No Fragments Archive 10: Diskmags/nf_archive_10.iso/MAGS/ST_USER/1990/USERNV90.MSA/TEXT_STOS.DOC

Text File | 1990-05-24 | 11KB | 247 lines

                           SPRITE PATH DEFINER

         This month's STOS feature is devoted to Phil Lawson's
        utility for creating sprite paths to include in your own
                         mega-blasting arcade games.

Well, I've promised it for long enough, so now here it is. If you recall the
articles from a couple of months back, you'll remember we discovered how to
control the sprites by using an array, holding the X and Y coordinates of every
position the sprite goes though.

The Sprite Path Definer, (SPD), does a similar thing, except the actual
starting position is the only real coordinate present. All the rest are offsets
from the previous position. For example:

Start pos = 100,100
If next values are 2 and 4, the actual coordinates will be 102 and 104. If the
next two were 4 and -3, the coordinates would be (102+4,104-3) 106 and 101.

The reason for this small alteration in storing the sprite positions is that it
drastically reduces the amount of space needed to save the data on disk. I'm
not going to start explaining the workings of 16-bit arithmetic, so you'll just
have to believe me on this one. Anyway, enough of the waffle, let's get down to
business.

The easiest way for you to understand what each option does is for me to
explain them in turn, so here goes.

                                MENU 1
                                ¯¯¯¯ ¯

LOAD - Loads a previously saved set of sprite paths

SAVE - Once you've created the sprite paths, it would be a shame if you
were not able to save them.

UNUSED - There are two of these, mainly because I designed the screen and then
couldn't think of anything else to include. No doubt you'll have your own ideas
of what these should be used for, so I've left them for you to alter the
program and do with them what you will. (STOS owners only).

SET X and SET Y  - Several options require one or two parameters to be set. For
example, if you wished to copy only a section of a path to the end of another
one, you would set the X and Y values to the start and finish number of the
section to copy, and then use the COPY X/Y TO END option.

Each of the nine paths has its own unique X and Y values, which are
automatically displayed whenever you select a new path. To increase the number,
position the pointer over the required option and press the right mouse button.
To decrease it press the left button.

Notice that under each of the nine paths, selected by clicking in one of the
boxes, is a 3-digit number. This is the number of coordinates that each path
currently contains. This will range from 0, (meaning empty), to 999.

When trying to set the X and Y values, the numbers will whizz round pretty
quickly. To allow a bit of "fine-tuning" to select a specific number, press the
S key. To get things going at full speed again press F.

RESET - This will reset everything to zero, including speed, x and y values and
the contents of all the paths. Since selecting this by mistake would be very
upsetting, especially after several hours work, both mouse buttons have to be
pressed before the reset is performed.

SELECT SPRITE - There's no point in defining paths if you cannot use your own
sprites, therefore this option will allow you to choose one of the sprites
currently stored in the sprite-bank.

LOAD SPRITES - Quite obvious really. When the program is fully running, a lot
of memory is taken up. Therefore try and keep the sprite banks to a minimum.
One way of doing this is to create a second file containing only the main
sprites you intend to use in the your game.

LOAD BKGND .PI1 - Since most arcade games contain some form of terrain that the
sprites are to pass over, it would be nice to make sure they're going where
they're supposed to. This option will load a standard Degas PI1 picture, and
can then be used to check on the movement of the sprite.

LOAD BKGND .MBK - Exactly the same as the above option, except it will load a
screen that has been compacted with the STOS accessory, COMPACT.ACB. If you're
not intending to use the game in STOS, this option can be disabled. See STOS /
OTHER.

DELETE ONE PATH  - Does just what it says. The current sprite path will be
deleted. Take care though, once it's gone there's no way of getting it back
again. To change the current path, position the pointer over one of the nine
boxes at the top of the screen and press the left mouse button. The selected
path number will be shown at the far right of the screen.

DELETE ALL PATHS - Go on, try and guess what this one does.

MENU 2 - Displays the second set of options available.

BACKWARDS - If selected, the PLAY commands will result in the sprite going
backwards. Can be used to give the illusion of two paths being defined by one
set of data. Just have one type of sprite going forwards, and another going
backwards. With a little work and thought, this can produce really amazing
effects.

FORWARDS - The normal way of moving a sprite. See BACKWARDS

STOS / OTHER. Since the main program has been compiled, it follows that anyone
can use it to define their own sprite paths. In this case, they may not have
access to sprite banks and compressed picture screen. By selecting the other
option they can turn off the STOS only options.

PLAY - Use this option to check the movement of a sprite along a previously
defined path. Before using, seek the PLAY OPTIONS in menu 2.

