│         W E L C O M E         │
                   │  To the VGA Trainer Program   │ │
                   │              By               │ │
                   │      DENTHOR of ASPHYXIA      │ │ │
                   ╘═══════════════════════════════╛ │ │
                     ────────────────────────────────┘ │

                           --==[ PART 12 ]==--

■ Introduction

Hello! :-)

Well, a lot has happened since the last trainer, which is the reason for
the amazingly long delay. First, the elections. These were quite easy
actually, I went and watched a move (Demolition Man) (Election day
special, all movies R2, which is about 50 US cents), then went and voted
after most voters had gone home, so no long lines ;-). Soon after were
exams. These did not go too well, and I am not looking forward to the
results. Finally, I got measles and pneumonia at the same time and was
sent off to hospital for a few days. All in all, not events which are
conducive to coding! This has meant that the trainer has been delayed,
and ASPHYXIA was not able to enter into the local democompo, Dexterity
'94, which we were hoping to do well in. Oh well, onward and upward!

This trainer is on full screen scrolling in Chain-4, by request. This is
actually very easy to do (and smooth), and holds a lot of potential, as
I am sure you can immediately imagine.

A few more things : People have been saying they have had hassles
sending me email, and I have found that this is because they forget the
numbers in my name. They send mail to smith@batis... which does not
exist, or smith@beastie... which is my brothers account. He is getting a
bit sick of forwarding my mail to me ;). The two address are :

I have lost about 200k worth of email, chalk it up to my beginner status
at Unix. The test to see if your mail got through? I have answered
_every_ email message sent to me (no easy task), so if you haven't got a
reply, please resend the message.

You can now also send a group message to all members of Asphyxia. Just
send mail to asphyxia@beastie.cs.und.ac.za and we will all get a copy
... which could mean numerous replies to one querey ;)

If you would like to contact me, or the team, there are many ways you
can do it : 1) Write a message to Grant Smith/Denthor/Asphyxia in private mail
                  on the ASPHYXIA BBS.
            2) Write to Denthor, EzE, Goth, Fubar or Nobody on Connectix.
            3) Write to :  Grant Smith
                           P.O.Box 270 Kloof
                           South Africa
            4) Call me (Grant Smith) at (031) 73 2129 (leave a message if you
                  call during varsity). Call +27-31-73-2129 if you call
                  from outside South Africa. (It's YOUR phone bill ;-))
            5) Write to smith9@batis.bis.und.ac.za in E-Mail.
            6) Write to asphyxia@beastie.cs.und.ac.za

NB : If you are a representative of a company or BBS, and want ASPHYXIA
       to do you a demo, leave mail to me; we can discuss it.
NNB : If you have done/attempted a demo, SEND IT TO ME! We are feeling
        quite lonely and want to meet/help out/exchange code with other demo
        groups. What do you have to lose? Leave a message here and we can work
        out how to transfer it. We really want to hear from you!

■  What is full screen scrolling?

I seem to recall doing this in a previous tut, but here goes again! Full
screen scrolling is when the entire screen moves in a particular
direction, with the new picture scrolling on to the screen. Um. Think of
movie credits. The screen, filled with text, is scrolled off the top of
the screen while the new text is scrolled on from the bottom. This is
full screen scrolling.

Full screen scrolling is not limited to movie credits. Games like Raptor
have you flying over a scrolling landscape while you are shooting down
the bad guys. In this tutorial we will be doing vertical scrolling, but
the code can very easily be altered for horizontal scrolling too.

Remember that we will be using Chain-4 to do our scrolling, so you may
want to brush up on tut 10 in which that was covered. I will assume a
brief knowledge of how chain-4 works for this tutorial.

■  The theory

The theory behind full screen scrolling in Chain-4 is acually very

Picture if you will, a screen that is two monitors high. Chain-4
actually has four, but for this we only need two. Now, for this screen
that is two monitors high, we can only see one monitors worth. Here it
is in ASCII

    +-------------+  Screen two monitors high
    |             |
    |             |
    |             |
    |             |
    ||           ||
    ||           ||<- This is the bit we can see, one monitors worth
    ||           ||

We can move the bit we can see up or down the enlarged screen. So, for
example, if the screen two monitors high had a picture on it, we could
move the bit we see up and down to try glimpse the entire picture. Think
of it in this way : The screen is a large painting, but we can only see
though a small magnifing glass. We can move this magnifing glass around
the painting, but can never see the painting all at once.

This actually works in our favour. Anything done outside the bit we are
looking through cannot be seen, so we can do our work without changing
our screen.

On to scrolling. The method we will use this time is as follows :

1) Draw the next line to be seen just above and just below the part we
   can see.

      +------------+ The enlarged screen
      |            |
      |            |
      |111111111111|  The new part of the picture
      ||          || The bit we can see
      |111111111111|  The new part of the picture

2) Move the view up one pixel so that the new part of the picture is
   visible at the top of the screen.

