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Creating a Picture with a Host Application Program
A host application program uses GDDM to create a picture. GDDM clips
and transforms it to match the presentation space in the terminal, and
sends the picture segments to the work station in the form of structured
fields within the 3270 data stream.
The Graphics Control Program puts the picture segments in the session's
segment store. The picture is then further clipped and transformed in the
work station to match the window in which it is to be displayed. Finally,
the picture is drawn on the screen using the display unit's vector-to-raster
conversion logic.
Updating a Picture with a Host Application Program
Using GDDM, a host application program can update the picture by adding,
changing, or deleting segments. Subsequently, only the changed segments
need to be retransmitted to the work station. There is no need to
retransmit the entire picture.
Creating or Updating a Picture with a Local Application Program
Local, PC DOS application programs have their own interface, the Graphics
Procedure Interface (GPI), which gives the PC DOS application program
access to the full graphics function of the work station. The application
program uses this interface to create picture segments and to issue the
commands that will draw the picture. The local application program can
subsequently update the stored picture segment by segment.
Graphics Input for Host Application Programs
To use a logical input device such as a locator, a host application program
sends the work station the control data that is needed to initialize the
logical device:
•
What type of logical device is it?
•
What type of hardware device will be used to control the logical device?
•
In what form is input to be echoed on the screen?
Now, as the operator moves, say, the mouse, the Graphics Control Program
uses the data to echo the movement on the screen. The host processor is
not involved and the vectors that describe the picture on the screen are not
at this stage changed.
When the operator completes the move by selecting, the appropriate data is
returned to the host, where the application program can update the picture
and retransmit segments that have changed.
44
Introducing the IBM 3270 Personal Computer/G and /GX Work Stations
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