Extended PAL Format

The extended PAL format includes the following:

• A palette-header chunk

• A data chunk containing an RGB palette (consisting of a LOGPALETTE structure) or some other palette type, including YUV and XYZ palettes.

For an RGB palette, the extended PAL format is represented as follows:

RIFF('PAL' plth( <palette-header> ) data( <LOGPALETTE-data> ))

For a YUV palette, the extended PAL format is represented as follows:

RIFF('PAL' plth( <palette-header> ) yuvp( <YUV-LOGPALETTE-data> ))

Both the <LOGPALETTE-data> and <YUV-LOGPALETTE-data> use the Windows 3.0 LOGPALETTE structure, described in "Simple PAL Format," earlier in this section. The <YUV-LOGPALETTE-data> contains YUV values instead of RGB valves.

The "plth" chunk is defined as follows:

<plth-ck>  PLT( struct {
           ULONG  ulMapType;
           USHORT usWhite;          /* Fields from this point on are     */
           USHORT usBlack;          /* optional. If they are included    */
           USHORT usBorder;         /* but not used, set them to 0xFFFF. */
           USHORT usRegisteredMap;
           USHORT usCustomBase;     /* If an application encounters a    */
           USHORT usCustomCnt;      /* 'PLT' chunk smaller than shown    */
           USHORT usRsvBase;        /* here, it should treat the missing */
           USHORT usRsvCount;       /* fields as unused.                 */
           USHORT usArtBase;
           USHORT usArtCnt;
           USHORT usNumIntense;
} )

The structure fields are described in the following:

Field

Description

FOURCC code specifying the type of palette. Currently, the following palette types are identified:

Code

Description
'data'
Specifies an RGB palette. Data chunk contains a LOGPALETTE structure.
'yuvp'
Specifies a YUV palette. Data chunk contains a YUV palette.
'xyzp'
Specifies an XYZ palette. Data chunk contains a XYZ palette.
usWhite, usBlack
Specify palette-map indices corresponding to the closest value of white and black. These identify the pair of colors with the best contrast for use in cursors, calibration, etc. These values are usually changed if the palette changes. Ignore these fields if they contain 0xFFFF.
usBorder
Specifies the index of the palette entry to be used for any display-border regions, if supported by the display device. Ignore this field if it contains 0xFFFF.
usRegisteredMap
Specifies how many palette entries correspond to a registered color map. Registered entries are stored at the front of the palette. Ignore this field if it contains 0xFFFF.

Registered map entries are always stored at the beginning of the palette, so usRegisteredMap also indicates the index of the first custom color in the palette. Registered color maps include predefined palettes for general use, forest/nature, or seasides. Currently defined values are the following:

PAL_UNREGISTERED (0xFFFF)

Color map does not contain colors from a registered color map.
PAL_VGA (0x0000)
Color map contains the standard 16 VGA colors.
PAL_AVC198 (0x0001)
Standard AVC 198-entry palette.
usCustomBase
Specifies the index of the first custom color of the palette. The beginning of the palette contains the entries of the registered map, so usCustomBase also indicates the number of entries in the registered palette. Map entries starting with usCustomBase comprise additional custom colors used in the bitmap. Ignore this value if usRegisteredPalette is PAL_UNREGISTERED, or if usCustomBase contains 0xFFFF.
usCustomCnt
Specifies the number of custom colors in the palette. Ignore this value if usRegisteredPalette is PAL_UNREGISTERED, or if this field contains 0xFFFF.
usRsvBase
Specifies the index of the first reserved color of the palette. Reserved colors are those reserved for menus, text, and other screen elements. Reserved colors must be stored contiguously. Ignore this field if it contains 0xFFFF.
usRsvCnt
Specifies the number of reserved entries. Ignore this field if it contains 0xFFFF.
usArtBase
Specifies the index of the first art color of the palette. Art colors are colors used for text and drawing. Art colors consist of a number of hues, each of which has multiple intensities. The various intensities are used for anti-aliasing, a method of using different shades of a color to improve the quality of images displayed on low-resolution devices.

For example, if the first art color is red anti-aliased to black with three intensities, the first three entries in the palette would be dark red, medium red, and bright red. The art colors constitute an array, and all hues have the same number of intensities. The user can set both the number of hues and the number of intensities. Ignore these fields if they contain 0xFFFF.

usArtCnt
Specifies the number of art colors. Ignore this field if it contains 0xFFFF.
usNumIntense
Specifies the number of palette entries reserved for the anti-aliased levels of a given art color. This field must be present if usArtBase is present. Ignore this field if it contains 0xFFFF.

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