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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
Showing 442-443 out of 1137
Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 442
SEC. 5.6
USER INTERFACES: KEYBOARD, MOUSE, MONITOR
411
A complete treatment of the GDI is out of the question here. For the interested
reader, the references cited above provide additional information. Nevertheless,
given how important it is, a few words about the GDI are probably worthwhile.
GDI has various procedure calls to get and release device contexts, obtain infor-
mation about device contexts, get and set device context attributes (e.g., the back-
ground color), manipulate GDI objects such as pens, brushes, and fonts, each of
which has its own attributes. Finally, of course, there are a large number of GDI
calls to actually draw on the screen.
The drawing procedures fall into four categories: drawing lines and curves,
drawing filled areas, managing bitmaps, and displaying text. We saw an example
of drawing text above, so let us take a quick look at one of the others. The call
Rectangle(hdc, xleft, ytop, xright, ybottom);
draws a filled rectangle whose corners are (
xleft
,
ytop
) and (
xright
,
ybottom
). For
example,
Rectangle(hdc, 2, 1, 6, 4);
will draw the rectangle shown in Fig. 5-37. The line width and color and fill color
are taken from the device context. Other GDI calls are similar in flavor.
0
0
1
2
3
4
5
6
7
1
2
3
4
5
6
7
8
Figure 5-37.
An example rectangle drawn using
Rectangle
. Each box represents
one pixel.
Bitmaps
The GDI procedures are examples of vector graphics. They are used to place
geometric figures and text on the screen.
They can be scaled easily to larger or
smaller screens (provided the number of pixels on the screen is the same).
They
are also relatively device independent.
A collection of calls to GDI procedures can
be assembled in a file that can describe a complex drawing. Such a file is called a
Page 443
412
INPUT/OUTPUT
CHAP. 5
Windows
metafile
and is widely used to transmit drawings from one Windows pro-
gram to another. Such files have extension
.wmf
.
Many Windows programs allow the user to copy (part of) a drawing and put it
on the Windows clipboard. The user can then go to another program and paste the
contents of the clipboard into another document. One way of doing this is for the
first program to represent the drawing as a Windows metafile and put it on the clip-
board in
.wmf
format. Other ways also exist.
Not all the images that computers manipulate can be generated using vector
graphics. Photographs and videos, for example, do not use vector graphics.
In-
stead, these items are scanned in by overlaying a grid on the image. The average
red, green, and blue values of each grid square are then sampled and saved as the
value of one pixel. Such a file is called a
bitmap
.
There are extensive facilities in
Windows for manipulating bitmaps.
Another use for bitmaps is for text. One way to represent a particular character
in some font is as a small bitmap.
Adding text to the screen then becomes a matter
of moving bitmaps.
One general way to use bitmaps is through a procedure called
BitBlt
. It is cal-
led as follows:
BitBlt(dsthdc, dx, dy, wid, ht, srchdc, sx, sy, rasterop);
In its simplest form, it copies a bitmap from a rectangle in one window to a rectan-
gle in another window (or the same one).
The first three parameters specify the
destination window and position. Then come the width and height.
Next come the
source window and position. Note that each window has its own coordinate sys-
tem, with (0, 0) in the upper left-hand corner of the window. The last parameter
will be described below. The effect of
BitBlt(hdc2, 1, 2, 5, 7, hdc1, 2, 2, SRCCOPY);
is shown in Fig. 5-38. Notice carefully that the entire 5
×
7 area of the letter A has
been copied, including the background color.
BitBlt
can do more than just copy bitmaps. The last parameter gives the possi-
bility of performing Boolean operations to combine the source bitmap and the
destination bitmap. For example, the source can be ORed into the destination to
merge with it.
It can also be EXCLUSIVE ORed into it, which maintains the char-
acteristics of both source and destination.
A problem with bitmaps is that they do not scale.
A character that is in a box
of 8
×
12 on a display of 640
×
480 will look reasonable. However, if this bitmap is
copied to a printed page at 1200 dots/inch, which is 10,200 bits
×
13,200 bits, the
character width (8 pixels) will be 8/1200 inch or 0.17 mm.
In addition, copying
between devices with different color properties or between monochrome and color
does not work well.
For this reason, Windows also supports a data structure called a
DIB
(
Device
Independent Bitmap
). Files using this format use the extension
.bmp
.
These files
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