2D Game Physics - Part I

Ben Jimenez-2010

---

2D Game Physics

You can easily add basic physics to your games using these examples below. Original examples found at http://www.rodedev.com/tutorials/gamephysics/ . The examples there are very good,but can confuse you. I will try to give examples that I think are easier to understand.

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_x and velocity_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 cos() and sin() we will convert our angle value to give us another value that we will call the scale_x and 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 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.

'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
 

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.

'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
 
 
 

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.

'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
 

Third Part Title

Text here.

'code here