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QuickDraw

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QuickDraw is the 2D graphics library and associated Application Programming Interface (API) which is a core part of the classic Mac OS operating system. It was initially written by Bill Atkinson and Andy Hertzfeld. QuickDraw is the part of the Macintosh Toolbox that performs graphics operations on the user's screen. All Macintosh applications use QuickDraw indirectly whenever they call other Toolbox managers to create and manage the basic user interface elements (such as windows, controls, and menus, as described in Inside Macintosh: Macintosh Toolbox Essentials).

Overview

As the Macintosh has evolved toward greater graphics capabilities, QuickDraw has grown along with it. Each new generation of QuickDraw has maintained compatibility with those that preceded it, while adding new capabilities and expanding the range of possible display devices. This evolutionary approach has helped to ensure that existing applications, written for earlier Macintosh models, continue to work as more powerful computers are developed.

The development of QuickDraw has progressed along these three main evolutionary stages:

  • Basic QuickDraw, which was designed for the earliest Macintosh models with their built-in black-and-white screens. System 7 added new capabilities to basic QuickDraw, including support for offscreen graphics worlds and the extended version 2 picture format. Basic QuickDraw is still used in more recent black-and-white Macintosh systems such as the Macintosh Classic and PowerBook 100 computers.
  • The original version of Color QuickDraw, which was introduced with the first Macintosh II systems. This first generation of Color QuickDraw could support up to 256 colors.
  • The current version of Color QuickDraw, which was originally introduced as 32-Bit Color QuickDraw and is now part of System 7. This version has been expanded to support up to millions of colors.

Applications that use only basic QuickDraw routines are compatible with all Macintosh systems. However, applications that use routines specific to Color QuickDraw cannot run on computers supporting only basic QuickDraw.

QuickDraw's Coordinate Plane

Your application typically uses Window Manager routines to create graphics ports in the form of windows. Your application can draw into a window without regard to its location on the screen--even if the window spans more than one screen. This is possible because QuickDraw maintains a global coordinate system for a computer's entire potential drawing space and a different local coordinate system for every window displayed in this space.

The Macintosh screen (or screens) on which QuickDraw displays your images represents a small part of a large global coordinate plane. Coordinates in the global coordinate system reflect the entire potential drawing space on this plane. The (0,0) origin point of the global coordinate plane is assigned to the upper-left corner of the main screen--that is, the one with the menu bar--while coordinate values increase to the right and (unlike a Cartesian plane) down. Any pixel on the screen can be specified by a vertical coordinate (ordinarily labeled v) and a horizontal coordinate (ordinarily labeled h).

Windows are rectangular areas that are subsets of the global coordinate plane. Each window represents its own QuickDraw graphics port. When you create a window, the Window Manager uses QuickDraw to create a graphics port in which the window's contents are displayed. (See the chapter "Window Manager" in Inside Macintosh: Macintosh Toolbox Essentials for a complete description of creating and managing windows.)

When your application creates a new graphics port, QuickDraw defines a boundary rectangle, which, by default, is the entire main screen; this rectangle links the local coordinate system of a graphics port to QuickDraw's global coordinate system and defines the area of the pixel image or bit image into which QuickDraw can draw. A boundary rectangle is stored in the pixel map for a color graphics port or in the bitmap for a basic graphics port.

The graphics port includes a field called portRect, which defines a rectangle to be used for drawing. In a graphics port that represents a window, the portRect rectangle--or simply, the port rectangle--represents the window's content region.

When you use the Window Manager to place a window on the screen, you specify the location of its port rectangle in global coordinates. However, within the port rectangle, the drawing area is described using a local coordinate system. You draw into a window in local coordinates, without regard to the window's location on the screen.

Images

QuickDraw provides a plethora of routines for drawing different kinds of images. These routines typically require that you start at a particular location in a graphics port and then move the graphics pen. The graphics pen is a metaphorical device for performing drawing operations onscreen. Your application can set this pen to different sizes, patterns, and colors.

You specify where to begin drawing by placing the pen at some location in the window's local coordinate system, and then specifying an act of drawing, usually from there to another location. Take, for example, the following two lines of code:

MoveTo(20,10);
LineTo(50,30);

The MoveTo procedure places the graphics pen at a point with a horizontal coordinate of 20 and a vertical coordinate of 10 in the local coordinate system of the graphics port, and the LineTo procedure draws a line from there to a point with a horizontal coordinate of 50 and a vertical coordinate of 30.

Colors

The earliest Macintosh models all used basic QuickDraw to draw to built-in screens with known characteristics. The Macintosh II computer introduced Color QuickDraw, which supports a variety of screens of differing sizes and color capabilities. With Color QuickDraw, users can choose from a wide range of screen options, from simple 12-inch black-and-white screens to full-page grayscale monitors to large two-page displays capable of presenting millions of colors. Users can even connect two or more separate screens to the same computer and simultaneously view different portions of the system's global coordinate plane.

A pixel, which is short for picture element, is the smallest dot that QuickDraw can draw. On a black-and-white monitor, a pixel is a single-color phosphor dot that displays in two states--black and white. On a color screen, three phosphor dots (red, green, and blue) compose each color pixel.

A pair of fields in a graphics port, fgColor and bkColor, specify a foreground and background color. The foreground color is the color used for bit patterns and for the graphics pen when drawing. By default, the foreground color is black. The background color is the color of the pixels in the bitmap or pixel map wherever no drawing has taken place. By default, the background color is white. However, when there is a color screen your application can draw with a color other than black by changing the foreground color, and your application can draw into a background other than white by changing the background color. For example, by changing the foreground color to red and the background color to blue before drawing a rectangle, your application can draw a red rectangle against a blue background.

On a color screen, you can draw in color even when you are using a basic graphics port. Although basic QuickDraw graphics routines were designed for black-and-white drawing, they also support an eight-color system that basic QuickDraw predefines for display on color screens and color printers. Because Color QuickDraw also supports this eight-color system, it is compatible across all Macintosh platforms.

See Also