gameLoop()... why do we need to know last loop time?

[Rob Poulos]

so im checking out the SpaceInvaders tutorial found here:

http://www.cokeandcode.com/node/6

im trying to wrap my head around why you need to know the last loop time to calculate movement? i would think that you would just need to look at the current position and increment that x-axis and/or y-axis by .01, 1, 30 or whatever needed for each loop.

below are a few snippets of the code listed in the above link:

 while (gameRunning) {
	// work out how long its been since the last update, this
	// will be used to calculate how far the entities should
	// move this loop
	long delta = System.currentTimeMillis() - lastLoopTime;
	lastLoopTime = System.currentTimeMillis();
   

   public void move(long delta) {
	// update the location of the entity based on move speeds
	x += (delta * dx) / 1000;
	y += (delta * dy) / 1000;
}
 

public void move(long delta) {
	// if we're moving left and have reached the left hand side
	// of the screen, don't move
	if ((dx < 0) && (x < 10)) {
		return;
	}
	// if we're moving right and have reached the right hand side
	// of the screen, don't move
	if ((dx > 0) && (x > 750)) {
		return;
	}
	
	super.move(delta);
}

[fred rosenberger]

All the loops may not be constant in time, depending on what else the machine is doing, but you still want your sprites to look like they are moving at a constant rate.

so if i update the screen more often (i.e. shorter loop times), i want them to move less. 5 frames per second need to move further between each re-paint than 20 frames per second.