3) Repeat Steps 1) and 2) until the whole screen is filled. Our screen
   will look as follows :

     ||2222222222222|| Viewscreen

Check this picture with steps 1) and 2), you will see that this is

4) Set our viewport to the bottom of the enlarged screen.

     ||2222222222222|| New position of viewscreen

As you can see, the bit we will be looking at is exactly the same as
before, we are now just at the bottom of the larger screen instead of
the top!

5) Jump back to 1). The entire sequence can begin again, and we can have
   infinate scrolling in the upward direction. Clever huh?

■  Our code

In the sample code, we have 21 different icons. What we do is decide
what the next row of icons is going to consist of. We then draw the next
line of pixels above and below the viewscreen according to what icons we
are displaying. We then scroll up one pixel and begin again. When we
have completed a row of icons, we randomly select a new row and begin
again. Our icons are 16x16, so exactly 20 fit across a 320 pixel screen.

When we hit the top of our enlarged screen, we flip down to the bottom
which looks exactly the same as the screen we have left. In this manner
we have obtained smooth, infinate full screen scrolling!

■  Extra bits

As you will see from the code, it would be the work of but a few minutes
to extend our landscape across the two unused screens, thereby allowing
limited horizontal movement along with our vertical movement. In fact,
the entire routine could easily be made to be a horizontal scrolling

A map of sorts could be generated, with one byte equalling one terrain
type. In this manner, the terrain scrolled over could be set, as in a
flying game (Flying Shark, Raptor etc). The terrain could also easily be
replaced with letters for our movie-style credits.

Free direction scrolling, ie scrolling in all directions, is a very
different matter, with very different methods to get it to work. Perhaps
this will be discussed in a later trainer. But for now, work with this,
know it, understand it, and think up many great things to do with it!
How about a full screen text scrolly? A game? Go wild!

■  In closing

Well, I hope you enjoyed this, the latest trainer. The sample program is
a little short, but that is because the concept is so simple. Attached
is a file, PICCS.DAT, which contains the terrain and letters for the
sample program. They were .CEL's, which I loaded into the des^ variable,
which I then dumped to disk, for easy access later. The .CEL's were
drawn on short notice, but still produces some nice terrain.

I have recieved a few requests for future trainers, and most of them are
for effects, so I guess that is what will be done from now on. A
surprising number have told me not to do a sound trainer and stick with
graphics stuff, with only a few asking for sound stuff, so I will hold
off on that until there is more of a demand.

I am still open to suggestions for future trainers, and of course
suggestions for improving the series. Leave me mail!

Hmm. A quote? Okay, let me think ....

        [    The little devil sat atop the alpine slopes, frolicking in the
           snow.  He threw a snowball at a nearby squirrel, which
           missed. The resulting avalance buried two villages and a ski
             The little devil was scared. Avalances were bad for
           business. The locals would form team spirit, be nice to
           each other and work together and free those trapped beneath
           the snow, which created even more goodwill. The man
           downstairs didn't like goodwill. He didn't like it at
             In the blink of an eye the devil was in his penthouse
           apartment, dressed in his usual suit. He picked up the phone.
           Dialing was for mortals.
             "Hello, Micros..."
             "This is Mister Crowley", interrupted the devil.
             There were sounds of thumping on the other side of the
           phone, then there was a new voice. "Hello, Bill here, we
           haven't heard from you in a while, Mister Crowley." The fear
           of the man on the other end was almost tangible. The devil
             "Hello Bill. Something has come up."
             "No!" The man on the other side almost shouted with terror.
           "Not Win..."
             "Yes, Bill. It is time."
             "Havn't I paid enough for my sins? Just that one night..."
           The man was almost sobbing.
             "You are touching me, Bill. But nevertheless, it is time."
             "No." The man sounded beaten, alone.
             "Yes. Bill, it is time for a new update."
                                                      - Grant Smith

See you next time!
  - Denthor

The following are official ASPHYXIA distribution sites :

║BBS Name                  ║Telephone No.   ║Open ║
║ASPHYXIA BBS #1           ║+27-31-765-5312 ║ALL  ║
║ASPHYXIA BBS #2           ║+27-31-765-6293 ║ALL  ║
║C-Spam BBS                ║410-531-5886    ║ALL  ║
║Connectix BBS             ║+27-31-266-9992 ║ALL  ║
║POP!                      ║+27-12-661-1257 ║ALL  ║
║Pure Surf BBS             ║+27-31-561-5943 ║A/H  ║
║Soul Asylum               ║+358-0-5055041  ║ALL  ║
║Wasted Image              ║407-838-4525    ║ALL  ║

Leave me mail if you want to become an official Asphyxia BBS
distribution site.