ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
;
;     TITLE: Horizontal Star field
;WRITTEN BY: DRAEDEN
;      DATE: 04/16/93
;
;     NOTES: Need a 286+ to execute.
;
;ASSOCIATED FILES:
;
;       HSTAR.ASM   =>  ASM code for horizontal starfield
;
;       HSTAR.BAS   =>  Basic program that generates a set of 'random' star
;                       locations.  Creates the file 'HSTARS.DW'
;
;       HSTAR.TXT   =>  This file
;
;       HSTARS.DW   =>  Holds the star location data (the structured info)
;                       Created by HSTAR.BAS
;
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

    The horizontal starfield is simply a series of dots that are moved in the
X direction in either the positive or negative direction.  That is all that
this ASM file does.  It simply takes a (X,Y) position, adds a velocity to the
position and keeps it in bounds.  Different from before, I've introduced a
well known (to graphics programmers) thing.  A look-up chart.  In this case
I use the look up chart to find the offset for a particular Y pos.  This way
is MUCH faster than the old [imul di,320] and it is more flexable.
Here's how I did it in detail:

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

    mov di,[Ypos]               ;get the ypos
    add di,di                   ;multiply the ypos by 2, so I can use it to
                                ; look up WORD sized data
    mov di,[YOffsetChart + di]  ;and grab the offset and put it in DI

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

    I can use DI as both a source and a destination for much the same reason
that you can do that in any language.  I have to use DI to get the data
before it can be put in DI.  Hmmm... That may be confusing.  Nevermind.

    ANYway, if you've already looked at the source, you've probably noticed
that the Display procedure is awfully large.  This is because I chose to just
repeat the same block over 4 times instead of having to get clever and write
a little loop.  Everyone knows what happens when you get clever.  You lose a
foot. :)

    All the data was created by the small BASIC program that was included.
If you don't like the current layout of the stars, run the basic program-
it'll give you a new set.

    This code is not 100% perfect.  It's up to you to fix it up.  I did that
'cause I was feelin' awfully lazy last night when I decided to finally write
this.  Here are some suggestions on how to fix it up:

    1)  Fix the main subroutine so that instead of being 4 repeated blocks, 
        its just 1 small loop.

    2)  Change it so that you can have "fractional" speeds.  This would be
        done by just changing the size of the Xpos from a WORD to a DWORD and
        changing the velocity to a DWORD.  Then you'd set 10000h = 1 unit
        (just use the high word as the Xpos.)

            For instance, a velocity of 1/2 would be = 08000h

    3)  After you fix it into a loop and give it a non-integer velocity, 
        make it so you can add multiple layers. (more than 4)

    
    After you make these improvements, you'd have a way-cool h-starfield.


    Kinda a short DOC, but then again, the program isn't that complicated nor
are the concepts contained within. 

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

   Well, that's it for now.  See INFO.VLA for information on contacting us.

   I would like some suggestions on what to write code for.  What would you
   like to see done?  What code would you like to get your hands on?

   Send question, comments, suggestions to draeden@u.washington.edu or post
    on Phantasm BBS.