Using an index into a logical palette allows your application greater control over the colors displayed. This method becomes impractical, however, when dealing with a device that has 2^24 colors with no system palette. On a device capable of supporting full 24-bit color, this limits your application to displaying only the colors in your logical palette. By specifying palette colors indirectly, you can avoid this limitation.
You specify a palette color indirectly by using a palette-relative RGB COLORREF value instead of a palette index. A palette-relative RGB value is a 32-bit value that has the second bit in the high-order byte set to 1 and one-byte values for red, green, and blue in the remaining bytes. The PALETTERGB macro accepts three values that indicate the relative intensity of red, green, and blue, and returns a palette-relative RGB COLORREF value, which, like a palette-index COLORREF value, you can use in place of an explicit RGB COLORREF value for functions that require a color.
By specifying a palette-relative RGB value instead of a palette index, your application can draw to an output device by using palette colors, without having to determine first whether the device supports a system palette. The following table shows how Windows interprets a palette-relative RGB value.
| Device supports a system palette? |
How Windows uses a palette-relative RGB value |
| Yes | Windows matches the RGB information to the nearest color in the currently selected logical palette and uses that palette entry as though the application had directly specified the entry. |
| No | Windows uses the RGB information as though the palette-relative RGB were an explicit RGB value. |
For example, assume your application includes the following statements:
pLogPal->palPalEntry[5].pRed = 0xFF;
pLogPal->palPalEntry[5].pGreen = 0x00;
pLogPal->palPalEntry[5].pBlue = 0x00;
CreatePalette((LPSTR) &pa);
crRed = PALETTERGB(0xFF, 0x00, 0x00);If the target output device supports a system palette, crRed would be equivalent to this:
crRed = PALETTEINDEX(5);If the output device does not support a system palette, however, crRed would be equivalent to this:
crRed = RGB(0xFF, 0x00, 0x00);Even when using a logical palette, an application can use an explicit RGB value to specify color. In such cases, Windows displays the color as it would for an application that does not use a color palette, by displaying the nearest color in the default palette. If an application creates a solid brush by using an explicit RGB value, Windows simulates the color by dithering—that is, producing a pattern of pixels made up of colors in the default palette.