Q71227: INFO: Inverting Color Inverts Palette Index, Not RGB Value
Article: Q71227
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.1,95; winnt:3.5,3.51,4.0
Operating System(s):
Keyword(s): kbOSWinNT350 kbOSWinNT351 kbOSWinNT400 kbOSWin95 kbSDKWin16
Last Modified: 11-MAY-2001
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The information in this article applies to:
- Microsoft Windows Software Development Kit (SDK) 3.1
- Microsoft Win32 Application Programming Interface (API), used with:
- Microsoft Windows NT Server versions 3.5, 3.51, 4.0
- Microsoft Windows NT Workstation versions 3.5, 3.51, 4.0
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SUMMARY
=======
Performing any bitwise logical operator on a color, such as inversion, does not
modify the color's RGB value; it instead changes the index into the system
palette. This applies also to the ROP codes associated with the blt functions
(BitBlt, StretchBlt, and PatBlt) and in the SetRop2 function. For display
devices with hardware palettes [generally, devices with fewer than 24
Bits-Per-Pixel (BPP)], this can produce unexpected or undesirable results.
MORE INFORMATION
================
Suppose the system palette contained the following colors:
Entry #
Color 0 1 2 3 4 5 6 7
-----
Red = 0 0x80 0 0 0x80 0x80 0 0x80
Green = 0 0 0x80 0 0x80 0 0x80 0x80
Blue = 0 0 0 0x80 0 0x80 0x80 0x80
Entry #
Color 8 9 A B C D E F
-----
Red = 0xC0 0xFF 0 0 0xFF 0xFF 0 0xFF
Green = 0xC0 0 0xFF 0 0xFF 0 0xFF 0xFF
Blue = 0xC0 0 0 0xFF 0 0xFF 0xFF 0xFF
The inversion of colors would look like this: (Half = half intensity, Full = full
intensity)
Color Index Inverse Color Index
----- ----- ------------- -----
Black 0 White F
Half Red 1 Full Cyan E
Half Green 2 Full Magenta D
Half Blue 3 Full Yellow C
Half Yellow 4 Full Blue B
Half Magenta 5 Full Green A
Half Cyan 6 Full Red 9
Dark Gray 7 Light Gray 8
This obviously is not the desired effect. Inverting a full-intensity color such
as red will not invert to full-intensity cyan; instead, it is inverted to
half-intensity cyan.
This is also true for any logical operations performed on the bits of a bitmap,
pen, or flood fill through ROP codes. All operations are done on the index of
the color and not its RGB value.
Note that when using custom palettes on a palette capable device, the application
does not have control over the precise mapping between logical palette indexes
and system palette indexes. The results of bitwise logical operations are
unpredictable in such a case.
The only way for an application to precisely control color mixing is to perform
the operation on RGB values, then translate the RGB result back into the most
appropriate palette index.
For example, one way to do this is to mix colors in a 24 BPP Device-Independent
Bitmap (DIB), then set the DIB bits into the Device Context (DC) again when
finished. Another method is to query the RGB color of pixels to modify, do the
mixing, and then use the SetPixel function to apply the change to the DC.
Additional query words: 3.00 3.10 3.50 4.00 win16sdk
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Keywords : kbOSWinNT350 kbOSWinNT351 kbOSWinNT400 kbOSWin95 kbSDKWin16
Technology : kbAudDeveloper kbSDKSearch kbWin32sSearch kbWin32API kbWinSDKSearch
Version : WINDOWS:3.1,95; winnt:3.5,3.51,4.0
Issue type : kbinfo
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