PRB: UnrealizeObject() Causes Unexpected Palette Behavior (86800)
The information in this article applies to:
- Microsoft Windows Software Development Kit (SDK) 3.1
- Microsoft Win32 Application Programming Interface (API)
This article was previously published under Q86800
3.00 3.10 4.00 | 3.50 3.51
WINDOWS | WINDOWS NT
kbgraphic kbprb
SYMPTOMS
In an application for the Microsoft Windows graphical environment,
when a logical palette (HPALETTE) is used with a Device-Dependent
Bitmap (DDB) and the application realizes the palette, the DDB is
painted in incorrect colors.
CAUSE
The UnrealizeObject function was previously called to unrealize the
palette.
RESOLUTION
Modify the code to remove the call to UnrealizeObject()/
STATUS
This behavior is by design.
MORE INFORMATION
Because the colors of a DDB are stored using indices into the system
palette rather than explicit RGB colors, proper DDB rendering depends
on the colors of the system palette being set properly. An application
sets up the system palette to display a DDB by realizing a logical
palette. The realization process changes the colors in the system
palette and creates a mapping between entries in the logical palette
and entries in the system palette.
When an application first renders a logical palette with RealizePalette(), Windows sets an internal flag to indicate that the palette has been realized and stores the current mapping from logical palette entries to physical palette entries. When an application realizes the palette again (for example, after another application modifies the palette), Windows restores the effected entries of the system palette to the state they had when the logical palette was realized for the first time.
This mechanism allows a bitmap first realized with a specific palette
to display correctly when the same palette is realized subsequently.
If the application calls UnrealizePalette() on a logical palette,
Windows discards the stored state information for the palette. If the
application realizes the palette subsequently, its colors may map into
new locations in the system palette. Because the bitmap contains
indices into the old system palette, it may display incorrectly.
To address this situation, do not call UnrealizeObject() on a palette if the application has a DDB that depends on that palette.
| Modification Type: | Minor | Last Reviewed: | 2/11/2005 |
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| Keywords: | kbprb KB86800 |
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