Amiga Computing - Commodore Is Awesome

Phil South looks at
plotting and
drawing functions
in AMOS
The fun part about being your friendly
local AMOS columnist is that I have
absolutely no idea of your level of
understanding on AMOS. This is both
a blessing and a curse. Do you want machine
code widgets or do you want basic code gener
ation? Do you want graphics or procedures? My
answer is to put a wide range of things into the
column and let them find their own level. From
the mail I receive via post and the Internet, this
approach seems to be working, as almost
everyone gets what they want eventually, and
as long as it's entertaining you don't seem to
mind what goes on.
AMOS is one of those things which is all
things to all men. Some view it as a means of
putting ideas into action, others see it as per
sonal development, a way to stretch them
selves. Either approach is worthwhile and I'm
sure you'll enjoy the bits and bobs I've got lined
up for the next few months. In the meantime,
thanks for all your mail - let's get going with
this months AMOS curiosity.
Top of the Plots
Something a lot of beginners ask me about is
how to make marks on the screen such as
lines, dots, squares etc. The reason I get asked
this has something to do with the old Amiga
Basic, as this was the only kind of graphics you
could produce. I like to call it Etch a Sketch
graphics, as it is based on lines and dots and
boxes, just like the old two knob drawing toy.
Ifs easy. The first instruction you should look
at is PLOT. This is the easiest command of all
and simply places a dot, in the currently select
ed ink colour, on the screen, like so:
Ink 9 : Plot 30,56
This places a dot of colour 9, 30 pixels along
the screen and 56 pixels down. There is a easy
way to specify the colour, if you want it differ
ent from the Ink colour:
Plot 30,56,9
This is a useful command tf you want to create
a starfield in your game, like in the old classic
Galaxians (which may or may not be a bit
before your time...} like SO:
Do
Cls : Curs Off
Loop
Plot ,Rnd(255), Rnd(IS)
Going dotty.
Amiga Computing
MARCH 1997
Plots
Thicken
This fills the screen with multicoloured dots. If
you don't want those of colour 3 to flash, add
the command:
Flash Off
Okay, that's dots, what about lines? We do this
in pretty much the same way:
Drau 30,56 To 150,200
This draws a line from the first point mentioned
to the last. Simple enough. You can change the
look of the line by adjusting the line style:
Cls : Ink 2
Set Line SFOFO
Box 50,100 To 150,UO
Set Line 11100110011001100
Box 60,110 To 160,160
The line style is set using the SET LINE com
mand and this allows you to set the variation of
the pixels in the line. This is a nice and very
underused feature, so write something that uses
it, eh?
The final bunch of features are the box and
circle commands. To make a box, like the one
we just saw in the SET LINE program, we have
to set the top left and bottom right corners of
the box. So:
Box 10,10 To 3D,30
makes a box 20 by 20 starting at 10,10 and
ending at 30,30. The circle command is similar
again, but this time you set the centre of the cir
cle and the radius (the distance from the mid
dle to the outside of the circle, for those of you
who fell asleep in maths). So you can do this:
Circle 100,100,50
Allied to this is the POLYLINE command, which
draws multiple lines. This is like drawing a line
with the DRAW command, only you keep going,
Polyline 10,10 To 2D,30 To 60,3D To 58,10 To
200,100
and so on. You can build up quite complicated
pictures like this, but it would take a bit of
graph paper and a lot of sweat to make a
human face.
There was a program once, 1 can't even
remember if I printed it, but it took a drawing
you made with the mouse and turned it into
the code you would need to draw it from poly
lines! tf anyone can find this program I'd be
grateful if they would e-mail it to me.
That's enough for now, I'll move on into
more complicated graphics next time, until then
Happy Amos!
Write
STUFF
If you have any other AMOS pro
grams or queries about AMOS,
please write to the usual address,
which is: Phil South, Amos Column,
Amiga Computing, Media House,
Adtington Park, Macclesfield, SK1O
4NP. Or you can send e-mail to me
on phil@webscape.co.uk, or via my
home page on http://www.webscape.
co.uk/phil/.
Please send routines on an Amiga
disk with notes on how the program
works on paper, not as text files on
the disk. Make the routines short
enough to appear in print, i.e. no
more than about 30-40 lines of code.
If possible make them use no exter
nal graphics, if they can't be used
without them, be sure to provide
them on the disk in native IFF for
mat, the same goes for sound files.