Important Notice

The pages on this site contain documentation for very old MS-DOS software, purely for historical purposes. If you're looking for up-to-date documentation, particularly for programming, you should not rely on the information found here, as it will be woefully out of date.

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                         About Painting and Drawing
 
This topic describes presentation spaces, device contexts, and window
regions, and how an application uses them for painting and drawing. You
should also be familiar with the following topics:
 
    ◄Frame windows►
    ◄Presentation spaces and device contexts►
    ◄Messages and message queues►
    ◄Graphics programming interface►
    ◄Drawing in windows►
 
An application typically maintains an internal representation of the data
that it is manipulating. The information displayed by a screen, window, or
by printed copy is a visual representation of some portion of that data. MS
OS/2 provides a rich environment for displaying information in a variety of
ways. This topic introduces concepts and strategies necessary to make your
application function smoothly and cooperatively in the MS OS/2 display
environment.
 
Presentation Spaces and Device Contexts
 
A presentation space is a data structure maintained by the operating system
that describes the drawing environment for an application. An application
can create and hold several presentation spaces, each describing a different
drawing environment. All drawing in an MS OS/2 application must be directed
into a presentation space.
 
Each presentation space is associated with a device context that describes
the physical device where graphics commands are displayed. The device
context translates graphics commands made to the presentation space into
commands that cause the physical device to display information. Typical
device contexts are the screen (windows), printers and plotters, and
off-screen memory bitmaps.
 
By creating presentation spaces and associating them with particular device
contexts, an application can control where its graphics output appears.
Because a presentation space and device context isolate the application from
the physical details of displaying graphics, the same graphics commands can
typically be used for many types of displays. This virtualization of output
can reduce the amount of display code that an application needs to support.
 
This topic discusses how an application sets up its presentation spaces and
device contexts before drawing and how to use window-drawing functions.
Other topics in this database discuss the individual graphics routines
available in MS OS/2.
 
Window Regions
 
A window and the presentation space associated with it have three regions
that control where drawing takes place in the window. These regions ensure
that the application does not draw outside the boundaries of the window or
intrude into the space of an overlapping window.
 
Region          Description
────────────────────────────────────────────────────────────────────────────
Update region   This region represents the area of the window that needs to
                be redrawn. This region changes when overlapping windows
                change their Z order or when an application explicitly adds
                an area to the update region to force a window to be
                painted.
 
Clip region     This region and the visible region determine where drawing
                takes place. Applications can change the clip region to
                limit drawing to a particular portion of a window.
                Typically, a presentation space is created with a clip
                region equal to NULL, which means that there is no clipping
                supplied by this region.
 
Visible region  This region and the clip region determine where drawing
                takes place. The system changes the visible region to
                represent the portion of a window that is visible.
                Typically, the visible region is used to mask out
                overlapping windows. When the application calls the
                WinBeginPaint function in response to a WM_PAINT message,
                the system sets the visible region to the intersection of
                the visible region and the update region to produce a new
                visible region. Applications cannot change the visible
                region directly.
 
Whenever drawing occurs in a window's presentation space, the output is
clipped to the intersection of the visible region and the clip region.
 
 
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