basics of game development - delta time and transformations

Welcome to this java game development series. I’m back with another post on the basics of game programming.

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Last time we made a MovingShip program which had a spaceship polygon running around the window when pressing the arrow keys. The movement of the ship was dependent on the frames i.e 1px per frame update.

But now the movement of the ship will depend on the time so on the different systems your app shows you the same result. No matter that the game is running either at 120fps or 60 fps the speed of the moving objects will be the same.

Through this post, you will also learn about these concepts of the game programming.

1. delta time
2. transformation
3. vectors
4. rotation transform
5. scale transform
6. skew transform

Delta time

We want to move the ship with a velocity of v unit per second instead of 1px per frame. To achieve this we need the time difference between the last and the current frame.

let say the ship is moving by 12px per second to the right and its world position for the frame f1 is (x, y).

Now, After some time of f1( dt - elapsed time ), the app is going to draw the next frame f2. Since the ship is moving to the right, therefore the value of x should be increased by the covered distance in time dt.

We can calculate the new position of the ship as follow-

dt = time at f2 - time at f1

dt = t2 - t1

covered distance = 12 * dt.

let say the app draws the next frame f2 after 15 milliseconds ie dt = 15ms.

So the new position of the ship would be (x', y).
where x' = x + 12 * 15 / 1000

it was so easy!

Let’s implement this to our MovingShip class and make its movement independent of the frame. We just have to make some changes to the previous example.

In order to get the current time we can call System.currentTimemillis() and System.nanoTime() methods which return long data type values.

Previously we had used these methods in the FrameRate class in the article designing the Framerate class.

Just make changes to the while loop As shown in this pseudo code bellow

// ct : current time
// lt : last time
// dt : delta time
while(running)
  dt = ct - lt
  updateObject(dt * 1E-9) // dt * 10 raise to -9
  renderFrame()
  lt = ct
      

Do not forget to save the current time to the last time at the end of each iteration.

Now we can use delta time dt in updateObject() method.

// velocity v = 60px/sec
// dt is in seconds
updateObject(dt)
  if (up) 
      ship.ty -= dt * v 
  if (down) 
      ship.ty += dt * v   
  if (left) 
     ship.tx -= dt * v 
  if (right) 
     ship.tx += dt * v

I uploaded the complete code to Github for MovingShip class.

continue...

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