Scrolling

By Alec Thomas

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

Incrementing the offset register by one actually scrolls by FOUR pixels (and there are FOUR planes on the VGA, what a coincidence)
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: 33h

and in real life, you use it like this ----------------- Pixel Panning --------------- IN PORT 3DAH (this clears an internal flip-flop of the VGA) OUT 33H 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