ID Number: Q71227
3.00 3.10
WINDOWS
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 reference words: 3.00 3.10 3.x