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GDI requires a display driver to support bit-block transfers by providing
the BitBlt and related bitmap functions. At the very least, a display driver
should provide a BitBlt function that can carry out bitmap operations on the
screen and in monochrome bitmaps. The brute functions provided by GDI and
used by many printer drivers require monochrome bitmap support.
 
GDI checks the dpRaster member in the GDIINFO structure to determine whether
the display driver supports bitmaps. If the driver does, GDI checks the
dpBitsPixel and dpPlanes members to determine the format of the screen
bitmap.
 
Bitmap Format
 
Most display devices are either planar or packed-pixel devices. A planar
device stores screen bits in separate bitmap planes, each representing a
distinct color. A packed-pixel device stores screen bits in a single plane,
but each pixel on the screen has a set of two or more corresponding bits
that define the pixel color.
 
A display driver specifies the type of display device by setting the
dpBitsPixel and dpPlanes members to appropriate values. For a planar device,
dpBitsPixel is 1 and dpPlanes specifies the number of planes. For a
packed-pixel device, dpPlanes is 1 and dpBitsPixel specifies the number of
bits per pixel.
 
These values also help determine the number of nondithered colors available
for the screen. A device with 4 planes is capable of 16 colors. A device
with 8 bits per pixel is capable of 256 colors. However, the exact color
specified by a combination of bits depends on whether the device supports a
color palette.
 
Pixel Output
 
A display driver must support setting and getting individual pixels by
providing the Pixel function. GDI uses Pixel to support a variety of
simulations. It also calls the function whenever an application calls
SetPixel (GDI.31) and GetPixel (GDI.83).
 
Bit-Block Transfers
 
A display driver can support bit-block transfers by providing a BitBlt
function. GDI uses BitBlt to copy a rectangular block of bits from bitmap to
bitmap, possibly applying a raster operation to the source and destination
bits as it copies. GDI checks the RC_BITBLT value in the dpRaster member to
determine whether the display driver supports the function. If it does, GDI
calls BitBlt whenever an application calls GDI functions such as BitBlt
(GDI.34) and PatBlt (GDI.29).
 
A display driver can provide additional bit-block transfers by providing a
StretchBlt function. GDI uses StretchBlt to stretch or compress a block of
bits to fit a given rectangle in a bitmap. Stretching and compressing
requires either adding or removing bits as defined by a stretching mode. GDI
checks the RC_STRETCHBLT value in dpRaster to determine whether the driver
provides the StretchBlt function. GDI calls the function whenever an
application calls StretchBlt (GDI.35).
 
If a display driver can carry out bit-block transfers on bitmaps that are
larger than 64K, it should set the RC_BITMAP64 value in the dpRaster member.
GDI checks this bit to determine whether large bitmaps are permitted.
 
Transparent-Block Transfers
 
In Windows 3.1, display drivers can indicate that they support
transparent-block transfers by setting the C1_TRANSPARENT value in the
dpCaps1 member of the GDIINFO structure. In a transparent-block transfer, a
driver excludes source and brush pixels from a BitBlt or StretchBlt
operation if those pixels have the same color as the current background
color for the destination device.
 
If a display driver supports transparent block transfers, the BitBlt
function must check the bkMode member of the DRAWMODE structure as well as
the Rop3 parameter of the BitBlt function to determine how to carry out the
transfer. If the bkMode member specifies the background mode TRANSPARENT1,
BitBlt must not transfer source and brush bits that have the same color as
the destination's background color (as specified by the bkColor member of
the DRAWMODE structure pointed to by the lpDrawMode parameter). In other
words, the corresponding destination bits must be left unchanged. Other
background modes do not affect the transfer.
 
Although a display driver may support transparent-block transfers, GDI does
not currently provide access to this support for Windows applications.
 
Fast Borders
 
A display driver can support fast-border drawing by providing a FastBorder
function. Windows uses the function to quickly draw borders for windows and
dialog boxes. GDI checks the RC_GDI20_OUTPUT value in the dpRaster member to
determine whether a display driver provides the FastBorder function. If it
does, GDI calls the function to draw the borders. Otherwise, it returns an
error value to direct Windows to use some other means to draw the borders.
 
A display driver also uses the RC_GDI20_OUTPUT value to specify whether it
supports the ExtTextOut function. In some cases, a display driver may
support ExtTextOut but not FastBorder. To account for this, the display
driver must provide a FastBorder function, but the function can immediately
return an error value to direct Windows to use some other means to draw
borders.
 
Saved Bitmaps
 
A display driver can permit Windows to temporarily save bitmaps in
off-screen video memory by providing the SaveScreenBitmap function. This
function allows the driver to take advantage of unused video memory and to
speed up drawing operations that require restoring a portion of the screen
that was previously overwritten.
 
GDI checks the RC_SAVEBITMAP value in the dpRaster member to determine
whether the driver supports SaveScreenBitmap.
 
Flood Fill
 
A display driver can support flood-fill operations by providing a FloodFill
function. GDI uses the function to quickly fill a region on the screen or in
a bitmap with a specified color. GDI calls the function whenever an
application calls FloodFill (GDI.25).
 
GDI checks the RC_FLOODFILL value in dpRaster to determine whether the
driver provides the function.
 
 
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