How Indexed Pixels Work

Indexed devices expect that the pixel maps pointed to by color grafPort and

graphics device records are based on an indexed pixel system, in which the

stored image values are indexes to a table, not the RGB values themselves.

The indexed pixel path

As illustrated in the figure above, the user tells the application to choose a

color for some object (1). The application calls the

Color Picker (2), which offers its color wheel dialog box to the user (3),

who selects a color. The Color Picker returns that color to the application as a

full 48-bit RGB value (4). The application tells Color QuickDraw to draw

the object in that color (5). Color QuickDraw asks the Color Manager to

determine what color in the card's color table comes closest to the color

requested (6).

At startup, the video card's declaration ROM supplied information for the

creation of a graphics device record for the card, describing its

characteristics. The resulting graphics device record contains a color table

that is kept synchronized with the card's CLUT. The Color Manager need only

look into the current graphics device record's inverse table to find what RGB

colors are currently available (7) and decide which comes closest in RGB

space to the color requested by the application. (The inverse table is described

in the Color Manager) The Color Manager gets the index value for the best

match in the table and returns that value to

Color QuickDraw (8), which puts the index value into those places in video

RAM that store the object (9).

The card continuously displays video RAM by taking the index values,

converting them to RGB colors according to the CLUT entry at that index (10),

and sending them to digital-to-analog converters (11) that produce a signal

for the screen (12).

Referenced by (1):