it provides the high-level interface,NSImage knows almost nothing about managing the actual image data. Instead, the NSImage class manages one or more image representation objects—objects derived from theNSImageRep class. Each image representation object understands the image data for a particular format and is capable of rendering that data to the current context.

The following sections provide insight into the relationship between image objects and image representations.

Image Representations

An image representation object represents an image at a specific size, using a specific color space, and in a specific data format. Image representations are used by anNSImage object to manage image data. An image representation object knows how to read image data from a file, write that data back to a file, and convert the image data to a raw bitmap that can then be rendered to the current context. Some image representations also provide an interface for manipulating the image data directly.

For file-based images, NSImage creates an image representation object for each separate image stored in the file. Although most image formats support only a single image, formats such as TIFF allow multiple images to be stored. For example, a TIFF file might store both a full size version of an image and a thumbnail.

If you are creating images dynamically, you are responsible for creating the image representation objects you need for your image. As with file-based images, most of the images you create need only a single image representation. BecauseNSImage is adept at scaling and adjusting images to fit the target canvas, it is usually not necessary to create different image representations at different resolutions. You might create multiple representations in the following situations, however:

• For printing, you might want to create a PDF representation or high-resolution bitmap of your image.

• You want to provide different content for your image when it is scaled to different sizes.

When you draw an image, the NSImage object chooses the representation that is best suited for the target canvas. This choice is based on several factors, which are explained in“How an Image Representation Is Chosen.” If you want to ensure that a specific image representation is used, you can use thedrawRepresentation:inRect: method of NSImage.