Advanced MFC Programming

Chapter 12. Screen Capturing & Printing
12.3 Simple Printing
Although we didn’t write a single line of code to implement printing feature, all our SID or MDI
samples have the default printing functionality. This includes default printer set up, print preview, and
printing the client window. Like display, printer is another type of graphic device that can be used to output
drawings. Its interface to the software programmer is similar to that of display: instead of writing code to
control the hardware directly, we can use DC to output drawings to the printers. Actually we can call
member functions of class CDC to output dot, line, curve, rectangle and bitmaps to a printer.
Mapping Mode
However, there are some differences between printing devices and display devices. One main
difference is that two devices may have different capabilities. Because all displays have similar sizes and
resolutions, it is relatively convenient to measure everything on the screen using pixel. For example, it
doesn’t make much difference if we display a 256×256 bitmap on an 800×600 display or a 1024×768
display. Since every window is able to display an object that is larger than the dimension of its client area
(using scroll bars), it is relatively easy to base every drawing on the minimum possible unit ⎯ pixel.
For printers, this is completely different. There are many types of printers in the world, whose
resolutions are remarkably different from one another. For example, there are line printers, one pixel on this
kind of printers may be 0.1mm×0.1mm; also, there are many types of laser printers, whose resolution can
be 600dpi, 800dpi or even denser. If we display a 256×256 image on the two types of printers, their sizes
will vary dramatically.
Anther difference between printing devices and display devices is that when doing the printing, it is
desirable to make sure that all the outputs fit within the device. For a window, since we can customize the
total scroll size, it doesn’t matter what the actual output dimension is (The scroll size can always be set to
the dimension of output drawings). For the printer, we need to either scale the output to let it fit within the
device or print the output on separate papers.
In order to handle this complicated situation, under Windows, OS and devices have some common
agreements. When we draw a dot, copy a bitmap to device, everything is actually based on logical units
(pixels). By default, the size of one logical unit is mapped to one minimum physical pixel on the device,
however, this can be changed. Actually class CDC has a function that let us customize it:
virtual int CDC::SetMapMode(int nMapMode);
Parameter nMapMode specify how to map one logical unit to physical units of the target device. By
default it is set to MM_TEXT, which maps one logical unit to one physical unit. It can also be set to one of the
following parameters:
Parameter
Meaning
MM_HIENGLISH One logical unit is mapped to 0.001 inch in the target device
MM_HIMETRIC One logical unit is mapped to 0.01 millimeter of the target device
MM_LOENGLISH One logical unit is mapped to 0.01 inch of the target device
MM_LOMETRIC One logical unit is mapped to 0.1 millimeter of the target device
MM_TWIPS One logical unit is mapped to 1/1440 inch
Instead of mapping one logical unit to a fixed number of pixels, it is mapped to a fixed size on the
target device. It is the device driver’s task to figure out the actual number of pixels that should be used for
drawing one logical pixel. By doing this type of mappings, the output will have the same dimension no
matter what type of target device we use.
There is one difference between MM_TEXT mapping mode and other modes: for MM_TEXT mode, the
positive y axis points downward. For other mapping modes, the positive y axis points upward. So if we
decide to use one of the mapping mode listed above, and the origin of the bitmap is still the same, we need
to use negative values to reference a pixel’s vertical position (Figure 12-5).
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