GRAPHPAC COMMAND MANUAL Page 3
20 FOR A=0 T0 30
30 PSET A,A
40 NEXT A
This short routine will draw a diagonal line on the screen.
PRESET (pixel reset)
To RESET a pixel.
X and Y are graphic co-ordinates.
20 FOR A=0 TO 30
30 PSET A,A
40 NEXT A
50 FOR A=O TO 30
60 PRESET A,A
7O NEXT A
80 GOTO 20
This routine will continuously draw and erase a line on the screen.
PTEST (pixel teat)
PTEST X,Y: A®PEEK (PVALUE)
To test if a specified pixel on the screen is dark or bright.
Normally PTEST is implemented as a BASIC function, so that the line
A=PTEST X,Y would be valid. It has been impossible in the space allocated for
Mbaslink to include functions in its command table, therefore this function
has to be completed in two stages. PTEST X,Y tests the specified pixel and
stores the result in a memory location. It is then the responsibility of the
BASIC program to retrieve this value by PEEKing this location. In the
statement line above, the numeric variable would hold the result of the pixel
teat. If the test confirmed that the pixel was bright, the result in A would
be 1. If the pixel was dark the result would be 0. If the specified X,Y
coordinate does not lie within the screen display area the result would be 2.
The address of (PYALUE), the location to PEEK is given in the Graphpac
GRAPHPAC COMMAND MANUAL Page 4
A collection of PEN commands have been made available in Graphpac. The
PEN commands control an invisible graphics cursor. The position of this cursor
we have chosen to call the CAP (Current Active Point). The CAP can be moved
around on or off the display area by various graphic statements and depending
on the PEN mode active at the time, a line may be drawn, erased, or inverted.
The commands that control the PEN mode may he abbreviated if required.
In the interests of speed Graphpac works in 16-bit arithmetic, this
limits the distance the CAP may be moved in the X or the Y plane, in one
operation to 32767. It is therefore probable that moving the CAP from the
centre of the screen to X,Y co-ordinates -32767,-32767 in one operation may
introduce a small error which can be confirmed by using the CAP command.
STARTAT (Start at)
STARTAT X,Y or SA X,Y
This command allows the CAP to be moved unconditionally to the
specified X,Y graphics co-ordinate aa if the PENUP mode is effective.
STARTAT would normally be used as part of the first graphic statement
of a program, to position the CAP ready for subsequent plotting. The current
PEN mode is not altered. PENRET will now be inactive until a subsequent
DRAWTO, DRAW or PLOT command is encountered.
PENUP or PU
To allow the CAP to be moved around the display area using DRAW,
DRAWTO or PLOT commands without producing a trace or changing the existing
PENDOWN or PD
All following DRAW, DRAWTO, PLOT or DOCAP commands will cause any
pixels the CAP passes through to be set bright.
PENFLIP or PF
All following DRAW, DRAWTO, DOCAP and PLOT commands will cause any
pixels the CAP passeg through to be inverted, that is, dark pixels will be SET
bright and bright pixels will be RESET dark.