Path: chuka.playstation.co.uk!news
From: richard.cutting@virgin.net (Richard Cutting)
Newsgroups: scee.yaroze.beginners
Subject: Re: Wheeee!!! Throwing actions !
Date: Mon, 05 Oct 1998 22:50:39 GMT
Organization: PlayStation Net Yaroze (SCEE)
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On Mon, 5 Oct 1998 22:31:38 +0100, "Tones" <caz.tones@cwmail.com>
wrote:

>Hello all,
>
>Once again I speak after having a little trouble with yet another trivial
>problem. This time it is programming a particular fuction.  Throwing an
>object! (I'm sure all you guys must think I am totally incompetant, having
>posted a number of requests for help b4 ('bout 3 I think?)!)
>
>Basically, when I press X I want to be able to throw an object, which then
>becomes totally independent in it's flight path.  I have used a fuction to
>tell the program that the object has now been thrown (I use a simple array
>to do this), but what method would I use to calculate it's trajectory from
>Player1.x and Player1.y if I want it to go up a little in the direction the
>player is facing, then obviously fall back to the ground according to Mr
>Isaac Newton's law of gravity ? I.e. an arc of some kind.
>
>I have tried a number of things, such as calulating the x and y positions,
>sticking these in a structured array, then access each successive
>co-ordinate on each pass of the fuction.  However, this take's up a bit of
>time (in programming terms, not actually on the screen - as I use a for
>loop).  I have also thought of using a Sine wave table, but this is a little
>beyond my capabilities.
>
>In quick terms, I need pointing (HaHa! Oops, no pun intended :(  ) in the
>right direction.
>
>Otherwise, you may be pleased to learn my game is coming on quite nicely -
>Apples & Pears!!!!
>
>Many thanks in advance,
>
>Tones  : )
>
>

All you need to do is to calculate the x and y velocities for the
object as if it were chucked in a straight line and then apply wind
and gravity so it falls back to earth. You can do this using some
simple maths.

To get the x and y velociities you use;

xv = sin( angle ) * magnitude
yv = cos( angle ) * magnitude

where angle is the angle relative to the horizon and magnitude is the
speed.

( I may have got sin and cos the wrong way round, but it's late and
I'm knackered.... )

Then to calculate the x and y positions you just apply wind to the x
and gravity to the y. I.e;

x_position = x_position + xv + wind
y_position = y_position + yv + gravity

( where gravity is a fixed constant and wind is whatever you want ! )

This would be done every game cycle.

For speed reasons you want to steer well clear of the Yaroze's sin and
cos library functions. I recommend that you grab James Shaughnessy's
SINCOS.H file from his home page (
http://www.netyaroze-europe.com/~shaughnj ) which gives you tables
containing a fixed point implementation. You have to do a bit of bit
shifting to use them but it's not difficult.

Richard ( Who's really needs to dig out his basic maths books !! )
http://www.netyaroze-europe.com/~rcutting