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GDI requires all display drivers to provide an Output function that supports
a minimum set of line, curve, and polygon drawing capabilities. In
particular, a display driver must be able to draw scan lines and polylines.
A scan line is a solid or styled, horizontal line that is exactly one pixel
wide. A polyline is a sequence of solid lines, each one pixel wide and each
connected at an endpoint to the next line in the sequence.
 
GDI may call the Output function whenever an application calls a function
that draws lines, curves, or polygons. If the display driver supports all
capabilities, GDI calls the Output function for each request. Otherwise, GDI
simulates output that the driver does not support by combining scan lines
and polylines.
 
Since GDI can use scan lines and polylines to simulate all other line,
curve, and polygon output, many display drivers do not support additional
capabilities unless the display hardware can produce faster and
higher-quality results than GDI. The only drawback to using display hardware
is that the display driver must be able to produce the same results in
memory bitmaps as on the screen.
 
GDI checks the dpCurve, dpLines, and dpPolygonals members of the GDIINFO
structure to determine what line, curve, and polygon capability the display
driver has.
 
Curves
 
GDI checks the dpCurve member to determine whether the display driver
supports circles, ellipses, pie wedges, and chord arcs. This member also
specifies whether the display driver can draw wide or styled borders for
curves and fill the interiors of curves.
 
If a display driver does not support circles, GDI can use an ellipse to draw
a circle. If the display hardware can fill ellipses, the display driver
should set the interiors bit. GDI can use an alternate-fill polygon to draw
wide borders (both solid and styled) just as efficiently as if the driver
supported them correctly.
 
Polylines
 
GDI checks the dpLines member to determine whether the display driver
support polylines. This member also specifies whether the display driver can
draw wide or styled lines and fill the interiors of wide lines. A display
driver must support polylines.
 
If a display driver supports styled lines, the line segments drawn by the
display hardware must be as specified by the dpStyleLen member. Although
wide, styled lines are used infrequently, the display driver should support
them if the display hardware can draw them and the effort to support memory
bitmaps is not too great. If necessary, GDI can simulate wide, styled
lines.
 
Polygons
 
GDI checks the dpPolygonals member to determine whether the display driver
supports rectangles, scan lines, alternate-fill polygons, and
winding-number-fill polygons. This member also specifies whether the display
driver can draw wide or style borders for polygons, and fill the interiors
of polygons. The display driver must support scan lines.
 
If the display driver does not draw its own wide borders, GDI simulates wide
borders using alternate-fill polygons. Some display drivers intentionally
take advantage of this by supporting alternate-fill polygons, but not wide
borders.
 
If the display hardware provides support for styled borders, the driver
should use the hardware to draw the borders. Although GDI can simulate
styled borders, such simulations are always slower than using hardware.
 
Hardware Capabilities
 
The hardware for some displays supports many of the line, curve, and polygon
capabilities that Windows itself supports or expects the display driver to
support. A display driver should take advantage of display hardware whenever
possible because it often dramatically improves performance.
 
The only disadvantage to using display hardware is if the hardware cannot
produce its output both on the screen and in memory bitmaps. Windows
requires that for any figure drawn on the screen, the display driver must
also be able to draw it in a memory bitmap. If the hardware cannot access
memory bitmaps, the display driver must include code that emulates the
algorithms used by the display hardware. Depending on the complexity of the
hardware code, this may be a difficult and costly task.
 
One alternative to emulating the display hardware is to use video memory as
a temporary bitmap. The driver copies the memory bitmap from system memory
to unused video memory, uses the display hardware to carry out the requested
graphic operations, and then copies the results back to system memory. The
efficiency of this method depends on the relative speed of the graphics
hardware, and the size of the bitmap. This method is not appropriate under
386 enhanced-mode Windows if the display driver does not have full access to
the hardware capabilities or to offscreen video memory while running in the
background, that is, while the user is running a non-Windows application in
text mode. This method is not appropriate if the display driver cannot
support monochrome bitmaps. (All display drivers must support output to
monochrome bitmaps regardless of whether the display adapter is color or
monochrome.) Finally, this method is not appropriate for device-independent
bitmaps (DIBs).
 
Partial Support for Capabilities
 
A display driver does not have to provide complete support for a given
capability. Instead, a driver's Ouput function can provide support for a few
specific cases and return all others to GDI for simulation. In such cases,
the display driver sets bits in the GDIINFO structure as if it provided
complete support, but the driver's Output function returns -1 to GDI for all
cases that need simulation.
 
For example, if display hardware can draw polygons with 256 vertices but not
with 257, the Output function can use the display hardware to draw the
smaller polygons and return -1 to let GDI simulate the rest.
 
GDI does not simulate styled lines. If the display driver specifies support
for styled lines, it must provide complete support.
 
 
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