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Display devices that are capable of displaying 256 or more simultaneous
colors using a palette need to provide support for color palettes. A display
driver specifies that it has palette support by setting the RC_PALETTE value
in the dpRaster member in the GDIINFO structure. The display driver also
must set the dpPalColors, dpPalReserved, and dpPalResolution members.
 
The number of reserved colors on the palette is always 20, with 16
corresponding to the VGA colors and 4 special colors. Half of the reserved
palette colors are placed at the beginning and half at the end of the
palette.
 
If a display driver supports color palettes, it must export the SetPalette,
GetPalette, SetPalTrans, and GetPalTrans functions.
 
Hardware-Palette Initialization
 
A display driver should initialize the hardware palette when the driver
initializes the rest of the display hardware. The driver initializes the
palette for static and nonstatic colors.
 
Static colors are system colors that remain in the palette at all times and
are used for old-style RGB support. The number of static colors to be used
is specified in the dpPalReserved member in the GDIINFO structure. This
number must always be even. The colors are placed in the hardware palette so
that the first half is in the lowest entries, and the second half is in the
highest entries.
 
For Windows 3.x, 20 static colors should be set as follows:
 
; Lowest ten palette entries
 
db      0,   0,   0,   0        ; 0
db      080h,0,   0,   0        ; 1
db      0,   080h,0,   0        ; 2
db      080h,080h,0,   0        ; 3
db      0,   0,   080h,0        ; 4
db      080h,0,   080h,0        ; 5
db      0,   080h,080h,0        ; 6
db      0C0h,0C0h,0C0h,0        ; 7
db      192, 220, 192, 0        ; 8
db      166, 202, 240, 0        ; 9
 
; Highest ten palette entries
 
db      255, 251, 240, 0        ; 10
db      160, 160, 164, 0        ; 11
db      080h,080h,080h,0        ; 12
db      0FFh,0,   0,   0        ; 13
db      0,   0FFh,0,   0        ; 14
db      0FFh,0FFh,0,   0        ; 15
db      0,   0,   0FFh,0        ; 16
db      0FFh,0,   0FFh,0        ; 17
db      0,   0FFh,0FFh,0        ; 18
db      0FFh,0FFh,0FFh,0        ; 19
 
These colors consist of the 16 standard VGA colors and four other special
colors used by Windows.
 
The nonstatic colors included in the hardware palette need not be
initialized. GDI manages these and initializes them when needed.
 
The RGBs for an 8bpp system palette should exactly match those reported to
GDI by the 8514/a driver. Using these RGBs ensures that colors will be
preserved when an image created with solid colors is viewed on a 4bpp
display.
 
An 8bpp display driver should not necessarily program the DAC with the exact
RGBs as in the GDI system palette. Instead, DAC values should be used that
result in on-screen system colors reasonably close to those shown by the
Windows 8514/a display driver. This accounts for differences in color
representation by DACs on different display adapters.
 
Palette-Translation Table
 
The driver has to maintain a palette-translation table to translate the
logical color indexes, passed to it by GDI, into the actual physical color
indexes. The translation has to occur before any raster operation (ROP) is
performed. ROPs are always applied to physical colors.
 
Whenever a display driver function receives a DRAWMODE, LPEN, PBRUSH, or
PBITMAP structure, the driver may need to translate the logical colors in
these structures to physical colors before using the colors.
 
An application has to perform color translation only when the physical
device is involved. In other words, if a line is drawn into a memory bitmap
or a bitmap is block transferred into another memory device, no color
translation is required. On the other hand, if a bitmap is transfered to or
from the screen into a memory bitmap or a line is drawn directly onto the
screen, color translation is required. In the case of a block transfer from
the screen to the screen (where the physical device is both the source and
destination of the block transfer), color translation is not needed since
all the color indexes are already translated into physical indexes.
 
Color specifications are passed to display drivers as either color indexes
or RGB values. A color index is a 32-bit value in which the high 16-bits is
set to 0xFF00 and the low 16-bits is the actual index. An RGB value is a
32-bit value as specified by the RBGQUAD structure. When an RGB value is
specified, it should match this color as closely as possible among the 20
reserved colors. In the case of a brush, the color may be dithered with the
16 VGA colors.
 
A palette-translation table is an array of 16-bit indexes, each mapping a
logical color index to a physical color index. A display driver uses the
translation table to translate color indexes in physical pens and brushes
and in the DRAWMODE structure to the actual color indexes used by the
hardware palette.
 
GDI calls the GetPalTrans and SetPalTrans functions to get and set the
translation table. The translation table has the number of elements
specified by the dpPalColors member. In constructing the inverse table, the
driver may come across ambiguities because different logical colors can map
to the same physical color. It is up to the driver to decide how to resolve
these cases since the result will look the same no matter how such
ambiguities have been resolved. The driver can also set accelerator bits to
bypass the various translations. For BitBlt, bypassing color translation
results in substantial performance improvements.
 
UpdateColors Function
 
If a display driver supports color palettes, it must include the
UpdateColors function. GDI calls this function to direct the driver to
redraw a region on screen using the translation table passed to the
function. For each pixel in the region, the function retrieves the pixel's
color index, translates the index, and writes the translated index back to
the given pixel.
 
Black-and-White Palette Entries
 
Display drivers that support color palettes must make sure that the index
for the palette entry that corresponds to black must be the one's complement
of the index for the palette entry for white. Black and white must be static
palette entries, meaning the driver sets the indexes for these colors during
initialization and does not change the indexes.
 
 
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