INFO: Background, Foreground, and System Palette Management (72386)

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When a logical palette is selected into a DC, it can be selected as either a foreground or a background palette. Setting the bForceBackground parameter of the SelectPalette() API to TRUE selects the palette as a background palette. If this parameter is FALSE, the palette can be selected as a foreground palette. A palette will be selected as a foreground palette only if the DC into which the palette is selected is one of the five cached DCs managed by the GetDC() API and the DC is retrieved on behalf of the active window. If the DC is returned by CreateDC() or CreateCompatibleDC() or if the window is not the active window, the palette will be forced into the background.

The status as a foreground or a background palette affects how the colors in the logical palette are mapped into the system palette when the logical palette is realized.

When a foreground palette is realized, every entry in the system palette that can be modified by an application is accessible to the logical palette. Logical palette entries are mapped into the system palette starting at the first available entry. Because a logical palette entry that exactly matches a reserved system palette entry is mapped to that system entry, it does not consume a separate palette slot. If the logical palette has more entries than available slots in the system palette, the available slots are filled, in order, from the logical palette. The remaining logical palette entries are mapped to the closest colors already present in the system palette. There is one exception to this rule: if a logical palette entry is marked with the PC_RESERVED flag, no colors will be mapped to that entry. If all available system palette entries are reserved, additional colors will not be mapped to any entry and will be displayed as black on the screen.

A palette entry marked as PC_NOCOLLAPSE will always take a separate slot if available, just as for PC_RESERVED. Unlike a PC_RESERVED color, if no slots are available, it will map to the nearest color, and other colors may map onto it.

The first available entry in the system palette is the first palette entry not marked as used. For example, assume a device with 256 palette entries, 20 of which are reserved for the system. An application realizes a palette of 36 colors on this device; therefore, the first 36 entries are marked used. Another application realizes a 100-entry palette; therefore, the next 100 entries are marked used. If a third application receives the input focus and realizes a foreground palette with 236 entries, Windows maps the first 100 colors into the remainder of the system palette. Each of the remaining 136 colors of the logical palette is mapped into the closest color available in the system palette.

When a background palette is realized, any empty positions in the system palette are filled. Any colors that remain are mapped to the closest color in the system palette. A background palette entry cannot overlay a foreground entry in the system palette; however, a foreground palette entry can overlay a background entry in the system palette.