Inside Macintosh: QuickDraw GX Objects /

Chapter 4 - Colors and Color-Related Objects / About Color in QuickDraw GX

Color-Component Values, Color Values, and Colors

Each of the color spaces described in this chapter requires one or more numeric values in a particular format to specify a color. This section describes the data types and structures with which QuickDraw GX describes colors in its color spaces.

Each dimension, or component, in a color space has a color-component value. In the fundamental, unpacked QuickDraw GX color spaces--those with 16 bits per component--each color-component value is of type gxColorValue:

typedef unsigned short gxColorValue;

A color-component value can vary from 0 to 65,535 (0xFFFF), although the numerical interpretation of that range is different for different color spaces, as has been noted in Table 4-1 through Table 4-7. In most cases, color-component intensities are interpreted numerically as varying between 0 and 1.0; for that reason, QuickDraw GX provides the constant gxColorValue1 to represent 0xFFFF.

Depending on the color space used, one, two, three, or four color-component values combine to make a color value. A color value is a structure; it is the complete specification of a color in a given color space. QuickDraw GX supports 13 color-value formats, representing the fundamental 16-bits-per-component color spaces; all color operations in memory use one of those formats. The color-value formats are described in the section "Color Values" beginning on page 4-50. For example, an RGB color value has this format:

struct gxRGBColor{
   gxColorValue   red;
   gxColorValue   green;
   gxColorValue   blue;
};

This is exactly the storage format for colors in gxRGBSpace. However, colors stored in gxRGB16Space or gxRGB32Space have a packed storage format, and need to be converted to gxRGBColor format when they are used. QuickDraw GX takes care of this for you; as far as your application is concerned, you can always manipulate colors in the color space you have specified.

A color value plus a specification of the color space it belongs to (plus an
optional reference to a color profile to use for color matching) constitute a color in QuickDraw GX. A color is defined by the gxColor structure:

struct gxColor{
   gxColorSpace         space;
   gxColorProfile       profile;
   union {
         struct gxCMYKColor      cmyk;
         struct gxRGBColor       rgb;
         struct gxRGBAColor      rgba;
         struct gxHSVColor       hsv;
         struct gxHLSColor       hls;
         struct gxXYZColor       xyz;
         struct gxYXYColor       yxy;
         struct gxLUVColor       luv;
         struct gxLABColor       lab;
         struct gxYIQColor       yiq;
         gxColorValue            gray;
         struct gxGrayAColor     graya;
         unsigned short          pixel16;
         unsigned long           pixel32;
         struct gxIndexedColor   indexed;
         gxColorValue            component[4];
   } element;
};

Each gxColor structure holds the specification of a single color. Note that, besides the basic color-value formats such as gxRGBColor and gxXYZColor, a QuickDraw GX color can contain a 16-bit or 32-bit pixel value or an indexed color value, and you can also access the color as an array of color-component values. Each of the color values in the element union of the gxColor structure is described in the section "The Color Structure" beginning on page 4-53.