80Bus News 
March–April 1983, Volume 2, Issue 2 



Page 31 of 55 




GRAPHPAC COMMAND MANUAL  Page 5 

PENERA or PE
All following DRAW, DRAWTO, DOCAP and PLOT commands will cause any pixels the CAP passes through to be erased.
PENRET or PR
This allows the CAP to return to the coordinates that existed before the last DRAW, DRAWTO or PLOT commands.
DRAWTO X,Y
To draw, erase or invert a line based on coordinates specified in rectangular notation.
The CAP coordinates are changed to those specified by X,Y and a line may be drawn, erased or inverted (depending on the current PEN mode) to these coordinates from the previous CAP. The previous CAP coordinates are saved in case of a following PENRET command.
DRAW X,Y
To draw, erase or invert a line based on relative coordinates.
The DRAW command has a similar syntax to the DRAWTO command however the X,Y values supplied are added to the current CAP coordinates to produce a new CAP. Draw is useful if an object or shape has to be produced several times but at different screen coordinates. The shape is specified by a subroutine using the DRAW command and whenever the routine is called, the shape will be produced relative to the current CAP.
RATIO N
GRAPHPAC COMMAND MANUAL  Page 6 

To adjust for the height to width ratio of the screen when using PLOT or CIRCLE. The argument passed with the RATIO command will be used to modify the Y axis coordinates.
Normally the display on the screen is taller than it is wide and a circle plotted on the screen would appear oval with the sharp ends in the vertical plane. RATIO allows the user to introduce a correction factor whenever PLOT or CIRCLE are used so that a circle does in fact appear as a circle. RATIO is followed by a single argument. This argument should be in the range 0 – 511. If 511 is exceeded the value will be MOD 512 (513 will be treated as 1). A value of 512 (or zero) will cause no correction to take place and a true (distorted) display will be produced. A value of one will cause maximum correction to take place and in fact a circle will appear as a single horizontal line on the screen. The optimum value is somewhere between the two and should be decided by the user for his particular hardware configuration.
PLOT A,D
To draw, erase or invert a line based on coordinates given in polar notation.
The new CAP coordinates are calculated, based on a specified angle “A” and distance “D”. “A” is an angle between 0 and 359 degrees, which is calculated clockwise, zero degrees being towards the top of the screen. This value can be exceeded as a wrap around effect takes place, 360 degrees being the same as 0 degrees, 450 degrees being the same as 90 degrees etc. “D” is a distance specifying the number of pixels. This distance is modified automatically by a value dependant on the argument given in a previous RATIO command and also the value of the angle A. As the argument given in the RATIO command only modifies calculations in the Y axis, there will be no modification to the length of lines plotted at angles 90 and 270 degrees, however as the angle nears the vertical (0 or 180 degrees) the modification will be at it’s greatest. The old CAP coordinates are saved in case of a following PENRET command.
CIRCLE R,A1,A2
To draw a circle or are on the screen.
A circle or arc is drawn the centre of which is the CAP and radius is R. Al and A2 must be positive values with A2 greater than A1. An arc will be drawn from angle 1 (A1) to angle 2 (A2) in a clockwise direction. If angle 1 is zero and angle 2 is 360 a circle will be drawn. The maximum radius is 511 and if this is exceeded the radius will be MOD 512 as for the distance in the PLOT command.



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