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Inside Macintosh: Imaging With QuickDraw /
Chapter 1 - Introduction to QuickDraw


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.

The basic QuickDraw color values consist of 1 bit for normal black-and-white drawing (black on white), 1 bit for inverted black-and-white drawing (white on black), 3 bits for the additive primary colors (red, green, blue) used in video display, and 4 bits for the subtractive primary colors (cyan, magenta, yellow, black) used in printing. Basic QuickDraw defines a set of constants for those standard colors:

 \xF0blackColor   =\xF033;
 \xF0whiteColor   =\xF030;
 \xF0redColor     =\xF0205;
 \xF0greenColor \xF0 =\xF0341;
 \xF0blueColor    =\xF0409;
 \xF0cyanColor    =\xF0273;
 \xF0magentaColor =\xF0137;
 \xF0yellowColor  =\xF069;
These are the only colors available in basic QuickDraw (or with Color QuickDraw drawing into a basic graphics port).

In Color QuickDraw, however, a color pixel represents up to 48 bits in memory. On a grayscale screen, a white phosphor dot whose intensity can vary is a pixel that usually represents 1, 2, 4, or 8 bits in memory.

To remove (for most applications) the burden of worrying about screen capabilities, Color QuickDraw is device-independent. Your application can use an RGBColor record, a data structure of type RGBColor, to specify a color by its red, green, and blue components, with each component defined as a 16-bit integer. Color QuickDraw compares the resulting 48-bit value with the colors actually available on a video device--such as a plug-in video card or a built-in video interface--at execution time and then chooses the closest match.

What the user finally sees depends on the characteristics of the actual video device and screen. Screens may display color or black and white; the video devices that control them may have indexed colors that support pixels of 1-bit, 2-bit, 4-bit, or 8-bit depths, or direct colors that support pixels of 16-bit or 32-bit depths. Color QuickDraw automatically determines which method is used by the video device and matches your requested color with the closest available color.

Indexed Colors

Some video devices use indexed colors to support a maximum of 256 colors at any one time. The indexed color system was created when the Macintosh II computer was introduced, at a time when memory was scarce and moving megabyte images around was impractical. With indexed color, the maximum value of a pixel in a PixMap record is limited to a single byte. Each pixel's byte can specify one of 256 (28) different values. Video devices implementing indexed color contain a data structure called a color lookup table (or, more commonly, a CLUT). The CLUT, in turn, contains entries for all possible color values.

For example, an indexed video device supporting 2 bits per pixel provides indexes into a table of four colors; if two colors are black and white, however, only two other hues can be shown.

An indexed video device supporting 4 bits per pixel can provide indexes into a table of 16 colors, a number sufficient for many straightforward graphics, such as those used in charts, presentations, or games.

An indexed video device supporting a byte (8 bits) for each pixel allows 256 colors to be displayed, which for many images is enough to produce near-photographic quality. The problem is that the colors needed for one near-photographic image may not be appropriate for another. The prevailing shades of browns necessary for displaying a painting by Rembrandt aren't appropriate for the prevailing shades of blues in a Monet painting. Because most indexed video devices use a variable CLUT (rather than a fixed one), you can display a Rembrandt painting with one set of 256 colors, then use system software to reload the CLUT with a different set of 256 colors for a Monet painting. If your application needs this sort of control on indexed video devices, you can use the Palette Manager (as described in the chapter "Palette Manager" in Advanced Color Imaging on the Mac OS) to arrange palettes--that is, sets of colors--for particular images and for video devices of differing color capabilities.

Some Macintosh computers, such as grayscale PowerBook computers, have a fixed CLUT, which your application cannot change.
If your application uses a 48-bit RGBColor record to specify a color, the Color Manager examines the colors available in the CLUT on the video device. If the video device supports 8 bits per pixel, for example, it contains a CLUT with 256 entries. Comparing these entries to the RGBColor record you specify, the Color Manager determines which color in the CLUT is closest, and the Color Manager gives Color QuickDraw the index to this color. Color QuickDraw then draws with this color.

Direct Colors

Video devices that implement direct color eliminate the competition for limited table spaces and remove the need for color table matching. By using direct color, video devices may support a maximum of thousands or millions of colors. When you specify a 48-bit RGBColor record, Color QuickDraw truncates the least significant bits of its red, green, and blue components to either 16 bits (5 bits each for red, green, and blue, with 1 bit unused) or 32 bits (8 bits each for red, green, and blue, with 8 bits unused). Using 16 bits, direct video devices can display over 32,000 colors; using 32 bits, direct video devices can display as many as 16 million different colors.

Using direct color not only removes much of the complexity of the CLUT mechanism for video device developers, but it also allows the display of thousands or millions of colors simultaneously, resulting in near-photographic realism.

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© Apple Computer, Inc.
7 JUL 1996