managing_framerate.rdoc

Path: doc/managing_framerate.rdoc
Last Update: 2009-04-27 14:23:49 -0500

Managing Framerate

This is a guide to using the Rubygame::Clock class in your games to monitor and limit the game’s framerate. It also gives tips and good practices for dealing with framerate and time units in your application.

The theory and general information here is applicable to most game development platforms, so you may find this guide useful even if you are not using Rubygame.

The Short Version

  1. When your game launches, ceate a Clock instance, set the target framerate, enable tick events, and calibrate it.
  2. If your game uses multiple ruby threads, set Clock#nice to true.
  3. Call Clock#tick once per frame. It returns a Rubygame::Events::ClockTicked instance.
  4. Use ClockTicked#seconds to see how much time passed since the previous frame. Use that number to calculate how far your characters moved, etc.
  5. You can check the framerate with Clock#framerate.

Usage Example

  require "rubygame"

  $clock = Rubygame::Clock.new()

  # Set the desired framerate.
  $clock.target_framerate = 20

  # Make Clock#tick return ClockTicked (Rubygame 2.5+)
  $clock.enable_tick_events()

  # Allow Clock to work nicely with multiple ruby threads.
  # You can skip this if you don't use multiple threads.
  $clock.nice = true

  # Optimize clock for this computer.
  puts "Calibrating... "
  $clock.calibrate()
  puts "New granularity: %d ms"%[$clock.granularity]

  i = 0

  catch :quit do
    loop do

      tick_event = $clock.tick()  # call tick once per frame.

      # Give tick_event to an event handler or queue, or whatever.

      fps = $clock.framerate     # current framerate
      diff = tick_event.seconds  # time since previous call to tick

      puts "Tick %0.2d = %0.2d fps (%0.4f s since previous)"%[i, fps, diff]

      i += 1
      throw :quit if( i > 50 )

    end
  end

Background

Almost all games and other interactive applications have a main loop, code that is repeated many times per second to take user input, update the status of the game, and refresh the screen, among other things. Each repetition of this loop is one frame, and the number of frames that occur in one second is called the framerate or FPS (frames per second).

Effects of Framerate

In general, the higher the framerate, the smoother the motion in your game seems.

At low framerates (generally less than 12-15 FPS), the game will feel “choppy” or “jerky” to the user. At even lower framerates (less than 5), the user may find it frustrating to interact with the game. Obviously, that’s bad!

High framerates appear smoother, but only up to a point. For example, a user will almost certainly notice a big difference between 5 and 10 FPS, but would probably not notice much difference between 50 and 100 FPS. Framerates above 60 FPS are usually just a waste, as they use much more CPU power but have very little gained smoothness.

So, it’s best to find a balanced framerate, which will appear smooth to the user, without being wasteful. A good framerate is usually in the range of 20-40 FPS, but varies from game to game. For example, a fast-paced action game may need 60 FPS, while a slow-paced puzzle game may be fine with only 10.

Framerate Monitoring

How to Enable Framerate Monitoring

Rubygame’s Clock class makes it simple to monitor (measure) framerate. Simply call Clock#tick exactly once in your main game loop. Clock will measure the time since the previous tick. From that information, collected over many frames, it can determine the framerate that your game is running at.

You can get the average recorded framerate, in frames per second, with Clock#framerate.

If you prefer, you can get the frametime (the average time it took to make one frame), measured in milliseconds per frame, with Clock#frametime. Clock#frametime is equivalent to 1000.0 / Clock#framerate.

Finding Out How Long a Frame Took

The Clock#tick method returns information about how long it has been since the previous frame. You can use this information to decide how far objects in your game should move. (See the Framerate Independence section, below).

For backwards compatibility, Clock#tick by default returns the number milliseconds since the previous frame as an integer. But if you call Clock#enable_tick_events (in Rubygame 2.5+), Clock#tick will instead return an instance of ClockTicked containing that information. You can then get that information with either ClockTicked#milliseconds or ClockTicked#seconds, whichever is more convenient for you.

Framerate Limiting

Why Limit the Framerate?

If you don’t limit the framerate of your game, the main loop will repeat as quickly as the computer can run it. This will make the game run as smoothly as possible, but it will also use as much of the computer’s CPU power as it can!

This can make other applications on the computer run more slowly or choppily. Also, running the CPU full throttle uses more electricity, so laptops and other mobile devices will lose battery charge faster. It can also make the laptop get uncomfortably warm, or trigger the fans on the laptop to turn on, causing unwanted noise.

Another benefit of framerate limiting is that it can make your game run at a stable, consistent framerate. However, it is poor practice to assume that every frame will take the same amount of time. Instead, you should use the data returned by Clock#tick to see how long the frame took. The “Framerate Independence” section below further explains why and how to do that.

