


      SSSSS   CCCCC  RRRRR    OOOOO  LL    LL    IIIIII NN    NN  GGGGG
     SS   SS CC   CC RR  RR  OO   OO LL    LL      II   NNN   NN GG   GG
     SS      CC      RR   RR OO   OO LL    LL      II   NNNN  NN GG
      SSSSS  CC      RR  RR  OO   OO LL    LL      II   NN NN NN GG
	  SS CC      RRRRR   OO   OO LL    LL      II   NN  NNNN GG  GGG
     SS   SS CC   CC RR  RR  OO   OO LL    LL      II   NN   NNN GG   GG
      SSSSS   CCCCC  RR   RR  OOOOO  LLLLL LLLLL IIIIII NN    NN  GGGGG

		    by Alec Thomas (Kestrel) of FORGE Software Australia
					  (c9223826@cs.newcastle.edu.au)


------------
INTRODUCTION
------------
Okay, here it is fans (and air conditioners, open windows...geez I hate that
joke!), how to do scrolling using either X-mode (and associated variants) and
standard mode 13h (not hard but I thought I'd put it in anyway :) as well as
the basics of parallax scrolling...

First things first - X-mode. Throughout this little dissertation, I'm going
to assume that you know the basics of X-mode (or mode-X or mode-Y or
whatever you want to call it) such as how to get into it, how to set the
offset register, etc. and just get on with the scrolling :) I'm not trying
to teach you X-mode, but SCROLLING!!

One further thing. I'm not saying that the methods I'll explain below are
the best method of scrolling, I'm just showing how I got it to work myself
in the hope that someone out there can use it. Anyway, enough of this crap,
on with the STUFF!!!

(just a little note, when I'm talking about rows, they number from 0-199 and
the same with columns (except 0-319), etc. unless otherwise stated)

********************************************************************************
*			       X-MODE SCROLLING			       *
********************************************************************************
------------------
VERTICAL SCROLLING
------------------
Ok, this is the easiest form of scrolling using the VGA hardware...fast and
clean. The following example assumes you are using 320x200 X-mode with the
visible page starting at the top of the first page (offset 0).

To scroll what is on the screen up off the top, you simply add 80 (decimal)
to the screen offset register. This causes the screen to jump up by one
row. However, it also causes whatever is off the bottom of the screen
(the next page!) to become visible...not a desireable effect.

Easily fixed however. Draw the image you want to scroll, on the row that
will scroll on. So, when the screen offset is changed to scroll the screen
up, the new data is already there for all to see. Beautiful!!!

----------- Scrolling A (up) --------------
OFFSET = 0
WHILE NOT FINISHED DO
  OFFSET = OFFSET + 80
  DRAW TO ROW 200
  SET VGA OFFSET = OFFSET
END WHILE
-------------------------------------------

Bzzzzz! Wrong! This works fine, until you have scrolled down to the
bottom of page 4. Because you're effectively off the bottom of the VGA
window (starting at segment A000h), you can't write to the rest of the
VGA memory (if there is any - only SVGA's have more than 256K on board
memory) and so, you'll be viewing garbage.

No problem. The way around it is to only use two pages!!! "What?" I hear
you say. In fact, by using only two pages for scrolling, you gain two
major advantages: page flipping (because you're only using two pages for
the actual scrolling, you can use the spare two to perform page flipping)
and infinite scroll regions.

You perform the infinite scrolling in exactly the same way as before, with
two minor additions: after changing the offset register, you copy the row
just scrolled on to the row just scrolled off. Also, after you have scrolled
a full page, you reset the offset to the top of the original page.

----------- Scrolling B (up) --------------
OFFSET = 0
WHILE NOT FINISHED DO
  OFFSET = OFFSET + 80
  IF OFFSET >= (200 * 80) THEN OFFSET = 0
  DRAW TO ROW 200
  SET VGA OFFSET = OFFSET
  DRAW TO ROW -1 (was row 0 before scroll)
END WHILE
-------------------------------------------

Ok, so that's how to do vertical scrolling, now on with horizontal scrolling.



--------------------
HORIZONTAL SCROLLING
--------------------
Horizontal scrolling is essentially the same as vertical scrolling, all
you do is increment or decrement the VGA offset register by 1 instead of
80 as with vertical scrolling.

However, horizontal scrolling is complicated by two things

  1. Incrementing the offset register by one actually scrolls by FOUR
     pixels (and there are FOUR planes on the VGA, what a coincidence)

  2. You can't draw the image off the screen and then scroll it on
     because of the way the VGA wraps to the next row every 80 bytes
     (80 bytes * 4 planes = 320 pixels), if you tried it, you would
     actually be drawing to the other side of the screen (which is
     entirely visible)

I'll solve these problems one at a time.

Firstly, to get the VGA to scroll by only one pixel you use the horizontal
pixel panning (HPP) register. This register resides at

  PORT:     3C0H
  INDEX:    13h

and in real life, you use it like this

----------------- Pixel Panning ---------------
IN PORT 3DAH (this clears an internal
  flip-flop of the VGA)
OUT 13H TO PORT 3C0H
OUT value TO PORT 3C0H (where "value" is the
  number of pixels to offset)
-----------------------------------------------

To implement smooth horizontal scrolling, you would do the following:

-------------- Horizontal Scrolling ------------
FOR X = 0 TO 319 DO
  SET HPP TO ( X MOD 4 )
  SET VGA OFFSET TO ( X/4 )
END FOR
------------------------------------------------

Okay, no problem at all (although I think you might have to fiddle
around with the HPP a bit to get it right...try different values and
see what works :).

