IBM 2250 3 Manual page 43

the Transfer on Deferred Detects order. Thus, all
elements are logically a single entity for detection.
A single Defer Response to Light Pen Detects order
could serve for all entities. The Enter Graphic
Mode, Absolute Point Plotting order initiates the
entity at an absolute point so that it can only be
moved on the screen by a change in the data con-
tained within the entity. The Enter Graphic Mode,
Absolute Vector order also has been used. The
X- Y value may be modified directly by a Store X-Y
Deflection Registers in Buffer order in the track-
ing subroutine. The Enter Vector Incremental or-
der maintains the physical relationship of elements
of an entity while taking advantage of the compact
data storage format of two-byte vectors.
When no element of the entity is within the light-
pen field of view, the Transfer on Deferred Detects
order is not executed, and the next order will be
the Enable No Switch Detects Operation order.
When any element of the entity is within the light-
pen field view, the buffer program is transferred
to location E. When an entity has been detected
and a Transfer on Deferred Detects order has been
executed, it is necessary to perform such opera-
tions as storing entity identification. In many ap-
plications, an immediate interruption is not desired.
The Di.sabled Pen Detects order permits the buffer
program to proceed to the next entity without loop-
ing on the Transfer on Deferred Detects order.
The Move Immediate Data order stores the entity
identification, LABEL, in a program-specified
location for future input to the CPU. The Enable
No Switch Detect Operation order resets the Dis-
able Pen Detects mode before proceeding to the
next entity. Then, the Transfer Unconditional
order links each entity to the next (or back to)
Start Regeneration Timer order. Additional data
can be added to the buffer program by modifying
the address in the Transfer Unconditional order
location of the last entity in the current program.
Buffer Subroutines
Individual image subroutines, programmed with
incremental orders, can be displayed in multiple
screen locations by positioning the beam initially
with an absolute blanked vector and transferring
control to the subroutine. The subroutine is set
up initially by moving a return address into a
transfer instruction which is the last executable
order in the subroutine.
Input by Light Pen Search
The simplest form of graphic input is implemented
by touching the light pen to the screen, indicating
the entry of a single graphic coordinate. The oper-
ator positions the pen and presses the light-pen
switch to indicate to the buffer program that the
pen position is to be read.
An incremental Graphic mode order or a Char-
acter mode order can be used to generate a raster
scan of the display screen. The CPU will be inter-
rupted upon detection of a raster element, and the
X-Y position of the light pen can be determined
from the character, point, or vector end point
position by executing a Read X-Y Position Regis-
ters command.
Input By Light Pen Tracking
Many graphic input applications require feedback
to the user. This can be accomplished by light-pen
tracking with a buffer program that displays a pat-
tern at the current light-pen position. Key elements
of light-pen tracking are the display of a position-
indicating pattern, sensing light-pen positions rela-
tive to the current pattern position, updating the
pattern position to the new location, and storing the
coordinates of the new pattern position for future
updating or input to the CPU. This tracking opera-
tion can be coupled with a light-pen search routine
if the initial light-pen position is not known. A
sample tracking routine is shown in Table 12.
Feedback by Light Pen Tracking History
In sketching- or drafting-type operations, the user
requires feedback that indicates not only where
the light pen is but, also, where it has been. The
simplest form of tracking history, a straight line,
can be programmed in conjunction with the tracking
subroutine. This can be accomplished by trans-
ferring to a tracking subroutine immediately after
displaying the starting point of the straight line.
The location of the pen pattern on the screen is
specified by an unblanked absolute vector instead
of by a point. Each time the location of the pattern
is updated, the end point is established automati-
cally at the new position. This technique is gen-
erally known as "rubber-banding".
Tracking history for curved lines and for
groups of straight lines can be displayed in a
variety of ways; these involve various amounts of
CPU assistance and buffer programs of varying
complexity. The key considerations of this opera-
tion are (1) when and where to fix a point on the path
of the light-pen movement (determined by the pro-
gram or by the user depressing a programmed
function key to indicate the need for storage to the
program) and (2) where to store the fixed points
(vector end points) in the buffer (controlled by
the program). (See Table 11 for sample routine
to accomplish this.)
2840 Operations with the Channel 43