Part I: Do the Twist!

DEFINITIVE TRICKS PART I - THE TWIST SCROLL
by Manikin of The Lost Boys (TLB)

Hi, we are the Lost Boys, a demo crew from London, England. We
have just released our newest and as yet biggest demo on the ST,
it is called the DEFINITIVE demo. If you haven't got the demo yet
then you are unlikely to know that there are 4 members of the
Lost Boys. The youngest member is Spaz, age 15, he is our
graphixx, sample and MIDI specialist. Next comes Sprog, coder
extrordinaire (shit at everything else but a very good machine
code programmer!! age 15). Sammy Joe, age 15 (nearly at the age
of consent!!), he is a general dogsbody and the man with the
filofax!! Finally there is me, Manikin, age 18, machine code
programmer and also a little bit of graphixx!!.

Before we start in ernest I must warn you that this scroll is
really pretty complicated to describe and also to program. So I
advise that if you have never written a scroll before then you
certainly should not start with this one. YOU HAVE BEEN WARNED!

This article concerns one of the many screens on our DEF demo:
The Twist screen, programmed by Manikin, the original idea for
this scrolline appeared on the Amiga a little while ago, the
first ST coders to attempt it were XXX- International (ex UNION).
The idea of the scroll is to give the appearance of twisting a
scroll around an imaginary bar, it looks very much like twining
two pieces of string around each other, the scroll rotates around
and appears on the back of the bar but upside down and then moves
back around the bar again.
In case you hadn't noticed this is really very hard to
describe!! Therefore the best thing to do would be to assemble
the source on this disk to see what the final effect is, or refer
to a copy of our DEF demo, you'll see what I'm trying to say!!
But anyway I shall continue bashing away at this keyboard while
being deafened by some really naff heavy metal music that Spaz
has just started to play outside my bedroom. I believe it is by
Helloween and it really is incredibly bad.

Sammy Joe and Spaz: "Oh no it Isn't!!"
Manikin: "Oh Yes it is!!"
Sammy Joe and Spaz: "Oh no it Isn't, OUCH !!"

Normally I (Manikin) would not resort to physical violence but
this really was bad. Anyway now the heavy metal is going to be
switched off and replaced by some S-Express.
"Ahhhhh! Decent music at last!", says Manikin.
"Aaarrghhh!", says Sammy Joe and covers his ears.

Anyway enough of the spurious talk and back to the Twist scroll.
The trick for this scroll lies in the construction of the fonts
and how the original scroll is drawn out. Before being drawn onto
the screen around the imaginary bar the scroll is constructed in
a buffer. This buffer is in fact just a diagonal scroll which
wraps around after 80 lines have been drawn. If you assemble the
source code which is present on this disk (done with DEVPAC 2.08
ASSEMBLER) then the buffer is the top part of screen.
As I have said the trick in this scrolline lies in the
construction of the fonts.

The original font is simply a file containing 1-plane graphixx
for the 60 characters from the " " to the "Z" in the standard
Atari font. Each character is 36 lines high but there are
graphics in only the top 28 lines, for reasons that will be
explained in a moment. The characters are 16 pixels wide, this is
1 word or 2 bytes. So the format is 60 (characters) x 36 lines x
2 bytes (1 word) = 4320 bytes.
(The standard font should be on the disk if those ST NEWS guys
have been kind and allowed us the space!!)
The font is then copied out 8 times in memory, but with one
extra word before every word that contains graphixx. Each
character in the first font is then moved down by one line. In
the second font each is moved down by 2 lines and in the third
font, 3 lines etc. This is why the empty lines were needed in the
original font. Now every character in the second font must be
shifted over by 2 pixels. this is done using the instructions

lsl.w and roxl.w (see source for full example)

This is why the extra words in the copies of the font were
required. The characters in the third copy of the font are
shifted over by 4 pixels, the fourth by 6 pixels etc.
This now means that we have 8 copies of the original font each
copy shifted over by 2 pixels and down by 1 line. when animated
these characters will appear to move diagonally!

Well as before this description really is very badly worded and
complicated so for a better idea of whats happening please refer
to the source code. Oh god I think my brain is going to frazzle
if I write much more of this, still I know everyone will be very
dissapointed if I don't carry on, won't you?
No?
Oh go on just a little dissapointed!

Thanks I feel a lot better now.

Right, on we must press I guess, Manikin is a poet and he
doesn't know it!
The scroll is constructed in the buffer. Basically the scroll
is drawn on the screen from left to right, the first word of
every character is ADD.W 'ed to the screen while the 2nd word is
MOVE.W 'ed and hence the scroll is constructed (For full and more
accurate details take a look at the source code).

And now the actual making of the twist scroll. The scroll is
constructed from the buffer in the following fashion.
A data table in the program tells the copying routine how many
times to display each line of the buffer, by missing out lines or
by displaying them more than once it is possible to give the
impression that the scroll has been wrapped around the bar. The
buffer is actually copied twice to the screen, once in plane 1,
the correct way up and secondly in plane 2 but upside down.

And there you have it: The twist scroll. By changing the palette
so that the color for plane 1 set and for plane 1 & 2 set is the
same the correct way up scroll will then appear to be in front of
the upside down one. Other additions that you could make to this
scroll would be a raster display to shade the scroll thus making
it appear more 'bar like', or perhaps continued movement up and
down, it is really up to you.
The fonts are also important, to increase the effect of the
scroll you could possibly draw the font at a slant (the standard
font that we use is drawn like this). You still must not use more
than the top 28 lines of each character, but draw them with a
slant of 2 pixels per line and they will then appear to move
smoothly along the diagonal!

I'm sorry if this article seems a little garbled and perhaps a
bit rushed, but you find me (Manikin) at a time of great stress
and very little free time for article writing, in less then 6
weeks I have A-levels (Maths, Physics and Chemistry but not
Computer Studies!!) so I'm really rather worried about them, but
I did promise the guys at ST NEWS that I'd write them a little
article about my Twist scroll and as they say the rest is
history.

If you have any comments etc. on this article then please write
to ST NEWS. With luck, this will not be our last article for
them, and so until next time, Goodbye!!

The files belonging to this article are "TWIST.ASM" and
"TWIST.DAT", both in the "PROGRAMS" folder of this issue of ST
NEWS. The latter file should be copied into the directory in
which your assembler is located, after which the .ASM file can be
assembled and executed. The program only functions properly in
low res.