GamePhysics1

=2D Game Physics - Part I= 
//[[user:benjamin805]]//
[[toc]]
----
==Angle & Velocity== 
You can easily add basic physics to your games using these examples below. Original examples found at [[@http://www.rodedev.com/tutorials/gamephysics/]] .

In 2D games there are two position points that are used to place your sprite or drawing on the screen. They are known as the x axis and the y axis. In this example we will call them

object.x ' x axis
object.y ' y axis

We will use two variables called velocity_xvelocity.x and velocity_yvelocity.y to move the object on the screen.

velocity.x ' x axis movement increment
velocity.y ' y axis movement increment

We must be able to determine the direction and speed of the object moving on the screen. To do this we will use two variables called angle and speed.

angle ' direction of object in degrees (0-360)
speed ' The speed at witch the object moves

Using the Liberty BASIC commands [[http://en.wikipedia.org/wiki/Cos%28x%29|cos()]] and [[http://en.wikipedia.org/wiki/Sin%28x%29|sin()]] we will convert our angle value to give us another value that we will call the scale_xscale.x and scale_y.scale.y.

scale.x ' cos() of angle
scale.y ' sin() of angle

Since angle values can not be used directly with cos() and sin() we must first convert the angle to a [[http://en.wikipedia.org/wiki/Radian|radian]] value. We do this like so.

angle=angle/57.29577951

This gives us our radian value witch we can use with cos() and sin() to determine our scale values. The scale values will move the object in the direction we have chosen using our angle value.

scale.x=cos(angle)
scale.y=sin(angle)

Once we have our scale values we can now combine them with our speed value to get our velocity values

velocity.x=speed*scale.x
velocity.y=speed*scale.y

Remember that with each new direction the velocity values must be recalculated before moving the object.

Our code so far.
[[code format="vbnet"]]
'2D Game Physics - Part I

object.x=400
object.y=300

angle=45
r.angle=angle/57.29577951
speed=5

scale.x=cos(r.angle)
scale.y=sin(r.angle)

velocity.x=speed*scale.x
velocity.y=speed*scale.y

[[code]]

==Drawing the object== 
Now we will draw our object on the screen. For this example I will use a simple line to represent our object. By using the velocity values I can determine the line position and length. Below is an updated version of the code above.
[[code format="vbnet"]]
'2D Game Physics - Part I

object.x=400
object.y=300

angle=45
r.angle=angle/57.29577951
speed=5

scale.x=cos(r.angle)
scale.y=sin(r.angle)

velocity.x=speed*scale.x
velocity.y=speed*scale.y

nomainwin

WindowWidth=800
WindowHeight=600
open "2D Game Physics - Part I" for graphics as #main
#main "trapclose [quit]"
#main "color red;size 5;down"
#main "line ";object.x;" ";object.y;" ";object.x+(velocity.x*2);" ";object.y-(velocity.y*2)

wait


[quit]
close #main
end



[[code]]

==Moving the object== 
We will now move our object on the screen in the direction we have chosen (45 degrees) until it reaches the edge of our window.
[[code format="vbnet"]]
'2D Game Physics - Part I

object.x=400
object.y=300

angle=45
r.angle=angle/57.29577951
speed=5

scale.x=cos(r.angle)
scale.y=sin(r.angle)

velocity.x=speed*scale.x
velocity.y=speed*scale.y

nomainwin

WindowWidth=800
WindowHeight=600
open "2D Game Physics - Part I" for graphics_nsb as #main
#main "trapclose [quit]"
#main "color red;size 5;down"
#main "line ";object.x;" ";object.y;" ";object.x+(velocity.x*2);" ";object.y-(velocity.y*2)

timer 100,[move]
wait


[move]
object.x=object.x+velocity.x   'increase object.x value
object.y=object.y-velocity.y   'increase object.y value

'check for edge of window
if object.y<10 then object.y=10
if object.y>WindowHeight-20 then object.y=WindowHeight-20
if object.x>WindowWidth-15 then object.x=WindowWidth-15
if object.x<10 then object.x=10

'draw object
#main "cls"
#main "line ";object.x;" ";object.y;" ";object.x+(velocity.x*2);" ";object.y-(velocity.y*2)

wait
[quit]
close #main
end

[[code]]


==Controlling the object== 
Let's now add some basic movement control to our object. In this example we will use the arrow keys on your keyboard to change the angle of the object witch will change it's direction. I have also created a sub called setangle witch can be used to change the velocity values for each new direction we want to move in.
[[code format="vbnet"]]
'2D Game Physics - Part I

object.x=400
object.y=300

angle=45
speed=10

call setangle velocity.x,velocity.y,angle,speed

nomainwin

WindowWidth=800
WindowHeight=600
open "2D Game Physics - Part I" for graphics_nsb as #main
#main "trapclose [quit]"
#main "when characterInput [getkey]"
#main "color red;size 5;down"
#main "line ";object.x;" ";object.y;" ";object.x+(velocity.x*2);" ";object.y-(velocity.y*2)

timer 100,[move]
wait

[getkey]
key$ = Inkey$
    if len(key$) < 2 then

      else
        if right$(key$,1)=chr$(_VK_UP) then
             if moving=0 then
                moving=1
             else
                moving=0
             end if
        end if

        if right$(key$,1)=chr$(_VK_RIGHT) then
           angle=angle-5
           if angle<0 then angle=360
           call setangle velocity.x,velocity.y,angle,speed
        end if

        if right$(key$,1)=chr$(_VK_LEFT) then
           angle=angle+5
           if angle>360 then angle=0
           call setangle velocity.x,velocity.y,angle,speed
        end if

    end if


wait

[move]
#main "setfocus"
scan

if moving=1 then
object.x=object.x+velocity.x   'increase object.x value
object.y=object.y-velocity.y   'increase object.y value

'check for edge of window
if object.y<10 then object.y=10
if object.y>WindowHeight-40 then object.y=WindowHeight-40
if object.x>WindowWidth-15 then object.x=WindowWidth-15
if object.x<10 then object.x=10
end if

'draw object
#main "cls"
#main "line ";object.x;" ";object.y;" ";object.x+(velocity.x*2);" ";object.y-(velocity.y*2)

wait
[quit]
close #main
end

sub setangle byref velocity.x, byref velocity.y,angle,speed

r.angle=angle/57.29577951

scale.x=cos(r.angle)
scale.y=sin(r.angle)

velocity.x=speed*scale.x
velocity.y=speed*scale.y

end sub


[[code]]
I hope you find this example helpful in your game programming and I hope to add more parts as I finish them.