GamePhysics2

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guest (198.147.225.46) guest (198.147.225.46) Mar 8, 2012

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2D Game Physics - Part II - Acceleration

- benjamin805 benjamin805
Note to author: In physics, acceleration is the rate of change of velocity with time. In your article, velocity (speed) increases, but acceleration remains constant. It does not increase, as you've written. Please make changes to correct this assertion. Thanks. Object Acceleration We will now look at how you can add acceleration to your game objects. It's very easy to do and should not take much programming on your part. An example of when acceleration is needed could be when driving a car, flying a ship, rolling down hill, etc. Acceleration is something that affects an objects speed and can be added to the speed or subtracted from it. Imagine a car speeding up or slowing down, or a rocket ship flying through space or coming to a stop. In the examples here we will be working with racing cars. Four racing cars will take off from the starting line and race to the finish line. Their acceleration will increase as they move forward or as the drivers step on the gas pedal. We will make a change to our sub setangle by adding acceleration to it. We will also change our speed variable to be byref so that our sub can change this value. 
 sub setangle byref velocity.x, byref velocity.y,angle,byref speed,acceleration speed=speed+acceleration 'we now add in the acceleration 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
Setup acceleration starting values So the first part of setting up our example is to setup the cars starting positions and direction. I put this in a gosub since we will call it each time a race starts. We will set the starting speeds for each car to zero since they will increase with the acceleration value. Each cars acceleration will be chosen randomly so you will not know the winner until you press go! [carsetup] car1.x=20 car1.y=300 car1.vx=0 ' velocity.x car1.vy=0 ' velocity.y car1.speed=0 car2.x=20 car2.y=330 car2.vx=0 ' velocity.x car2.vy=0 ' velocity.y car2.speed=0 car3.x=20 car3.y=360 car3.vx=0 ' velocity.x car3.vy=0 ' velocity.y car3.speed=0 car4.x=20 car4.y=390 car4.vx=0 ' velocity.x car4.vy=0 ' velocity.y car4.speed=0 'get random acceleration value, we do not want any cars to have the same ' value 'we also do not want to use the int value for this example. [doover] car1.a=rnd(1)*1 'acceleration for car 1 car2.a=rnd(1)*1 'acceleration for car 2 if car2.a=car1.a then [doover] car3.a=rnd(1)*1 'acceleration for car 3 if car3.a=car1.a or car3.a=car2.a then [doover] car4.a=rnd(1)*1 'acceleration for car 4 if car4.a=car1.a or car4.a=car2.a or car4.a=car3.a then [doover] 'draw cars at starting position #main "cls" #main "color red;line ";car1.x;" ";car1.y;" ";car1.x+10;" ";car1.y #main "color blue;line ";car2.x;" ";car2.y;" ";car2.x+10;" ";car2.y #main "color green;line ";car3.x;" ";car3.y;" ";car3.x+10;" ";car3.y #main "color brown;line ";car4.x;" ";car4.y;" ";car4.x+10;" ";car4.y return Increasing acceleration When the race starts we will increase the acceleration of each car by recalling our setangle sub over and over. What this does is increases the speed of each car by adding more acceleration to the speed value with each call. Take a look at the game loop. [move] 'increase each cars acceleration call setangle car1.vx,car1.vy,angle,car1.speed,car1.a call setangle car2.vx,car2.vy,angle,car2.speed,car2.a call setangle