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How the output functions are implemented depends on whether or not the
device uses banding. Banding devices have their output stored in a metafile.
This metafile is replayed for every band that is rendered (either by GDI or
applications that wish to implement banding). Therefore, output coordinates
must be mapped into the current band, and output outside of the band must be
clipped.
 
Nonbanding devices perform output to the device in one pass. Therefore, the
device must have access to the entire display surface. Drivers must be able
to perform all the output functions to both the display surface and to
memory bitmaps. This restriction would make it very difficult for devices
that supported complex drawing primitives if it were not for the help that
GDI and the display driver supply.
 
In the GDIINFO structure, banding color drivers should use the same
bits-per-pixel and planes values used for band bitmaps.
 
Devices choose numbers that match how they output color. Color printers that
use the color dot-matrix libraries specify the same values as the bitmap
format. The color library in this DDK uses one bit-per-pixel in three
planes. The driver developer must modify the library to use another format.
 
Raster vs. Vector Devices
 
Many printers (such as dot-matrix and most laser printers) are raster
printers, that is, they print out text and graphics as bitmaps or raster
lines. Other devices (such as plotters and PostScript-based printers) are
vector devices, which draw text and graphics as a sequence of vectors or
lines. (Although PostScript printers are based on raster engines, the
language itself is vector oriented except where bitmaps are concerned.)
 
Raster devices usually have constraints that cause problems for implementing
the full GDI device model. Raster devices, for example, do not implement any
vector graphics operations. Therefore, all vector graphics must be drawn as
a bitmap before printing. Some devices, such as dot-matrix printers, do not
allow the driver to print anywhere on the page. They require that text and
graphics be output in the order of the print direction position on the
page.
 
These bitmaps can be enormous for a device such as a 300 dpi laser printer.
In such cases, the driver breaks up the page into smaller rectangles that
are printed individually. For each of the rectangles, GDI or the application
will draw all the graphics that fit in each rectangle into a bitmap and then
print each individual bitmap.
 
These rectangles are called bands, and the printing process that uses these
bands is called banding. It is usually necessary to band raster printers;
however, banding is not necessary for vector devices.
 
For vector devices (that is, nonbanding devices), the application calls GDI
graphics functions, which are translated into device-driver graphics
primitives. After each page is printed, the application uses the NEWFRAME
escape to eject the page.
 
An application can either treat the driver as if it were a nonbanding device
by calling the GDI functions and ending each page with the NEWFRAME escape,
in which case GDI performs the banding, or it can handle the banding
itself.
 
The NEXTBAND Escape
 
Before any graphics are drawn, the driver is called upon to perform the
NEXTBAND escape. When the Control function is called for the NEXTBAND
escape, lpInData points to a POINT structure, and lpOutData points to a
RECT structure.
 
The driver should initialize its band bitmap and set the RECT structure to
the size, in device coordinates, of the rectangle that the band represents
on the page.
 
GDI adds the POINT structure to determine the scaling factor for graphics
output. Some devices support the use of graphics at a lower resolution than
text to allow for faster output. The x-coordinate of the POINT corresponds
to horizontal scaling and the y-coordinate to vertical scaling.
 
The value in the structure corresponds to a shift count. A point of (0,0)
specifies graphics at the same density as text, whereas a point of (1,1)
specifies half-density graphics in both directions, for example, a 300 dpi
laser printer printing bitmaps at 150 dpi.
 
GDI then calls the driver's Output function to draw text or graphics in the
band bitmap. When all drawing for the band is finished, GDI calls the driver
with another NEXTBAND escape. The driver draws the band in the band bitmap,
reinitializes the bitmap, sets a new rectangle, and continues with the next
band as it did with the first.
 
When all the bands on the page are exhausted, and the driver receives a
NEXTBAND escape, it should output the last graphics band and then set the
rectangle pointed to by the lpOutData parameter to an empty rectangle to
indicate that there are no more bands on the page. It should also perform
all the processing necessary to eject the completed page. The next NEXTBAND
escape will correspond to the first band of the next page.
 
If the application performs banding, it will call the Escape function to get
the band rectangles. If GDI is handling banding for an application, then GDI
collects all the graphics functions on a page into a metafile, that is, a
temporary file containing a list of the graphics functions and their
parameters. When the application calls Escape to carry out the NEWFRAME
escape, GDI turns this escape into a sequence of NEXTBAND calls to the
Control function. GDI sets the clip region for the actual printer device
context to the band rectangle and then plays back the metafile, which
recreates all of the application's output in the band bitmap. GDI does this
for each band until the band rectangle returned by the driver is empty.
 
Some devices, such as raster laser printers, allow text to be placed
anywhere on the page at any time. Furthermore, these printers do not place
text into the band bitmap, since all the device fonts exist in printer or
cartridge memory. To optimize text output, their drivers use a single,
full-page band for all the text output and a sequence of smaller bands for
bitmapped graphics.
 
As an optimization, some of these drivers maintain a flag to detect whether
or not any output, other than text, is attempted during the first, full-page
band. If not, the driver skips the graphics bands.
 
The BANDINFO Escape
 
Some devices, such as laser printers, can print text and graphics anywhere
on the page but still require banding support for vector graphics
operations. Since these devices usually use their own internal device fonts,
they can greatly improve their text printing performance by using a single,
full-page band for text only as the first band. The driver ignores graphics
calls during this band and handles only the ExtTextOut or StrBlt functions.
Graphics are printed on subsequent, smaller bands.
 
An application that is aware of this process can speed up its printing
operation by determining whether text or graphics will be printed on the
current band. It may do so using the BANDINFO escape. The application can
also use BANDINFO to optimize the banding process.
 
 
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