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Drawing Destinations: The Graphics Context

A graphics context is an opaque data type (CGContextRef) that encapsulates the information Quartz uses to draw images to an output device, such as a PDF file, a bitmap, or a window on a display. The information inside a graphics context includes graphics drawing parameters and a device-specific representation of the paint on the page. All objects in Quartz are drawn to, or contained by, a graphics context.

You can think of a graphics context as a drawing destination, as shown in Figure 1-3. When you draw with Quartz, all device-specific characteristics are contained within the specific type of graphics context you use. In other words, you can draw the same image to a different device simply by providing a different graphics context to the same sequence of Quartz drawing routines. You do not need to perform any device-specific calculations; Quartz does it for you.

These graphics contexts are available to your application:

• A bitmap graphics context allows you to paint RGB colors, CMYK colors, or grayscale into a bitmap. A bitmap is a rectangular array (or raster) of pixels, each pixel representing a point in an image. Bitmap images are also called sampled images. The CMYK bitmap graphics context is available starting with Mac OS X v10.3. See “Creating a Bitmap Graphics Context.”

• A PDF graphics context allows you to create a PDF file. In a PDF file, your drawing is preserved as a sequence of commands. There are some significant differences between PDF files and bitmaps:

  • PDF files, unlike bitmaps, may contain more than one page.

  • When you draw a page from a PDF file on a different device, the resulting image is optimized for the display characteristics of that device.

  • PDF files are resolution independent by nature—the size at which they are drawn can be increased or decreased infinitely without sacrificing image detail. The user-perceived quality of a bitmap image is tied to the resolution at which the bitmap is intended to be viewed.

  See “Creating a PDF Graphics Context.”

• A window graphics context is a bitmap graphics context that you can use to draw into a window. Note that because Quartz 2D is a graphics engine and not a window management system, you use one of the Mac OS X application frameworks to obtain a graphics context for a window. See “Creating a Window Graphics Context” for information on how to obtain a graphics context in Carbon and Cocoa applications.

• A layer context (CGLayerRef) is an offscreen drawing destination designed for optimal performance. Introduced in Mac OS X v10.4, a layer context is a much better choice for offscreen drawing than a bitmap graphics context. See “CGLayer Drawing.”

• A PostScript graphics context is managed by the printing framework. See “Obtaining a Graphics Context for Printing” for more information.

As of Mac OS X v10.4, Quartz has CGLayer objects, which are drawing layers that are associated with a graphics context. Drawing to a layer destination improves performance for certain types of drawing. For more information, see “CGLayer Drawing.”

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