All Databases Inside Mac - Basic Toolbox

About the Graphics Overview

This topic presents an overview of Macintosh graphics. It surveys the

components and processes of Color QuickDraw, the other graphics managers

with which it interacts, and how colors flow from your application to the

screen.

Read the information given here if your application uses color or gray scales.

To use the Graphics Overview, you should have a basic understanding of the

original QuickDraw. The next figure below charts your path into the world of

Macintosh graphics.

Macintosh graphics begin with QuickDraw and end with glowing phosphor.

The early Macintosh systems, with their built-in screens and integral

graphics hardware, made well- defined and comparatively limited demands on

your understanding of graphics: if you learned QuickDraw, you were set.

The Macintosh II computer introduced two features that greatly increased

graphics capabilities: slots and color. Slots allow the addition of specialized

hardware to the system. (In the original Macintosh II, the graphics hardware

was always added in NuBus standard slots. Current systems allow a number of

hardware expansion modes in addition to NuBus slots.)

With slots, the characteristics of the output device can vary from machine to

machine. With color, the extent of variation can be very great: screens not only

can vary in horizontal and vertical dimensions, but they can also vary in

depth. To the single-bit-per-pixel depth of the original Macintosh systems,

the Macintosh II added pixel depths of 2, 4, and 8 bits. Output devices range

from black-and-white systems to cards and screens capable of presenting

hundreds of colors from palettes of millions. Furthermore, users can combine

screens: a user may move your application's window so that it overlaps

screens of very different characteristics.

To remove the burden of worrying about output devices (for most

applications), Color QuickDraw is device-independent. Applications using

color can work in an abstract color space defined by three axes of red, green,

and blue (RGB). Your application can specify a color as an RGB value, in which

each component is defined as a 16-bit integer. Color QuickDraw compares

such a 48-bit value with the colors actually available on the hardware at

execution time and chooses the closest match. Precolor applications and those

not concerned with color or gray-scale graphics need not change, and those

concerned only with straightforward color usage can ignore the problems of

output devices.

When the Macintosh II was introduced, the maximum pixel value was limited

to a single byte. Each pixel's byte can specify one of 256 (28) different

values, and, rather than simply truncating the least significant bits of each

component to get a color, Color QuickDraw treats such pixel values as

indexes into a color table. If your application asks for a 48-bit RGB color, the

Color Manager examines the colors available in the card. If the video device

supports 8 bits per pixel, the card contains a color look-up table (CLUT) with

256 entries, each entry an RGB value. The Color Manager determines which

RGB value is closest to the requested color and tells Color QuickDraw what

the index for that color is.

Storage and movement of such indexed color values require a maximum of 8

bits, rather than 48, saving space and time. (The RAM needed to hold a

640-by-480 pixel screen at 8 bits per pixel is about 300,000 bytes.) And

because the table is variable-it can be loaded with different

colors- applications can display up to 16 million colors, although only 256

different colors can appear at once.

With the addition of direct pixel values, first made available with system

software version 6.0.5, the 256 simultaneous color limit has been removed.

Color QuickDraw can now also process direct pixel values, which use 16

or 32 bits to directly represent a color. Using direct color not only removes

much of the complexity of the color table mechanism, but it also allows the

display of thousands or millions of colors simultaneously, resulting in

near-photographic realism.

The device-independence of Color QuickDraw is such that on a

three-screen system-for example, with displays for gray-scale indexed

pixels, color indexed pixels, and direct pixels-the user can move your window

to span all three devices and each will show its best representation of your

image.

This overview introduces Color QuickDraw and the related graphics

managers with which it works.

• Color QuickDraw calls upon the Color Manager to map color

requests to the actual colors available. Most applications never need to

call the Color Manager directly.

• The Palette Manager allows your application to specify the set of

colors that it needs on a window-by-window basis, and makes the

colors available (within application- determined ranges) in a graceful

manner.

• The Color Picker Package allows your application to solicit a color

choice from the user in a standard way.

• The Picture Utilities Package provides routines with which your

application can extract information, such as pixel depth and colors

used, from pixel maps and pictures.

• The Graphics Devices Manager offers routines for preparing

images off screen, and it manages the data structures that track the

characteristics of the graphics hardware of a particular system.

• The Slot Manager controls communication with expansion boards of

all types, including video cards.