SuperHack/SuperHack CD.bin/CODING/GRAPHICS/SCROLL.ZIP/SCROLL.TXT

Text File | 1996-04-26 | 12.5 KB | 535 lines

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      SSSSS   CCCCC  RR   RR  OOOOO  LLLLL LLLLL IIIIII NN    NN  GGGGG


                    by Alec Thomas (Kestrel) of FORGE Software Australia

                                          ([e-mail])



------------

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