How to Enable Framerate Limiting

Enabling framerate limiting in Rubygame is easy as pie. Just set Clock#target_framerate= to the maximum framerate for your game. So if you wanted your game to stay at 20 frames per second, you would do:

  my_clock = Rubygame::Clock.new
  my_clock.target_framerate = 20 # frames per second

There’s also another way to set the target framerate: Clock#target_frametime=. Instead of setting the number of frames per second, this allows you to set the number of milliseconds per frame. This has the same effect as setting the framerate, it’s just another way of expressing it. So, the following is exactly equivalent to the earlier example:

  my_clock = Rubygame::Clock.new
  my_clock.target_frametime = 50 # milliseconds per frame

To use framerate limiting, you also need to call Clock#tick once per frame, the same way you do for framerate monitoring (you should only call it once per frame, even if you’re doing both monitoring and limiting). When framerate limiting is enabled, the tick method pauses the program for a brief time. The length of the pause carefully calculated each frame to keep a consistent framerate.

However, framerate limiting only set the maximum framerate. In other words, it can only slow down your game if it’s running too fast, not make your game run faster. So if your game naturally runs at 25 FPS, setting a target framerate of 30 FPS will have no effect.

You can change the target framerate/frametime at any time. Setting the target back to nil will disable framerate limiting.

Improving Clock Efficiency

In Rubygame 2.5 and later, you can call Clock#calibrate to optimize the framerate limiting algorithm for the current computer. This minimizes wasted CPU power by setting Clock#granularity to match the system clock’s granularity. The improvement varies by operating system and processor, and will be most noticeable on systems with high-precision system clocks.

By default, the calibration will take up to a maximum of 0.5 seconds to complete. The needed amount of time will likely decrease in future versions of Rubygame. If you want to calibrate more quickly (but potentially less accurately), you can pass a different maximum time as a parameter to Clock#calibrate.

You can also set Clock#granularity directly, if you wish. Setting Clock#granularity to 0 will use the least CPU possible, but will lose accuracy.

Multithreaded Applications

By default, Clock’s framerate limiting feature will pause all ruby threads during the small pause each frame. This is a side effect of the SDL function used for the pause. It also affects Clock.delay and Clock.wait.

Starting in Rubygame 2.5, Clock has a new attribute, Clock#nice. If true, Clock will try to give the other threads an opportunity to run during the brief pause each frame when performing framerate limiting.

If you are calling Clock.delay or Clock.wait directly, you can get the same effect by passing true for the nice parameter.

NOTE: There is no guarantee or specification about how often other threads will allowed to run during the delay, or even that they will be allowed run during the delay at all. Setting nice to true simply tells the Clock to try to be nice to other threads, if it can do so without losing accuracy.

Framerate Independence

Why Be Framerate Independent?

Some programmers are tempted to use framerate limiting to control the speed of the action in their games. In other words, they assume that every frame will take the same amount of time on all computers, so they define movement speeds and other gameplay rates in “per frame” units: move 2 pixel per frame, lose 1% HP per frame when poisoned, etc. This is called “framerate dependence”, because the speed of the gameplay depends on how fast the framerate is.

You should *almost always* avoid this programming practice, because it’s not very robust, it usually makes the game more difficult to maintain or change in the future, and can lead to bugs and unwanted behavior.

For example, if you programmed your game so that the characters move 2 pixels per frame, and limited your game’s framerate to 30 FPS, the character would move 60 pixels in one second. But if you later decided to change the framerate to 60 FPS, the character would move 120 pixels each second — twice as fast! You would have to go through your entire game and adjust the speed for every character, which is a hassle.

Also consider the other end: slow computers. Your game may reliably run at 30 FPS on your computer, but an older or slower computer might only be able to run at 15 FPS. In that case, all the characters would move half as fast, which could make the game a lot less fun. Even worse, if more objects started to appear on the screen, the framerate would start to drop, and the gameplay would get even slower!

The solution to these problems is simple: define your gameplay in per-second or per-millisecond units, instead of per-frame units. You can then use the frame time to calculate how much the game changed in the past frame. This is called “framerate independence”.

Framerate Independence with Rubygame

It’s very easy to make your game framerate independent with Rubygame. So, you have no excuse not to do it, not even laziness!

The first step is making sure all your speeds and other gameplay rates are defined as per-second or per-millisecond units: move 60 pixels per second, lose 30% HP per second, etc. The choice to use seconds or milliseconds is up to you, and doesn’t make a difference as long as you use the same time scale everywhere.

The second step is to use the value returned by Clock#tick each frame to find out how long it has been since the previous frame, and use that time to calculate how far the character has moved in the past frame, how much HP they have lost, etc. (I will assume for this tutorial that you have called Clock#enable_tick_events, so that Clock#tick will return a ClockTicked event.)

This calculation is very simple: just multiply the number of seconds (or milliseconds) by the speed or rate. Remember to use the correct units! If your rates are defined in per-second units, multiply by ClockTicked#seconds; if they are per-millisecond units, multiply by ClockTicked#milliseconds.

  tick_event = Clock.tick
  movement = speed * tick_event.seconds

For a full example of how to apply this concept in code, see samples/framerate.rb in the Rubygame source distribution.

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