So, the next problem is with drawing the images off the screen where
they aren't visible and then scrolling them on!!! As it turns out,
there's yet ANOTHER register to accomplish this. This one's called the
offset register (no, not the one I was talking about before, that one
was actually the "start address" register) and it's at

  PORT:     3D4H/3D5H
  OFFSET:   13H

and here's how to use it

-------------- Offset Register ---------------
OUT 13H TO PORT 3D4H
OUT value TO PORT 3D5H
----------------------------------------------

Now, what my VGA reference says is that this register holds the number
of bytes (not pixels) difference between the start address of each row.
So, in X-mode it normally contains the value 80 (as we remember,
80 bytes * 4 planes = 320 pixels). This register does not affect the
VISIBLE width of the display, only the difference between addresses on
each row.

When we scroll horizontally, we need a little bit of extra working space
so we can draw off the edge of the screen.

Perhaps a little diagram will clarify it. The following picture is of a
standard X-mode addressing scheme with the OFFSET register set to 80.

      ROW    OFFSET
      0	 0 ========================
      1	80 [		      ]
      2       160 [		      ]
      ..       .. [       VISIBLE	]
		  [	SCREEN	]
		  [		      ]
		  [		      ]
      ..       .. [		      ]
      199   15920 ========================

and the next diagram is of a modified addressing scheme with the OFFSET
register set to 82 (to give us 4 extra pixels on each side of the screen)

ROW    OFFSET
0	 0 ------========================------
1	82 |   V [		      ]   V |
2       164 |   I [		      ]   I |
..       .. | N S [      VISIBLE	 ] N S |
	    | O I [       SCREEN	 ] O I |
	    | T B [		      ] T B |
	    |   L [		      ]   L |
..       .. |   E [		      ]   E |
199   16318 ------========================------

Beautiful!!!

As with vertical scrolling, however, you still have the problem of when
you reach the bottom of page 4...and it's fixed in the same manner.

I haven't actually managed to get infinite horizontal scrolling working,
but the method I have just stated will give you a horizontal scrolling
range of over 200 screens!!!! So if you need more (which is extremely
unlikely), figure it out yourself.


------------------
COMBINED SCROLLING
------------------
To do both horizontal and vertical scrolling, all you have to do is combine
the two methods with a few little extras (it's always the way isn't it).

You have to start off with the original screen on the current page and the
next page as well. When you scroll horizontally, you have to draw the edge
that's coming in to the screen to BOTH pages (that means you'll be drawing
the incoming edge twice, once for each page). You do this so that when you
have scrolled vertically down through a complete page, you can jump back
to the first page and it will (hopefully) have an identical copy, and you
can then continue scrolling again.

I'm sorry about this being so confusing but it's a bit difficult to explain.





********************************************************************************
*			    STANDARD VGA SCROLLING			    *
********************************************************************************
Without X-mode, there is no easy way to do scrolling using the VGA hardware.
So basically, you have to resort to redrawing the entire screen for every
frame. Several popular games (Raptor and Mortal Kombat spring to mind)
utilise this method with excellent effect, so it is quite effective.

Basically all you do to implement this is redraw the screen every frame
with a slightly different offset into the "map".

The following bit of pseudo-code will scroll down and to the right
through the map.

------------- Standard Scrolling ---------------
X = 0
Y = 0
WHILE NOT FINISHED DO
  DRAW TO SCREEN( 0, 0 ) FROM MAP( X, Y )
  X = X + 1
  Y = Y + 1
END WHILE
------------------------------------------------





********************************************************************************
*			      PARALLAX SCROLLING			      *
********************************************************************************
Parallax scrolling is when the "world" appears to have different levels
of perspective. That is, images further away from the viewer move
proportionately slower than images closer to the screen.

To implement parallax scrolling, you need two or more "maps". You start
from the most distant map and end with the closest map. When you scroll,
you offset the map furthest away by the smallest value and the map
closest to you by the largest value.

The following pseudo-code implements a 3 level parallax scrolling world,
scrolling (as above) down to the right.

--------------- Parallax Scrolling ------------------
X = 0
Y = 0
WHILE NOT FINISHED DO
  DRAW TO SCREEN( 0, 0 ) USING MAP_FAR AT ( X/4, Y/4 )
  DRAW TO SCREEN( 0, 0 ) USING MAP_MEDIUM AT ( X/2, Y/2 )
  DRAW TO SCREEN( 0, 0 ) USING MAP_NEAR AT ( X, Y )
  X = X + 4
  Y = Y + 4
END WHILE
-----------------------------------------------------

Obviously, with parallax scrolling, each successive map shouldn't delete
the previous map entirely. So you'll have to draw the maps using some
sort of masking (masking being where you can see through the background
colour to what was there previously).


********************************************************************************
*				  DISCLAIMER				  *
********************************************************************************
I'm sorry if any of this is confusing, but hey that's half the fun of it -
figuring out what the hell I'm raving on about :)

So, if you can figure it out, have fun and make games (preferably good ones!)

Later,
      Kestrel => FORGE Software Australia
