The Graphics Context

You can think of a graphics context as a drawing destination. A graphics context encapsulates all of the information needed to draw to an underlying canvas, including the current drawing attributes and a device-specific representation of the digital paint on the canvas. In Cocoa, graphics contexts are represented by the NSGraphicsContext class and are used to represent the following drawing destinations:

• Windows (and their views)

• Images (including bitmaps of all kinds)

• Printers

• Files (PDF, EPS)

• OpenGL surfaces

By far, the most common drawing destination is your application's windows, and by extension its views. Cocoa maintains graphics context objects on a per-window, per-thread basis for your application. This means that for a given window, there are as many graphics contexts for that window as there are threads in your application. Although most drawing occurs on your application's main thread, the additional graphics context objects make it possible to draw from secondary threads as well.

For most other drawing operations, Cocoa creates graphics contexts as needed and configures them before calling your drawing code. In some cases, actions you take may create a graphics context indirectly. For example, when creating a PDF file, you might simply request the PDF data for a certain portion of your view object. Behind the scenes, Cocoa actually creates a graphics context object and calls your view's drawing code to generate the PDF data.

You can also create graphics contexts explicitly to handle drawing in special situations. For example, one way to create a bitmap image is to create the bitmap canvas and then create a graphics context that draws directly to that canvas. There are other ways to create graphics context objects explicitly, although most involve drawing to the screen or to an image. It is very rare that you would ever create a graphics context object for printing or generating PDF or EPS data.

For information about graphics contexts, see Graphics Contexts.

The Graphics State

In addition to managing the drawing destination, an NSGraphicsContext object also manages the graphics state associated with the current drawing destination. The graphics state consists of attributes that affect the way content is drawn, such as the line width, stroke color, and fill color. The current graphics state can be saved on a stack that is maintained by the current graphics context object. Any subsequent changes to the graphics state can then be undone quickly by simply restoring the previous graphics state. This ability to save and restore the graphics state provides a simple way for your drawing code to return to a known set of attributes.

Cocoa manages some attributes of the graphics state in a slightly different way than Quartz does. For example, the current stroke and fill color are set using the NSColor class, and most path-based parameters are set using the NSBezierPath class. This shift of responsibility reflects the more object-oriented nature of Cocoa.

For more information about the attributes that comprise the current graphics state, and the objects that manage them, see Graphics State Information.

The Coordinate System

The coordinate system supported by Cocoa is identical to the one used in Quartz-based applications. All coordinates are specified using floating-point values instead of integers. Your code draws in the user coordinate space. Your drawing commands are converted to the device coordinate space where they are then rendered to the target device.

The user coordinate space uses a fixed scale for coordinate values. In this coordinate space, one unit is effectively equal to 1/72 of an inch. Although it seems like this might imply a 72 dots-per-inch (dpi) resolution for drawing, it is a mistake to assume that. In fact, the user coordinate space has no inherent notion of pixels or dpi. The use of floating-point values makes it possible for you to do precise layout in the user coordinate space and let Cocoa worry about converting your coordinates to the device space.

As the name implies, the device coordinate space refers to the native coordinate space used by the target device, usually a monitor or printer. Unlike the user coordinate space, whose units are effectively fixed, the units of the device coordinate space are tied to the resolution of the target device, which can vary. Cocoa handles the conversion of coordinates from user space to the device space automatically during rendering, so you rarely need to work with device coordinates directly.

For more information about coordinate systems in Cocoa, see Coordinate Systems Basics.

Transforms

A transform is a mathematical construct used to manipulate coordinates in two-dimensional space. Transforms are used extensively in graphics-based computing to simplify the drawing process. Coordinate values are multiplied through the transform's mathematical matrix to obtain a modified coordinate that reflects the transform's properties.

In Cocoa, the NSAffineTransform class implements the transform behavior. You use this class to apply the following effects to the current coordinate system:

• Translation

• Scaling

• Rotation

You can combine the preceding effects in different combinations to achieve interesting results. During drawing, Cocoa applies the effects to the content you draw, imparting those characteristics on your shapes and images. Because all coordinates are multiplied through a transform at some point during rendering, the addition of these effects has little effect on performance. In fact, manipulating your shapes using transforms is often faster than manipulating your source data directly.

For more information about transforms, including how they affect your content and how you use them, see Coordinate Systems and Transforms.

Color and Color Spaces

Color is an important part of drawing. Before drawing any element, you must choose the colors to use when rendering that element. Cocoa provides complete support for specifying color information in any of several different color spaces. Support is also provided for creating colors found in International Color Consortium (ICC) and ColorSync profiles.

Transparency is another factor that influences the appearance of colors. In OS X, transparency is used to render realistic-looking content and aesthetically appealing effects. Cocoa provides full support for adding transparency to colors.

In Cocoa, the NSColor and NSColorSpace classes provide the implementation for color objects and color space objects. For more information on how to work with colors in Cocoa, see Color and Transparency.

Geometry Support

Cocoa provides its own data structures for manipulating basic geometric information such as points and rectangles. Cocoa defines the data types listed in Table 1-1. The member fields in each of these data structures are floating-point values.

For information on how to manipulate point, rectangle, and size data types, see Manipulating Geometric Types.

Shape Primitives

Cocoa provides support for drawing shape primitives with the NSBezierPath class. You can use this class to create the following basic shapes, some of which are shown in Figure 1-2.

• Lines

• Rectangles

• Ovals and circles

• Arcs

• Bezier cubic curves

Bezier path objects store vector-based path information, making them compact and resolution independent. You can create paths with any of the simple shapes or combine the basic shapes together to create more complex paths. To render those shapes, you set the drawing attributes for the path and then stroke or fill it to “paint” the path to your view.

For more information about drawing shapes, see Paths.

Images

Support for images is provided by the NSImage class and its associated image representation classes (NSImageRep and subclasses). The NSImage class contains the basic interface for creating and managing image-related data. The image representation classes provide the infrastructure used by NSImage to manipulate the underlying image data.

Images can be loaded from existing files or created on the fly. Figure 1-3 shows some bitmap images loaded from files.

Cocoa supports many different image formats, either directly or indirectly. Some of the formats Cocoa supports directly include the following:

• Bitmap images, including the following image formats:

  • BMP

  • GIF

  • JPEG

  • JPEG 2000

  • PNG

  • TIFF

• Images based on Encapsulated PostScript (EPS) data

• Images based on Portable Document Format (PDF) data

• Images based on PICT data

• Core Image images

Because they support many types of data, you should not think of image objects strictly as bitmaps. Image objects can also store path-based drawing commands found in EPS, PDF, and PICT files. They can render data provided to them by Core Image. They can interpolate image data as needed and render the image at different resolutions as needed.

For detailed information about Cocoa support for images and the ways to use images in your code, see Images.

Gradients

In OS X v10.5 and later, you can use the NSGradient class to create gradient fill patterns.

Text

Cocoa provides an advanced text system for drawing everything from simple strings to formatted text flows. Figure 1-4 shows some basic examples of stylized text that you can create.

Because text layout and rendering using the Cocoa text system is a very complicated process, it is already well documented elsewhere and is not covered in great detail in this document. For basic information about drawing text and for links to more advanced text-related documents, see Text.