car3.vx,car3.vy,angle,car3.speed,car3.a call setangle car4.vx,car4.vy,angle,car4.speed,car4.a 'move cars car1.x=car1.x+car1.vx 'increase object.x value car1.y=car1.y-car1.vy if car1.x>finishline then timer 0 notice "Red car won!" wait end if car2.x=car2.x+car2.vx 'increase object.x value car2.y=car2.y-car2.vy if car2.x>finishline then timer 0 notice "Blue car won!" wait end if car3.x=car3.x+car3.vx 'increase object.x value car3.y=car3.y-car3.vy if car3.x>finishline then timer 0 notice "Green car won!" wait end if car4.x=car4.x+car4.vx 'increase object.x value car4.y=car4.y-car4.vy if car4.x>finishline then timer 0 notice "Brown car won!" wait end if 'draw cars #main "cls" #main "color red;line ";car1.x;" ";car1.y;" ";car1.x+10;" ";car1.y #main "color blue;line ";car2.x;" ";car2.y;" ";car2.x+10;" ";car2.y #main "color green;line ";car3.x;" ";car3.y;" ";car3.x+10;" ";car3.y #main "color brown;line ";car4.x;" ";car4.y;" ";car4.x+10;" ";car4.y wait Start the race! When all the new code is in place we can start the race! Here is a complete listing for the example program. '2D Game Physics - Part II nomainwin WindowWidth=800 WindowHeight=600 button #main,"Go!",[go],ul,10,10,35,30 open "2D Game Physics - Part II" for graphics_nsb as #main #main "trapclose [quit]" #main "size 20;down" finishline=750 angle=0 gosub [carsetup] wait [go] gosub [carsetup] timer 100,[move] wait [move] call setangle car1.vx,car1.vy,angle,car1.speed,car1.a call setangle car2.vx,car2.vy,angle,car2.speed,car2.a call setangle car3.vx,car3.vy,angle,car3.speed,car3.a call setangle car4.vx,car4.vy,angle,car4.speed,car4.a car1.x=car1.x+car1.vx 'increase object.x value car1.y=car1.y-car1.vy if car1.x>finishline then timer 0 notice "Red car won!" wait end if car2.x=car2.x+car2.vx 'increase object.x value car2.y=car2.y-car2.vy if car2.x>finishline then timer 0 notice "Blue car won!" wait end if car3.x=car3.x+car3.vx 'increase object.x value car3.y=car3.y-car3.vy if car3.x>finishline then timer 0 notice "Green car won!" wait end if car4.x=car4.x+car4.vx 'increase object.x value car4.y=car4.y-car4.vy if car4.x>finishline then timer 0 notice "Brown car won!" wait end if 'draw object #main "cls" #main "color red;line ";car1.x;" ";car1.y;" ";car1.x+10;" ";car1.y #main "color blue;line ";car2.x;" ";car2.y;" ";car2.x+10;" ";car2.y #main "color green;line ";car3.x;" ";car3.y;" ";car3.x+10;" ";car3.y #main "color brown;line ";car4.x;" ";car4.y;" ";car4.x+10;" ";car4.y wait [carsetup] car1.x=20 car1.y=300 car1.vx=0 ' car1.vy=0 ' car1.speed=0 car2.x=20 car2.y=330 car2.vx=0 ' car2.vy=0 ' car2.speed=0 car3.x=20 car3.y=360 car3.vx=0 ' car3.vy=0 ' car3.speed=0 car4.x=20 car4.y=390 car4.vx=0 ' car4.vy=0 ' car4.speed=0 [doover] car1.a=rnd(1)*1 'acceleration for car 1 car2.a=rnd(1)*1 'acceleration for car 2 if car2.a=car1.a then [doover] car3.a=rnd(1)*1 'acceleration for car 3 if car3.a=car1.a or car3.a=car2.a then [doover] car4.a=rnd(1)*1 'acceleration for car 4 if car4.a=car1.a or car4.a=car2.a or car4.a=car3.a then [doover] 'draw cars at starting position #main "cls" #main "color red;line ";car1.x;" ";car1.y;" ";car1.x+10;" ";car1.y #main "color blue;line ";car2.x;" ";car2.y;" ";car2.x+10;" ";car2.y #main "color green;line ";car3.x;" ";car3.y;" ";car3.x+10;" ";car3.y #main "color brown;line ";car4.x;" ";car4.y;" ";car4.x+10;" ";car4.y return [quit] timer 0 close #main end sub setangle byref velocity.x, byref velocity.y,angle,byref speed,acceleration speed=speed+acceleration 'we now add in the acceleration 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