Frame Rate Independent Movement

Frame Rate Independent Movement
by Ben Dilts

I’ve seen countless posts on this and other message boards, and even personal email has been sent to me, all asking the same question: How do I make it so my stupid objects move at the same speed if the frame rate rises or drops?

After a few months’ programming, I devised what I believe to be the most efficient and useful way to handle this problem. For the purposes of this article, I will encapsulate this functionality in its own class.

class framerate
{
public:
  float         targetfps;
  float         fps;
  LARGE_INTEGER tickspersecond;
  LARGE_INTEGER currentticks;
  LARGE_INTEGER framedelay;
	
  float         speedfactor;			

  void          Init(float tfps);
  void          SetSpeedFactor();
};

I leave all the members public because they work independently of each other and are all useful in some way to the outside program.

Here is the implementation, followed by some explanation

void framerate::Init(float tfps)
{
  targetfps = tfps;
  QueryPerformanceCounter(&framedelay);
  QueryPerformanceFrequency(&tickspersecond);
}
void framerate::SetSpeedFactor()
{
  QueryPerformanceCounter(¤tticks);
  //This frame's length out of desired length
  speedfactor = (float)(currentticks.QuadPart-framedelay.QuadPart)/((float)tickspersecond.QuadPart/targetfps);
  fps = targetfps/speedfactor;
  if (speedfactor <= 0)
    speedfactor = 1;

  framedelay = currentticks;
}

Some explanation:
targetfps is passed into the Init function, but can be set at any time. It is the target frame rate for the program. This is used in the SetSpeedFactor function to determine what the speedfactor will be. Look down at my explanation of speedfactor.

fps is the actual frame rate of the program. It is not really needed to make frame rate independent movement, but since it is so closely linked, I include it anyway.

tickspersecond is set in the Init function to be the number of ticks the high performance timer has per second.

currentticks is set in the SetSpeedFactor function to be the current high performance timer’s ticks.

framedelay is the previous frame’s currentticks.

speedfactor is the heart of this class. When it is set is SetSpeedFactor, it becomes a number that you multiply all your motion by. For instance, if the targetfps is 100, and the actual fps is 85, the speedfactor will be set to 100/85, or about 1.2. You then multiply all your motion is the game, at its lowest level, by this number.

For instance, rather than simply coding spaceship->MoveForward(5), I would code spaceship->MoveForward(5*framerate.speedfactor).

In conclusion:
This simple routine saves a whole heap of trouble. Just plug it into almost any game or other real-time program. Call Init at the beginning of the program, and then each frame call SetSpeedFactor. Then multiply all your movement by the speedfactor. It’s that simple. Now stop flooding those message boards :)

Questions? Comments? Complaints? Hate mail? benbeandogdilts@cs.com

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Date this article was posted to GameDev.net: 6/4/2001
(Note that this date does not necessarily correspond to the date the article was written)

See Also:
General
Sweet Snippets