From: Eleanor McHugh <eleanor@...>
Date: 2009-11-15T08:22:48+09:00
Subject: Re: Trajectories

On 13 Nov 2009, at 23:33, Thijs Leeflang wrote:
> hello,
>
> im having a problem with trajectories,
> im trying to make an arrow like movement
> i did find this but it's way over my head
>
> http://www.gamedev.net/reference/programming/features/physicsch6/ch06.pdf

This is mostly for people who want to model the real-world movement of  
projectiles, i.e. parabolic ballistic curves resulting from the action  
of gravity. The math isn't hugely complicated, but it does assume a  
basic familiarity with trigonometry.

> i have these values,
> @x and @y is the x and y the image is drawn
>
>    @targetx = targetx
>    @targety = targety
>    @fromx = fromx
>    @fromy = fromx
>    @x = fromx
>    @y = fromy
>
> can anybody help me with this?


I'm going to assume you want an OO solution that wraps stuff up in a  
nice reusable fashion, so here's an example for what such a solution  
might look like (it's not tested so treat with caution).

   Point = Struct.new(:x, :y)
   ComponentVector = Struct.new(:x, :y)

   # Assume bearing stored in radians (360 degrees = 2 * Pi radians)
   BearingVector = Struct.new(:bearing, :magnitude)

   class Entity
     attr_accessor :location, :forces

     def initialize location = nil
       @location = location || Point.new(0, 0)
       @forces = {}
     end

     def move timeslices = 1
       @forces.each do |f|
         case
         when f.bearing
           @location.x += Math.cos(f.bearing) * f.magnitude * timeslices
           @location.y += Math.sin(f.bearing) * f.magnitude * timeslices
         when f.x && f.y
           @location.x += f.x * timeslices
           @location.y += f.y * timeslices
         end
       end
     end
   end

The solution is generalised to allow more than one force to be applied  
to an individual entity, and to allow the force vector to be defined  
as either (x, y) components or as an angle and a magnitude.

   football = Entity.new(Point.new(100, 100)
   football.forces[:gravity] = ComponentVector.new(0, -9.81)
   football.forces[:kick] = BearingVector(0.5, 27)
   football.forces[:wind] = BearingVector(5, 3)
   football.move 3

Here I've set up a simple football scenario combining gravity, a kick  
and a prevailing wind. The scenario for an arrow would look very  
similar.


Ellie

Eleanor McHugh
Games With Brains
http://slides.games-with-brains.net
----
raise ArgumentError unless @reality.responds_to? :reason