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GETIING ACQUAINTED
UNDERSTANDING XV-COORDINATES
To enter graphics you will be using xy-coordinates
.
These are locations in the
terminal's drawing space
,
which is 4096 by 4096 points
.
To create graphics
images, you must specify xy-coordinates ranging from 0 to 4095 on the horizontal
(x) and vertical (y) axes of the screen.
The Graphics Position
The graphics position is the last
xy-coordinate
point specified in a graphics
command (in some cases
,
this point is implied
-
more on this
later).
The graphics
position becomes the starting point that the terminal uses when executing the next
graphics command
.
When you turn on the terminal
,
the graphics position is at the upper-left
corner
of
the screen (0,3071
).
When drawing an
image
,
the first thing you must do is specify
the graphics position - that is
,
where you want to start
drawing.
You can do this
in
a number of different ways, some of which you'll see
in
this exercise
.
CORRECTING ERRORS WHILE DEFINING PANELS AND
SEGMENTS
When creating a segment or panel
,
you might enter a command with parameters
that are not what you intended but that are acceptable to the terminal
;
for example
,
the color may be wrong or a line draws to the wrong point.
If you notice a wrong parameter before pressing Return
,
it's easy to correct
it
using
Rub Out or Ctrl-X
.
But if you have
already
pressed the Return
key,
you have two
options
:
•
If the mistake is in a FILLPATTERN
command (explained
below), and this is the
only command you have entered
,
then simply reenter the command
correctly.
The latest FILLPATTERN command setting overrides any previous settings
.
•
If the mistake is in an SGOPEN
,
BEGIN PANEL
, DRAW,
or CURVE command
(explained next), you'll have to delete that segment and begin again. To delete a
segment, enter SGDELETE and the number of the segment to be
deleted.
For
example:
SGDELETE 1
In this command, SGDELETE with the parameter of 1 deletes Segment
1
and
erases it from the screen
.
To start over after deleting the
segment,
you would begin with the SGOPEN
command (as described next) and reenter the entire sequence of correct
commands.
4106/4107/4109 OPERATORS
2-9
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