PLAY & PLOT - Just like PLAY, except each point the sprite passes through is
plotted on the screen.

SPEED - To closely check what the sprite is doing, change the value of speed.
As with SET X and SET Y, this is achieved with the left and right mouse
buttons. The speed value can range from 0 (fast) to 100 (zzzzz).

QUIT - Back to the desktop. As with RESET, this need both mouse buttons to be
pressed at the same time.

                                 MENU 2
                                 ¯¯¯¯ ¯

DELETE 1 to X - Deletes the whole of the current path from the first position
to the Xth position. All the ones from X onwards are moved down to fill the
gap.

DELETE X to END - Deletes everything from the Xth position to the end of the
current path.

KEEP X - Y - Everything apart from the data between X and Y is deleted.

PLAY OPTIONS

UNTIL MOUSEKEY - Will play the whole path until a mousekey is pressed or the
end of the path is reached.

FROM X to Y - Plays between X and Y only.

UNTIL X VALUE - Plays from the beginning, (or end if BACKWARDS is selected)
until the X value is reached.

With all these play options, once the sprite has finished moving, the current X
and Y coordinates of the sprite are displayed, along with the position number.
By setting the SET X and SET Y values to numbers slightly lower and higher
respectively, you can now examine the area of the path around where the sprite
was stopped.


BACKGROUND OPTIONS

BACKGROUND ON - If a background picture has been loaded, it will be displayed.

BACKGROUND OFF - Surprise, surprise. This option will stop any loaded pictures
from being displayed.

MENU 1 - Takes you back to the first set of options.

COPY OPTIONS

1 to X - Will copy the first X amount of data from the current path to the path
you next click on, which will be totally overwritten by the new data

X to Y - Copies the data between X and Y from the current path to the one you
next click. This option will also overwrite the original data held in the
selected path.

X/Y to END - Copies the data between X and Y to the end of the next selected
path. This option is best used to produce complicated paths by joining together
many smaller ones.


Your best chance of becoming famliar with SPD is to actually play around with
it, but some extra information may be of use later on.

1) When saving the path data to a file, the following format is used:

The first four bytes are 11, 22, 33 and 44. This is used to identify a sprite
path file when loading it back into SPD. The next byte is how many positions
are defined in this path.

The next four bytes are the actual starting position of the sprite, in hi-lo
order. For instance, if these were 01, 04, 00 and 95, the start position can be
found by:

       x = (01 * 256) + 04  =  260
       y = (00 * 256) + 95  =   95


The remainder is taken up with the offset values for each of the sprite
positions defined within this path. Note, if an offset value is greater than
127, treat it as a negative number. The way to calculate negatives is:

if num > 127 then num = 256 - num


All the above information, apart from the 11,22,33 and 44, is repeated for the
whole nine paths. If any of these is empty, the "how many" value will be zero.

To start using the program, first load your sprites. Non-STOS users will have
to make do with the three I've included as part of the program. These are 8 by
8, 16 by 16 and 32 by 32 pixels respectively.

You will now have to select which sprite to use. After choosing the SELECT
SPRITE option, pressing the right mouse button will display the next sprite in
the bank, while pressing the left button will display the previous one. To
select a particualr sprite, make sure it is shown on the screen and press the
return key.

To define the path, click once on the DEFINE PATH option and the screen will
clear. If a background picture has been loaded, and not turned off, it will now
be displayed. The mouse pointer now changes into the image of the selected
sprite and can be moved around the screen. To define a point press the left
mouse button. Note, you cannot define another point in the same place as the
last one.

The top of the screen will show the current X and Y coordinates of the sprite
along with how many points have been defined. Since trying to define a
previously defined path would result in the original data being lost, you will
have to press both mouse buttons at the same time when selecting the DEFINE
option if the current path already contains data.

Since large numbers are treated as being negative, don't try to move the sprite
in large jumps across the screen. Around 10 pixels is most a sprite should move
from one place to another, otherwise it will seem "jerky". When you've finished
defining the path, press the right mouse button.

Nine paths may not seem like very many, but few games have more. If you do need
some extra ones though, try creating small ones and joining them together with
the COPY X/Y to END option. It would be a simple matter to separate these when
you load them into the game.

===========================================================================

If any STOS users require the original Basic program, to either study or adapt
to their own specific needs, please send a cheque for £2.00 (to cover postage,
packing, cost of blank disc and wear & tear on a worn & torn disc drive), to:

PHIL LAWSON
7, Eaglescliffe Close
New Marske
Redcar
Cleveland
TS11 8BB

Please, do not include any other correspondance for the STOS column, since this
must be addressed to the magazine. (We've got to keep those editors busy).